IMS Structures and Functions

Part 2 in a series (Part  1 | Part  3)

This article discusses IMS database organization,  access methods, secondary indexes, and logical relationships. It covers the  following topics:

Control Blockshttp://www.dbazine.com/ofinterest/oi-articles/ims2#blocks

Access Methods

Hierarchic  Sequential Databases

Hierarchic  Direct Databases

Fast Path  Databases

The Role  of Secondary Indexes

The Role of  Logical Relationships

Control Blocks

When you create an IMS database, you tell IMS what the physical structure of  the database will be-the segment names, segment lengths, the fields that each  segment will contain, the segment's position in the hierarchy, and so on. You  also tell IMS what segments can be accessed, whether they can be updated,  deleted, or new ones inserted, and other access control specifications. You do  this through a series of specifications that will be contained in control  blocks, also called DL/I control blocks, because the DL/I command language is  used perform the data manipulation functions. Control blocks do just what the  name implies-they control the way in which IMS will structure and access the  data stored in the database.

The data structure and control specifications you write will be contained in  three major control blocks:

      • DBD, which describes the database organization and access methods

      • PSB, which describes an application program's view and use of the database

      • ACB, which combines information from the DBD and PSB

Database Description

A database description (DBD) is a series of macro statements that define the  type of database, all segments and fields, and any logical relationships or  indexing. DBD macro statements are submitted to the DBDGEN utility, which  generates a DBD control block and stores it in the IMS.DBDLIB library for use  when an application program accesses the database.

Figure 2-1 shows a sample DBD for an HDAM database. When the DBD is assembled  and link-edited, a load module is created and stored in an IMS DBDLIB library.  In the DBDGEN process, each segment is assigned a segment code, a one-byte value  in ascending sequence, that is used to identify the segment in physical  storage.

In the DBD statement, an IMS access method and a system access method are  specified (HDAM, OSAM in this example). The roles of the two access methods are  discussed in greater detail in "Access Methods."

Fields within each segment can be defined as key fields or non-key search  fields for use by application programs in retrieving segments. A key field is  used for searching and sequencing. Each segment occurrence will be placed in a  database record according to the sequence of the key fields. In Figure 2-1, the  statement for field COLLID (college ID) is defined as a sequence field (SEQ) and  as unique (U). Only fields that will be used in SSAs or that are key fields must  be defined in the DBD.

ims2-1.gif

Figure 2-1: DBD for an HDAM database with secondary  index.

 

The DBD contains the following statements:

 

DATASETDefines the DDname and block size of a data set. One DATASET statement is  required for each data set group.
SEGMDefines a segment type, its position in the hierarchy, its physical  characteristics, and its relationship to other segments. Up to 15 hierarchic  levels can be defined. The maximum number of segment types for a single database  is 255.
FIELDDefines a field within a segment. The maximum number of fields per segment  is 255. The maximum number of fields per database is 1,000.
LCHILDDefines a secondary index or logical relationship between two segments. It  also is used to define the relationship between a HIDAM index and the root  segment of the database.
XDFLDUsed only when a secondary index exists. It is associated with the target  segment and specifies the name of the indexed field, the name of the source  segment, and the field to be used to create the secondary index. See "The Role  of Secondary Indexes" for more information.
DBDGEN       Indicates the end of statements defining the DBD.
ENDIndicates to the assembler that there are no more  statements.

 

DBD Names the database being described and specifies its organization.

Program Specification Block

The program specification block (PSB) is a series of macro statements that  describe the data access characteristics of an application program. Among other  things, the PSB specifies:

      • all databases that the application program will access

      • which segments in the database that the application program is sensitive to

      • how the application program can use the segments (inquiry or  update)

A PSB consists of one or more program communication blocks (PCBs). The PCB  specifies the segments to which the application program can have access and the  processing authorization for each segment. You define a PCB for each database  (or each view of the database) accessed by the application program. In the  application program host code, you specify the PSB for that application.

 

For each PCB, you must code a corresponding block in the application  program's linkage section. These data communication I/O areas are used for  communication between IMS and the application. (There are actually two types of  PCBs, a database PCB and a data communications PCB.)

 

PCBs contain SENSEG (sensitive segment) and SENFLD (sensitive field)  statements. These statements allow you to specify which segments and fields the  application program will "see." If you define a segment as sensitive, it will be  accessible to the application. If you do not, it will be ignored by the  application program. This gives you great flexibility in creating the views that  application programs will have of your database.

 

The PSB macros are used as input to the PSBGEN utility, a macro assembler  that generates a PSB control block. The PSB control block is stored in the  IMS.PSBLIB library for use during database processing. There can be many PSBs  for one DBD.

Figure 2-2 shows the structure of PSB generation input.

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Figure 2-2: Sample PSBGEN generation input.

The PSB statements include the following:

 

PCBDefines the database to be accessed by the application program. The  statement also defines the type of operations allowed by the application  program. Each database requires a separate PCB statement. PSB generation allows  for up to 255 database PCBs (less the number of alternate PCBs defined).
SENSEGDefines the segment types to which the application program will be  sensitive. A separate SENSEG statement is required for each segment type. If a  segment is defined as sensitive, all the segments in the path from the root to  that segment must also be defined as sensitive. Specific segments in the path  can be exempted from sensitivity by coding PROCOPT=K in the SENSEG  statement.
SENFLDDefines the fields in a segment type to which the application program is  sensitive. Can be used only in association with field-level sensitivity. The  SENFLD statement must follow the SENSEG statement to which it is related.
PROCOPT    Defines the type of access to a database or segment. PROCOPTs can be used on  the PCB or SENSEG statements. Primary PROCOPT codes are as follows:
G    read only
R    replace, includes G
I     insert
D    delete, includes G
A    get and update, includes G, R, I, D

K    used on SENSEG statement; program will have key-only sensitivity  to this  segment

L    load database
     Secondary PROCOPT codes are as follows:
E    exclusive use of hierarchy or segments
O   get only, does not lock data when in use
P    must be used if program will issue path call using the D command  code

S    sequential (LS is required to load HISAM and HIDAM databases; GS gets in  ascending sequence)

Application Control Block

Application control blocks (ACBs) are created by merging information from  PSBs and DBDs. For online applications, ACBs must be prebuilt using the ACB  maintenance utility. For batch applications, ACBs can be built dynamically using  DBDLIB and PSBLIB as input (PARM=DL/I) or the prebuilt ACB from ACBLIB can be  used (PARM=DBB). The ACBGEN process is shown in Figure 2-3.

Figure 2-3: ACB generation.

Prebuilt ACBs require less time to schedule an application program and use  less storage. The ACB maintenance utility also provides some error-checking  capability.

ACBs can be built for all PSBs, for particular PSBs, or for all PSBs that  reference a particular DBD. Prebuilt ACBs are stored in the IMS.ACBLIB library.  During ACB generation, the ACB maintenance utility must have exclusive control  of the IMS.ACBLIB. Because of this, the utility must be executed using an  IMS.ACBLIB that is not currently allocated to an active IMS system. You can  execute the ACB maintenance utility against an inactive copy of ACBLIB, then use  the IMS Online Change function to make the new members available to an active  IMS online system.

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Access Methods

IMS accesses data after it has been retrieved from DASD and places it in a  buffer pool in memory. The task of retrieving the data from DASD is performed by  one of several system access methods. These should not be confused with IMS  access methods such as HSAM, HISAM, HDAM, HIDAM, and so on. IMS access methods  are actually types of database organizations. In IMS terminology, however,  databases often are referred to by their IMS access method. An IMS database  definition must always specify an IMS access method and a system access method.  In some cases, you can choose the type of system access method you want to use.  In other cases, the system access method is dictated by the IMS access method.  HISAM, for instance, uses only VSAM.

Both the system and IMS access methods are used for IMS database retrieval  and update. Application programs specify the data to retrieve and make a DL/I  call to the system access method. The system access method returns a block of  data to IMS. The IMS access method then locates the data within the block and  passes it to the application program. The IMS database types and their access  methods are shown in Table 2-1.

Table 2-1: IMS database and system access types.

VSAM

In the discussion on HISAM and HIDAM databases later in this chapter, you  will find reference to VSAM, particularly in association with VSAM key-sequenced  data sets (KSDSs) and entry-sequenced data sets (ESDSs), because of the way in  which certain databases use these data sets. Before discussing the various IMS  access methods, it is helpful to have an understanding of VSAM's role in the  storage and retrieval of data. VSAM performs the physical I/O of data for IMS.  It retrieves the data from DASD and places it in the main storage buffer pool  for use by IMS. When processing has been completed, VSAM returns the data to  DASD, where it is stored until needed again. To perform these functions, VSAM  uses its own set of data storage and retrieval structures.

A VSAM data set consists of a set of records. The records are grouped into  control intervals (CIs), which in turn are grouped into larger groupings called  control areas (CAs). The layout of a control interval is shown in Figure  2-4.

Figure 2-4: VSAM control interval layout.

A VSAM CI consists of records, free space, and control information. You can  determine the size of a CI or let VSAM do it for you. When you define the size  of a CI for a data set, all CIs in the data set will be the same size. When you  define the CI, you also determine the percentage of free space to be designated.  You will attempt to create enough free space to avoid CI splits while not using  so much free space that you waste DASD. CI splits occur when there is no room to  insert another record; consequently, VSAM moves half of the records from the CI  where the record was to be inserted to a new CI. CI splits are a costly  overhead, especially in high-activity systems. (You can correct CI splits by  reorganizing the database.)

CIs are grouped inside a control area (CA). The goal is to have enough unused  CIs to allow new data to be added without causing a CA split. CA splits are more  processing-intensive than CI splits. On the other hand, you don't want to waste  DASD by defining too many unused CIs. For information on calculating space  requirements, refer to the IBM manuals IMS/ESA Administration Guide: Database  Manager and IMS/ESA Administration Guide: System.

The control information portion of the CI contains two types of fields:

      • The record definition field (RDF) contains information on the records stored  in the CI, their length, and whether they are fixed or variable length.

      • The control interval definition field (CIDF) contains information on the CI  itself. It keeps track of the amount of free space available and where the free  space is located relative to the beginning of the CI. CIs have only one CIDF but  may have a variable number of RDFs, depending on whether the CI contains  fixed-length or variable-length records or a combination of the  two.

Sequence Sets and Indexes

For KSDSs, VSAM keeps track of all CAs and CIs through the use of two levels  of indexing-the sequence set and the index set.

VSAM maintains a sequence set record for each CA in the data set. The  sequence set record contains an entry for each CI in the CA. Each entry contains  the key of the highest record in the CI and a pointer to the CI. The index  contains an entry for each sequence set record. This gives the index an entry  for each CA, since there is a sequence set for every CA. Each index entry  contains the key of the highest record in its CA and a pointer to the sequence  set record for that CA.

By following the values of record keys from index to sequence set to CA to  CI, VSAM can locate any record in the data set. When VSAM reaches the CI, it can  obtain record information from the CIDF and RDFs of the CI. The example in  Figure 2-5 illustrates this concept.

Figure 2-5: Structure of VSAM index set and sequence set  records.

Key-Sequenced Data Sets

The data sets we have described so far have been key-sequenced data sets  (KSDSs). You can see that the name derives from the way VSAM stores and  retrieves records based on the record key.

VSAM can retrieve the records in a KSDS in a number of ways. The simplest and  most obvious way is to read each record in the logical order (lowest key to  highest key) in which they are stored. This is called sequential retrieval.  Obviously, this method has limitations if you want only some of the records or  if you want them in other than key sequence order.

VSAM can use the key sequence retrieval method to return only a portion of  the records. This method is called keyed sequential retrieval or skip sequential  retrieval. With this method, you specify the keys of the records you want  retrieved, but they must be in ascending order. Another method, addressed  sequential retrieval, locates the records to be retrieved by their RBA (relative  byte address-the number of bytes from the beginning of the data set to the  beginning of the record). You must supply the RBAs to VSAM. Addressed sequential  retrieval can be used with KSDSs but is primarily designed for ESDSs.

VSAM can also retrieve KSDS records directly. You provide the record key, and  VSAM uses the index set and sequence set to navigate its way to the correct CI  and to the record you requested. With this method, you can retrieve records in  any order.

VSAM can retrieve a record directly by its RBA. This method, addressed direct  retrieval, like addressed sequential retrieval, is designed primarily for  ESDSs.

Entry-Sequenced Data Sets

Entry-sequenced data sets (ESDSs) are stored in the order in which they are  loaded, rather than by key sequence. With ESDSs, VSAM does not create an index  and does not reserve free space. No index is needed because there are no record  keys to track. Likewise, free space is not needed because the next record added  to the data set is stored at the end of the existing set of records. If a record  is too large to fit in the CI being loaded, VSAM creates a new CI and puts the  record there. VSAM does not attempt to use space that may be left at the end of  each CI.

ESDSs are retrieved only by RBA using either addressed sequential retrieval  or addressed direct sequential retrieval. With addressed sequential retrieval,  you give VSAM the RBA of the first record. It retrieves the succeeding records  by computing their RBA based on the record length field of each record's RDF.  With the addressed direct method, you must supply VSAM with the RBA of each  record you want.

Because of their storage and retrieval mechanisms, ESDSs have certain  limitations that make them less attractive for many applications. Although  updating is relatively simple, adding and deleting records proves more  difficult. With updating, you read the record, enter changes, and rewrite it,  without changing the record length. To delete, you read the record and mark it  for deletion, but VSAM does not physically delete the record or reclaim the  unused space. To add a record, you must add it at the end of the data set.

QSAM

The queued sequential access method (QSAM) processes records sequentially  from the beginning of the data set. QSAM groups logical records into physical  blocks before writing them to storage and handles the blocking and deblocking of  records for you. QSAM is typically used by application programs that retrieve or  create a single member at a time within a partitioned data set (PDS). The  characteristics of a member of a PDS-which is a collection of sequentially  organized members-are the same as those of a sequential data set.

BSAM

The basic sequential access method (BSAM) allows you to read and write  physical records only. It does not perform blocking or deblocking of records.  With BSAM, you can begin processing a data set at any point

BDAM

The basic direct access method (BDAM) allows you to write or retrieve records  directly by address, using the physical track, relative track, or relative  record number.

OSAM

The overflow sequential access method (OSAM) was developed for use with DL/I  databases. It combines many features of sequential access methods and of BDAM.  To the operating system, an OSAM data set appears the same as a sequential data  set. An OSAM data set can be read with BSAM or QSAM. OSAM allows direct access  to records.

Database Organizations

The nine types of databases supported by IMS can be grouped by their IMS  access method. Hierarchic sequentially accessed databases include

      • HSAM

      • SHSAM

      • HISAM

      • SHISAM

      • GSAM

Hierarchic direct databases include

      • HDAM

      • HIDAM

Fast Path databases provide fast access with limited functionality

      • DEDB

      • MSDB

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Hierarchic Sequential Databases

The earliest IMS database organization types were based on sequential storage  and access of database segments. Hierarchic sequential databases share certain  characteristics. Compared to hierarchic direct databases, which we will discuss  later, hierarchic sequential databases are of simpler organization. The root and  dependent segments of a record are related by physical adjacency. Access to  dependent segments is always sequential. Deleted dependent segments are not  physically removed but are marked as deleted. Hierarchic sequential databases  can be stored on tape or DASD.

HSAM

In a hierarchic sequential access method (HSAM) database, the segments in  each record are stored physically adjacent. Records are loaded sequentially with  root segments in ascending key sequence. Dependent segments are stored in  hierarchic sequence. The record format is fixed-length and unblocked. An HSAM  database is updated by rewriting the entire database. Although HSAM databases  can be stored on DASD or tape, HSAM is basically a tape-based format. Figure 2-6  shows an HSAM database record and segment format.

Figure 2-6: HSAM database segment structure.

IMS identifies HSAM segments by creating a two-byte prefix consisting of a  segment code and a delete byte at the beginning of each segment. HSAM segments  are accessed through two operating system access methods:

      • basic sequential access method (BSAM)

      • queued sequential access method (QSAM)

QSAM is always used as the access method when the system is processing  online, but you can specify either method for batch processing through the  PROCOPT parameter in the PCB.

Entry to an HSAM database is through GET UNIQUE (GU) or GET NEXT (GN) calls.  The first call starts at the beginning of the database and searches sequentially  through the records until it locates the requested segment. Subsequent calls use  that position as the starting point for calls that process forward in the  database.

HSAM databases are limited by the strictly sequential nature of the access  method. DELETE (DLET) and REPLACE (REPL) calls are not allowed, and INSERT  (ISRT) calls are allowed only during the database load. Field-level sensitivity  is provided, but HSAM databases are limited in the number of IMS options they  can use.

Because of the numerous limitations, HSAM databases see limited use and are  reserved primarily for applications that require sequential processing only.

SHSAM

A simple HSAM (SHSAM) database contains only one type of segment-a  fixed-length root segment. Because there is no need for a segment code and  deletes are not allowed, there is no need for a prefix portion of a SHSAM  database segment. Because they contain only user data, SHSAM databases can be  accessed by BSAM and QSAM. The only DL/I calls used with SHSAM databases are the  GET calls. Like HSAM, SHSAM database segments can be deleted or inserted only  during a reload.

HISAM

The hierarchic indexed sequential access method (HISAM) database organization  adds some badly needed capabilities not provided by HSAM. Like HSAM, HISAM  databases store segments within each record in physically adjacent sequential  order. Unlike HSAM, each HISAM record is indexed, allowing direct access to each  record. This eliminates the need to read sequentially through each record until  the desired record is found. As a result, random data access is considerably  faster than with HSAM. HISAM databases also provide a method for sequential  access when that is needed.

A HISAM database is stored in a combination of two data sets. The database  index and all segments in a database record that fit into one logical record are  stored in a primary data set that is a VSAM KSDS. Remaining segments are stored  in the overflow data set, which is a VSAM ESDS. The index points to the CI  containing the root segment, and the logical record in the KSDS points to the  logical record in the ESDS, if necessary.

If segments remain to be loaded after the KSDS record and the ESDS record  have been filled, IMS uses another ESDS record, stores the additional segments  there, and links the second ESDS record with a pointer in the first record. You  determine the record length for the KSDS and the ESDS when you create the DBD  for the database.

If segments are deleted from the database, they are still physically present  in the correct position within the hierarchy, but a delete byte is set to show  that the record has been deleted. Although the segment is no longer visible to  the application program, it remains physically present and the space it occupies  is unavailable until the database is reorganized. The only exception to this is  the deletion of a root segment where the logical record in the VSAM KSDS is  physically deleted and the index entry is removed; any VSAM ESDS logical records  in the overflow data set are not be deleted or updated in any way.

Inserting segments into a HISAM database often entails a significant amount  of I/O activity. Because IMS must enforce the requirement for segments to be  physically adjacent and in hierarchic order, it will move existing segments  within the record or across records to make room for the insertion; however, any  dependent segments are not flagged as deleted. To facilitate indexing, HISAM  databases must be defined with a unique sequence field in each root segment. The  sequence fields are used to construct the index.

HISAM databases are stored on DASD, and data access can be much faster than  with HSAM databases. All DL/I calls can be used against a HISAM database.  Additionally, HISAM databases are supported by a greater number of IMS and MVS  options.

HISAM databases work well for data that requires direct access to records and  sequential processing of segments within each record.

Figure 2-7 shows the database structure for HISAM. Notice that four ESDS  records have been used in loading one logical record. The arrows represent  pointers.

Figure 2-7: HISAM database structure.

HISAM Segment Structure

Figure 2-8 shows the HISAM segment structure.

Figure 2-8: HISAM segment structure.

A HISAM segment contains the following fields:

 

Segment Code1 byte. The segment code byte contains a one-byte unsigned binary number  that is unique to the segment type within the database. The segments are  numbered in hierarchic order, starting at 1 and ending with 255 (X'01' through  X'FF').
Delete Byte1 byte. The delete byte contains a set of  flags.

Counters and Pointers

The appearance of this area depends on the logical relationship status of the  segment:

    • If the segment is not a logical child or logical parent, this area is  omitted.

    • If the segment is a logical child, and if a direct pointer (see "Pointer  Types") is specified (the logical parent must be in an HD database), the  four-byte RBA of the logical parent will be present.

    • If the segment is a logical parent and has a logical relationship that is  unidirectional or bidirectional with physical pairing, a four-byte counter will  exist.

If the segment is a logical parent and has one or more logical relationships  that are bidirectional with virtual pairing, then for each relationship there is  a four-byte RBA pointer to the first logical child segment (a logical child  first pointer) and, optionally, a four-byte RBA pointer to the last logical  child segment (a logical child last pointer), depending on whether you specified  LCHILD=SNGL or LCHILD=DBLE in the DBD.

There is only one counter in a segment, but there can be multiple logical  child first (LCF) and logical child last (LCL) pointers. The counter precedes  the pointers. The pointers are in the order that the logical relationships are  defined in the DBD, with a logical child first pointer before a logical child  last pointer.

Figure 2-9 shows a segment with multiple logical child pointers.

Figure 2-9: Multiple logical child pointers in a  segment.

Data

The length of the data area (which is specified in the DBD) can be a fixed  length or a variable length. For a logical child segment with symbolic keys  (PARENT=PHYSICAL on the SEGM statement), the concatenated key of the logical  parent will be at the start of the segment.

If the segment is variable length, the first two bytes of the data area are a  hexadecimal number that represents the length of the data area, including the  two-byte length field.

SHISAM

As is the case with SHSAM, a simple HISAM (SHISAM) database contains only a  root segment, and its segment has no prefix portion. SHISAM databases can use  only VSAM as their access method. The data must be stored in a KSDS. All DL/I  calls can be used with SHISAM databases, and their segments can be accessed by  DL/I calls and VSAM macros.

GSAM

Generalized sequential access method (GSAM) databases are designed to be  compatible with MVS data sets. They are used primarily when converting from an  existing MVS-based application to IMS because they allow access to both during  the conversion process. To be compatible with MVS data sets, GSAM databases have  no hierarchy, database records, segments, or keys. GSAM databases can be based  on the VSAM or QSAM/BSAM MVS access methods. They can have fixed-length or  variable-length records when used with VSAM or fixed-length, variable-length, or  undefined-length records when used with QSAM/BSAM.

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Hierarchic Direct Databases

Hierarchic direct access method (HDAM) and hierarchic indexed direct access  method (HIDAM) databases are referred to collectively as HD databases. The  hierarchic direct databases were developed to overcome some of the deficiencies  of sequential access databases. HD databases share these characteristics:

      • Pointers are used to relate segments.

      • Deleted segments are physically removed.

      • VSAM ESDS or OSAM data sets are used for storage.

      • HD databases are stored on DASD.

      • HD databases are of a more complex organization than sequentially organized  databases.

The following sections discuss HDAM, PHDAM, HIDAM and PHIDAM database  organizations. Because pointers play such an integral role in direct access  databases, they are referenced frequently in the text. Pointers are four-byte  address values that give the offset from the beginning of the data set of the  segment being addressed. They tie the segments together without the need for  segments to be physically adjacent. Segments can be inserted or deleted without  the need to move other segments. There are different types of pointers. Pointers  are discussed in greater detail in "Pointer Types" below.

HDAM

HDAM databases are typically used when you need fast access to the root  segment of the database record, usually by direct access. In a hierarchic direct  access method (HDAM) database, the root segments of records are randomized to a  storage location by an algorithm that converts a root's key into a storage  location. No index or sequential ordering of records or segments is involved.  The randomizing module reads the root's key and, through an arithmetic  technique, determines the storage address of the root segment. The storage  location to which the roots are randomized are called anchor points or root  anchor points (RAPs). The randomizing algorithm usually attempts to achieve a  random distribution of records across the data set. Theoretically, randomizing  the location of records minimizes the number of accesses required to retrieve a  root segment.

The randomizing technique results in extremely fast retrieval of data, but it  usually does not provide for sequential retrieval of records. This can be  achieved in HDAM databases through the use of secondary indexes or by using a  physical-key-sequencing randomizer module.

The advantage of HDAM is that it does not require reading an index to access  the database. The randomizing module provides fast access to root segments and  to the paths of dependent segments. It uses only the paths of the hierarchy  needed to reach the segment being accessed, further increasing access speed. The  disadvantage is that you cannot process HDAM databases in key sequence unless  the randomizing module stores root segments in physical key sequence.

An HDAM database consists of one data set split into two parts: the root  addressable area (RAA) and the overflow area. The data set can be a VSAM ESDS or  an OSAM data set. You specify which access method to use in the DBD ACCESS  parameter.

In designing an HDAM database, you decide the size of the RAA and the number  of CIs (or blocks, if you are using OSAM) that it will be broken down into.  Within a CI or block, you define the number of Raps The randomizer uses these  parameters in establishing a storage location for the root segment of the  record. The CI does not contain just RAAs and Raps It also is used for storage  of the record's dependent segments. Each CI begins with a free space element  anchor point (FSEAP) area. The FSEAP is used to locate free or unused space in  the block. When IMS inserts new segments in the block, it updates the FSEAP.

Figure 2-10 shows how a record appears in an HDAM database.

Figure 2-10: HDAM database structure.

Three CIs are in the RAA, and four CIs are in the overflow area. Together,  they make up the data set. Each CI in the RAA contains two Raps In the example,  the College record randomized to the second RAP in the second CI in the data  set. The RAP does not contain the root segment. It contains a four-byte pointer  that contains the address of the root segment.

After an initial load, segments within a database record are very likely to  be grouped together in blocks/CIs in hierarchic order. After update activity,  this may not be the case.

To insert a root segment, IMS invokes the HDAM randomizer that has been  specified in the DBD. This determines the RAP from which the root must be  chained.

When a segment has been inserted in an HDAM database, the segment is never  moved so that all the direct address pointers are preserved. The example assumes  physical child first pointers and twin forward pointers are specified for all  segment types. If, as in the case under consideration, there are more segments  in the database record at initial load than the BYTES parameter in the DBD will  allow to be inserted in the RAA, all other segments will be inserted into  overflow. This is also true for multiple inserts of segments within the same  database record during the same synchronization point. If the database were to  use multiple data set groups, each secondary data set group would have the  format of the overflow part of an HDAM database.

If segments are deleted from the database segment, they are physically  deleted from the data set and the space is designated as free space.

Bit Maps

In HD databases, bit maps are used to keep track of free space. A bit map is  a string of bits that indicate whether enough space is available in a CI or OSAM  block to contain an occurrence of the longest segment defined for the data set.  In a VSAM ESDS, the bit map is located in the second CI of the data set. (The  first CI is reserved.) In OSAM, bit maps are put in the first block of the first  extent of the data set.

As you read the bits in a bit map from left to right, they reflect the status  of the corresponding CI or block following the bit map. The bits are set as  follows:

      • 0 if not enough space exists in the CI or block

      • 1 if there is adequate space for the longest segment specified for the data  set.

For example, if the first bit is set to 1, the first CI or block following  the block containing the bit map will have space available. If the third bit in  the bit map is set to 0, the third CI or block following that containing the bit  map will not have sufficient space available, and so on. As data is added and  new CIs are created, the number of CIs may reach the size limit of the bit map.  If so, another bit map is created and the process repeated for CIs that are  added subsequently.

Free Space Element Anchor Point

Another specialized field, the free space element anchor point (FSEAP), is  used in determining the availability of free space within a CI or block. As  shown in Figure 2-10, the FSEAP is located at the beginning of each CI or OSAM  block in a data set. An FSEAP is made up of two 2-byte fields. The first field  contains the offset, in bytes, to the first free space element (FSE) in the  block. An FSE is associated with each area of free space within the block or CI.  The FSEs in a CI are chained together using pointers. The second field in the  FSEAP tells whether this block or CI contains a bit map.

Free Space Element

In addition to an FSEAP, each OSAM block or VSAM CI contains free space  elements (FSEs) that provide information about the free space available in the  block. The FSE makes up the first eight bytes of each area of free space. It  consists of the following fields:

      • Free space chain pointer (CP). This 2-byte field gives the offset in bytes  to the beginning of the next FSE in the block or CI. If it is the last FSE in  the block, it is set to zero.

      • Available length field (AL). This 2-byte field gives the length of the free  space area, including the length of the FSE.

      • Task ID field (ID). This 4-byte field contains the task ID of the program  that freed the space. This field is used to allow a program to reuse the same  space during a specified period without contending with other programs for the  space.

PHDAM

PHDAM databases are partitioned HDAM databases. Each PHDAM database is  divided into a maximum of 1001 partitions which can be treated as separate  databases. A PHDAM database is also referred to as a High Availability Large  Database (HALDB).

HIDAM

Unlike HDAM, HIDAM databases use an index to locate root segments. HIDAM  databases are typically used when you would like to access database records  randomly and sequentially and also access segments randomly within a record. The  index and the database are stored in separate data sets. The index is stored as  a single VSAM KSDS. The database is stored as a VSAM ESDS or OSAM data set. The  index stores the value of the key of each root segment, with a four-byte pointer  that contains the address of the root segment.

The root segment locations in the index are stored in sequential order,  allowing HIDAM databases to be processed directly or sequentially. A  disadvantage of HIDAM databases is that the additional step required to scan an  index makes access slower than with HDAM databases.

When you access a record by root key, IMS searches for the key in the index  and uses the pointer to go directly to the record. If the PTR =TB or PTR=HB  (twin backward pointer or hierarchic backward pointer) parameter is defined for  the root, the root segments are chained together in ascending order. Sequential  processing is done by following this pointer chain. In HIDAM, Raps are generated  only if you specify the PTR=T or PTR=H (twin pointer or hierarchic pointer)  parameter for the root. When either of these pointer parameters is defined, IMS  puts one RAP at the beginning of the CI or block. Root segments within the CI or  block are chained by pointers from the most recently inserted back to the first  root on the RAP. The result is that the pointers from one root to the next  cannot be used to process roots sequentially. Sequential processing must be  performed by using key values, which requires the use of the index and increases  access time. For this reason, you should specify PTR=TB or PTR=HB for root  segments in HIDAM databases.

Figure 2-11 shows how the database record exists in a HIDAM database.

Figure 2-11: HIDAM database structure.

After an initial load, segments within a database record are stored  physically in blocks/CIs in hierarchic order. After update activity, this may  not be the case.

When a segment has been inserted in a HIDAM database, the segment is never  moved to preserve all the direct address pointers. The example assumes that  physical child first pointers and twin forward pointers are specified for all  segment types.

If segments are deleted from the database segment, they are physically  deleted from the data set, and the space is designated as free space. If the  database has multiple data set groups, each secondary data set group has the  format of the main part of the HIDAM database.

PHIDAM

PHIDAM databases are partitioned HIDAM databases. Each PHIDAM database is  divided into a maximum of 1001 partitions which can be treated as separate  databases. A PHIDAM database is also referred to as a High Availability Large  Database (HALDB).

HDAM/HIDAM Segment Structure

Figure 2-12 shows the HDAM/HIDAM segment structure.

Figure 2-12: HDAM/HIDAM segment structure.

An HDAM/HIDAM segment contains the following fields:

 

Segment Code1 byte. The segment code byte contains a one-byte unsigned binary number  that is unique to the segment type within the database. The segments are  numbered in hierarchic order, starting at 1 and ending with 255 (X'01' through  X'FF').
Delete Byte1 byte. The delete byte contains a set of  flags.

Counters and Pointers

The following four-byte fields can be in this area and, if present, will  occur in the following order:

Counter

If the segment has a logical relationship that is unidirectional or  bidirectional with physical pairing, a four-byte counter will  exist.

Hierarchic or Physical Twin Pointers

A hierarchic forward pointer or a twin forward pointer must be present in the  segment prefix, unless PTR=NOTWIN was specified in the SEGM statement.  Hierarchic backward pointers or twin backward pointers can also be present  (PTR=HB or PTR=TB on SEGM statement).

Physical Parent Pointer

This pointer will be present if the segment is a logical child or has a  logical child below it in the hierarchy. It will also be generated if the  segment is a logical parent or has a logical parent below it in the  hierarchy.

It is also present in any segments that are target segments for a secondary  index or that lie on the hierarchical paths between the root segment and any  target segments.

Logical Twin Pointers

A logical twin forward pointer and a logical twin backward pointer exist only  in a logical child segment with a bidirectional logical relationship that is  virtually paired. A logical twin backward pointer will be present if LTWINBWD is  specified on the SEGM statement of the logical child segment.

Logical Parent Pointer

A logical child segment can have a direct pointer to its logical parent  (PHYSICAL specified on SEGM statement) if the logical parent is in an HD  database.

Logical Child Pointers

If the segment is a logical parent and has one or more logical relationships  that are bidirectional with virtual pairing, for each relationship there is a  four-byte RBA pointer to the first logical child segment (a logical child first  pointer). There can also be a four-byte RBA pointer to the last logical child  segment (a logical child last pointer), depending on whether you specified  LCHILD=SNGL or LCHILD=DBLE in the DBD. The pointers are in the order that the  logical relationships are defined in the DBD, with a logical child first pointer  before a logical child last pointer.

Physical Child Pointers

If physical pointers rather than hierarchical pointers have been chosen for  the segment, there is a four-byte RBA pointer to the first physical child  segment (a physical child first pointer). There can also be a four-byte RBA  pointer to the last physical child segment (a physical child last pointer). The  pointers are in the order that the physical children are defined in the DBD,  with a physical child first pointer before a physical child last  pointer.

Figure 2-13 shows a logical parent with unidirectional relationships, two  bidirectional virtually paired relationships (the second with backward  pointers), and three physical segments (the second with backward pointers).

Figure 2-13: Sample counter and pointers in a typical  HD-type segment.

 

Data         The data area can be a fixed length that is specified in the DBD or variable  length (described in "Variable-Length Segment Structure"). For a logical child  segment with symbolic keys (PARENT=PHYSICAL on the SEGM statement), the  concatenated key of the logical parent will be at the start of the  segment.
Data PadIf the segment length is an odd number of bytes, a one-byte pad will be  appended to the segment to ensure that all segments start on half-word  boundaries.

Variable-Length Segment Structure

Figure 2-14 depicts a variable-length segment (VLS) that can exist in HISAM,  HDAM, and HIDAM databases.

Figure 2-14: Variable length segment structure.

Variable-length segments contain the following fields:

 

Segment CodeSee the definition for the appropriate database organization.
Delete ByteSee the definition for the appropriate database organization.
Counters and PointersSee the definition for the appropriate database organization.
Length Field2 bytes. Signed binary number that specifies the length of the data portion  of the segment, including the length field itself.
Data              See the definition for the appropriate database organization. If a  variable-length segment in a HISAM database is replaced and is longer than it  was before, IMS moves the following segments to make room for the new segment.  IMS does not move HDAM and HIDAM database segments once they have been inserted.  Instead, it splits the segment, leaving the prefix part in the original location  and inserting the data part in another location. The two parts are connected by  a VLS pointer.

You can make a segment variable in length in one of two ways:

      • by specifically defining it to be variable in length by using the SIZE  operand in the SEGM macro of the DBD

      • by implicitly defining it to be variable length by using the COMPRTN  (compression routine) operand in the SEGM macro of the DBD

Use of a compression routine always makes the segment variable length in the  data, but may be presented to the user through a DL/I call as fixed length. How  the user sees the segment data is determined by the SIZE parameter in the  DBD.

Figure 2-15: Split variable length segment  structure.

Prefix Portion

The prefix portion contains the following fields:

Segment Code

See the definition for the appropriate database organization.

Delete Byte

See the definition for the appropriate database organization. BIT 4 is  on,

indicating that the prefix and data are separated. Typically this is  X'08'.

Counters and Pointers

See the definition for the appropriate database organization.

VLS Pointer

4 bytes. RBA of the data part of the variable length segment.

Free Space

Normal IMS free space (see "Free Space Element").

Data Portion

The data portion contains the following fields:

Segment Code

See the definition for the appropriate database organization.

Delete Byte

This will always be X'FF'.

Length Field

2 bytes. Signed binary number that specifies the length of the data portion  of the segment, including the length field.

Data                 

See the definition for the appropriate database  organization.

Pointer Types

Pointers play an important role in accessing data stored in hierarchic direct  access and indexed databases. An understanding of the various types of pointers  and how they are used is important in understanding how database segments are  accessed.

In HD databases, the records are kept in hierarchical sequence through the  use of pointers-four-byte fields that contain the address of another segment.  The prefix in every HD segment contains one or more pointers, and each pointer  contains the relative byte address (relative to the beginning of the database)  of the segment to which it points.

There are a number of types of pointers, and they can be mixed within a  database record to meet specific data access requirements.

Hierarchic Forward Pointers

Hierarchic forward (HF) pointers point to the next segment in the hierarchy.  Segments are located by following the HF pointers to the desired segment. This  is essentially the same as using a sequentially accessed database. As with a  sequential database, when HF pointers are used, all the segments in the  hierarchy must be searched until the requested segment is located. HF pointers  are most suitable for databases where segments are typically processed in  hierarchic sequence.

Figure 2-16 shows an example of hierarchic forward pointers.

Figure 2-16: Examples of Hierarchic Forward  Pointers.

Hierarchic Forward and Hierarchic Backward Pointers

With hierarchic forward (HF) and hierarchic backward (HB) pointers, each  segment points to the next segment and the previous segment in the hierarchy.  Because HF and HB pointers are four bytes each, combining them requires eight  bytes in each dependent segment. Root segments require 12 bytes, because the  root points forward to the next root, backward to the previous root, and forward  to the first dependent segment.

Physical Child First Pointers

With physical child first (PCF) pointers, each parent segment points to the  first occurrence of each of its immediate child segment types. No pointers exist  to connect occurrences of the same segment type under a parent, which means that  the hierarchy is not completely connected. Physical twin (PT) pointers can be  used to connect twin segments and complete the hierarchy.

Physical Child First and Physical Child Last Pointers

With physical child first (PCF) and physical child last (PCL) pointers, each  parent segment in a record points to the first and last occurrence of its  immediately dependent segment types. PCL pointers cannot be used alone; PCF  pointers must be used with them.

PCL pointers are most often used with PCF pointers in two cases:

      • when no sequence field is defined for the segment type

      • when new segment occurrences of a type are inserted at the end of existing  occurrences

PCL pointers give fast access to the last occurrence of a segment type,  a
feature that significantly speeds inserts applied at the end of  existing
occurrences. PCL pointers also provide fast retrieval of the last  occurrence of
a segment type.

Physical Twin Forward Pointers

With physical twin forward (PTF) pointers, each occurrence of a given segment  type under the same parent points forward to the next occurrence. PTF pointers  can be specified for root segments, but not in the same way as with other  segments. If PTF pointers are used alone, only part of the hierarchy is  connected. Parent and child segments are not connected. Physical child pointers  can be used to complete the hierarchy.

Physical Twin Forward and Backward Pointers

With physical child forward (PTF) and physical child backward (PTB) pointers,  each occurrence of a segment type under the same parent points forward and  backward to occurrences of the same type. As with other forward and backward  pointers, PTBs can be used only with PTFs. When PTF and PTB pointers are  specified for root segments, they point to the next root segment and the  previous root segment in the database.

Symbolic Pointers

IMS also allows the use of symbolic pointers. A symbolic pointer uses the  concatenated key of the segment it points to, rather than the segment's direct  address. A concatenated key consists of the key fields of the root segment and  successive child segments, to the key field of the accessed segment. For more  information on symbolic pointers, see "The Role of Secondary Indexes" and "The  Role of Logical Relationships."

Multiple Data Set Groups

This manual has discussed the use of two data sets that can be using for  storing databases-the primary data set and a secondary, or overflow, data set.  IMS also allows you to store HISAM and HD databases on up to 10 data set  groups-the primary data set group and a maximum of nine secondary data set  groups.

The use of multiple data set groups has a number of advantages. Primarily, it  allows you to create data set groups designed for the specific needs of various  application programs. By defining data set groups in different ways, you  can:

      • separate frequently used segments from those that are seldom used

      • separate segment types that are frequently added or deleted from those that  are seldom added or deleted

      • separate segments that vary greatly is size from the other segments in the  database

As an example, you may have designed a database so that the most frequently  accessed segment types are highest in the hierarchy, so that they can be  accessed more quickly. Later, you write another application program that  frequently accesses a number of segments that are scattered randomly throughout  the hierarchy. You know that these segments will take more time and processing  to access. To overcome this difficulty, you can define a data set group to  contain the segments accessed by the second application program.

You define data set groups in the DBD for the database.

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Fast Path Databases

The Fast Path feature of IMS is designed to give improved response times for  database inquiries and updates. It does so through the use of specialized  databases and the Expedited Message Handling (EMH) facility. EMH ensures that  Fast Path transaction messages bypass normal message queuing and the priority  scheduling process.

Two types of databases can be used with the Fast Path feature of IMS. They  are data entry databases (DEDBs) and main storage databases (MSDBs).

DEDBs

DEDBs are similar in structure to an HDAM database, but with some important  differences. DEDBs are stored in special VSAM data sets called areas. The unique  storage attributes of areas are a key element of the effectiveness of DEDBs in  improving performance. While other database types allow records to span data  sets, a DEDB always stores all of the segments that make up a record in a single  area. The result is that an area can be treated as a self-contained unit. In the  same manner, each area is independent of other areas. An area can be taken  offline, for example, while a reorganization is performed on it. If an area  fails, it can be taken offline without affecting the other areas.

Areas of the same DEDB can be allocated on different volumes or volume types.  Each area can have its own space management parameters. A randomizing routine  chooses each record location, avoiding buildup on one device. These capabilities  allow greater I/O efficiency and increase the speed of access to the data.

An important advantage of DEDB areas is the flexibility they provide in  storing and accessing self-contained portions of a databases. You might choose  to store data that is typically accessed during a specific period of the day in  the same area or set of areas. You can rotate the areas online or offline as  needed to meet processing demands. For example, you might keep all records of  customers located in one time zone in one set of areas and move the areas on and  offline to coincide with the business hours of that time zone. DEDBs also make  it easier to implement very large databases. The storage limit for a DEDB area  is 4 gigabytes (GB). By using a large number of areas to store a database, you  can exceed the size limitation of 232 bytes for a VSAM data set.

A DEDB area is divided into three major parts:

      • units of work (UOWs)

      • independent overflow part

      • sequential dependent part

Figure 2-17 illustrates the structure of a DEDB area.

Figure 2-17: DEDB area structure.

A Unit of Work (UOW) is further divided into the root addressable part and  the dependent overflow parts. Record storage in the root-addressable and  dependent overflow parts of a UOW closely resembles record storage in an HDAM  database. The root and as many segments as possible are stored in the root  addressable part, and additional segment occurrences are stored in the dependent  overflow part.

If the size of a record exceeds the space available in the root-addressable  and dependent overflow parts, segments will be added in the independent overflow  part.

Because a UOW is totally independent of other UOWs, you can process a UOW  independently of the rest of the DEDB. The ability to continue processing the  remainder of an area while reorganizing a single UOW significantly increases the  data availability of a DEDB database.

Figure 2-18 shows the configuration of a VSAM CI within a UOW.

Figure 2-18: Configuration of a control interval within a  unit of work.

FSEAP       

This field gives the offset in bytes to the first free space element (FSE).  If the CI is in the sequential dependent part, these two bytes are not  used.

CI TYP

This describes the type of CI.

1    base section

2    overflow section

3    independent overflow part

4    sequential dependent part

RAP

This contains the root anchor point if the CI is in the root addressable  part of the area. Only one RAP exists per CI. Other roots randomized to this CI  will be chained off of this RAP in ascending key sequence.

CUSN

CI update sequence number. This number is increased by one with each update  of the CI.

RBA

Relative byte address of this CI.

RDF

Record definition field. This contains information on the records stored in  the CI, their length, and whether they are fixed length or variable  length.

CIDF

CI definition field. This field contains information on the control interval  itself. It keeps track of the amount of free space available and where the free  space is located, relative to the beginning of the CI. Control intervals will  have only one CIDF but may have a variable number of RDFs, depending on whether  the CI contains fixed-length or variable-length records or a combination of the  two.

Sequential Dependent (SDEP)

DEDBs employ a special segment type called sequential dependent (SDEP).  Sequential dependents are designed for very fast insertion of segments or to  accommodate a very high volume of inserts. They must be located in the hierarchy  as the first child of the root segment, and they occupy their own space in an  area. Although SDEPs perform well for insert operations, they are not as  efficient at online retrieval. For this reason, SDEP segments are often  retrieved sequentially by using the SDEP Scan utility and are processed further  by offline jobs.

The sequential dependent part of the area is used solely for the storage of  sequential dependent segments. These segments are added in chronological order,  regardless of which root they belong to. They are chained back to the root in  reverse order by pointers.

The purely sequential nature of SDEPs allows rapid insertion of new SDEP  segment occurrences. SDEPs can only be written and read sequentially, and  REPLACE and DELETE calls are not allowed. When the sequential dependent part is  full, new segments must be added at its beginning. For this reason, the SDEP  area must be purged periodically. The SDEP Scan utility can be used to extract  SDEP segments from an area while online processing continues. The Sequential  Dependent Delete utility allows you to delete some or all of the SDEPs while  online processing continues.

SDEPs typically are used for temporary data. They are often used for  recording processing events that occur against a record during a particular time  period. A bank, for example, might record the activity of each customer account  during the day in SDEPs that are read and processed later offline.

Other DEDB segment types are direct dependent segments (DDEPs). They can be  stored and retrieved hierarchically and support ISRT, GET, DLET, and REPL calls.  IMS attempts to store DEDB DDEPs in the same CI as the root segment. If space is  not available in the root CI, IMS will search the dependent overflow and then  the independent overflow parts. You can define DDEP segments with or without a  unique sequence field. DDEPs are chained together by a PCF pointer in the parent  for each dependent segment type and a PTF pointer in each dependent segment.  Figure 2-19 illustrates the format of a DEDB record.

Figure 2-19: DEDB record format.

Root and direct dependent segments can be stored in the root addressable part  of a UOW or the dependent overflow part, if the root addressable part is full.  The independent overflow part consists of empty CIs that are not initially  designated for use by a specific UOW. Any UOW can use any CI in the independent  overflow part. When a UOW begins using a CI in the independent overflow part,  however, the CI can be used only by that UOW. The sequential dependent part of  an area stores SDEPs in the order in which they are loaded, without regard to  the root segment or the UOW that contains the root.

Although a DEDB can be compared in structure to HDAM, there are a number of  important differences between the two. DEDBs have the following  restrictions:

      • do not support secondary indexes

      • do not support logical relationships

      • allow unique key field or no key field for a segment; do not support  non-unique key fields

      • do not support batch processing

DEDBs are supported by DBRC and standard IMS logging, image copy, and  recovery procedures. DEDB areas are accessed by using VSAM improved control  interval processing (ICIP).

Randomizing Module

IMS uses a randomizing module to determine the location of root segments.

The randomizing module addresses records by:

      • ascending area number

      • ascending UOW

      • ascending key in each anchor point chain

Multiple Area Data Sets

IMS allows a DEDB area to be replicated (copied) up to seven times, thus  creating multiple area data sets (MADS). You can do this using the DEDB Area  Data Set Create utility, which lets you make multiple copies of an area without  stopping processing. The copies you create must have the same CI sizes and  spaces, but they can reside on separate devices. Although IMS allows seven  copies plus the original, for a total of eight iterations of the same area, most  IS shops use no more than two or sometimes three copies, to avoid excessive use  of DASD space.

As changes are made to data within one area of a MADS, IMS automatically  updates all copies to ensure the integrity of the data throughout the MADS. If  one area of the MADS fails, processing can continue with another copy of the  area.

If an error occurs during processing of an area, IMS prevents application  programs from accessing the CI in error by creating an error queue element (EQE)  for the error CI. IMS uses the EQE to isolate the CI in error while allowing  applications to access the remainder of the area. When IMS detects errors on  four different CIs within an area, or when an area has experienced more than 10  write errors, IMS stops the area. Because other copies of the area are  available, IMS can continue processing by using the same CI in one of the  copies.

MSDBs

Main storage databases (MSDBs) are so named because the entire database is  loaded into main storage when processing begins. This makes them extremely fast,  because database segments do not have to be retrieved from DASD. Most IS shops  reserve MSDBs for a site's most frequently accessed data, particularly data that  requires a high transaction rate. The fact that MSDBs require memory storage  limits their size.

In addition to being executed in main storage, MSDBs gain speed from a number  of other processing conventions. Update calls are not executed immediately;  rather, updates are held for simultaneous execution when a synchronization point  is reached. Also, MSDBs support field calls, a feature that allows MSDB segments  to be updated at the field level while permitting other programs access to other  fields within the segment at the same time. Finally, MSDBs are made up of  fixed-length root segments only. Because there are no dependent segments,  segment searches are simplified. There are two general types of MSDBs:  terminal-related and nonterminal-related.

Terminal-Related MSDBs

With terminal-related MSDBs, each segment occurrence is associated with, or  owned, by a specific logical terminal. Only the logical terminal that owns a  segment is allowed to update the segment, but all logical terminals are allowed  to retrieve all segments. The name of the logical terminal that owns the segment  is used as the key of the segment. The key of a terminal-related segment does  not actually reside in the segment. Terminal-related MSDBs are used when  segments contain data that can be associated with a specific user or  terminal.

Terminal-related MSDBs are divided into two types:

      • terminal-related fixed

      • terminal-related dynamic

Terminal-related fixed MSDBs do not allow segments to be inserted or deleted.  They can be retrieved and updated only. Terminal-related dynamic MSDBs allow  segments to be inserted or deleted.

Nonterminal-Related MSDBs

With nonterminal-related MSDBs, segments are not owned by any terminals. An  application program can read and update any segment; however, insert or delete  calls are not allowed.

There are two types of nonterminal-related MSDBs

      • nonterminal-related with terminal keys

      • nonterminal-related without terminal keys

With nonterminal-related with terminal keys, the name of a logical terminal  or a field in the segment is used as the key of the segment. As with  terminal-related fixed segments, if the terminal name is used as the key, it  does not reside in the segment. For nonterminal-related without terminal keys,  terminal names are not used as keys; a data value is used as key. The key can be  any part or all of the field, but it must reside in the sequence field of the  segment.

Nonterminal-related MSDBs are typically used when a large number of users  need to update data frequently, such as users at point-of-purchase  terminals.

Virtual Storage Option

The Fast Path Virtual Storage Option (VSO) for DEDBs allows you to achieve  performance improvements similar to those of MSDBs by loading one or more DEDB  areas into virtual storage. You do this by defining the DEDBs as VSO areas. VSO  areas must be registered with DBRC and do not allow block-level sharing while  the area is in virtual storage. Otherwise, they are the same as non-VSO DEDB  areas.

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The Role of Secondary Indexes

A secondary index lets you process a segment type in a sequence other than  that defined by the segment’s key. You can base a secondary index on any segment  in the database, including the root, and you can create more than one secondary  index on any segment. In fact, you can use up to 32 secondary indexes for a  segment type and a total of 1,000 secondary indexes for a single database. A  secondary index allows you to have an index based on any field in the database.  This section describes how secondary indexes can be used to solve access needs  in the sample College databases.

Secondary indexes can be used with HD databases and with HISAM. A secondary  index uses VSAM as its access method. A secondary index is a self-contained  database, with its own DBD, containing segments that point to a segment in the  primary database. Each secondary index’s DBD contains parameters that connect it  to the primary database.

Let’s look again at the College database created in Chapter 1, “IMS  Concepts.” It is shown again in Figure 2-20.

Figure 2-20: College database.

As part of our database, we decided to list the students enrolled in a  specific course. We handled this requirement by creating the Enroll segment as a  dependent of Course. Suppose that the Enroll segment has the data fields shown  in Figure 2-21.

Figure 2-21: Data fields of the enroll segment.

If we retrieve the occurrences of the Enroll segment in key sequence order,  we will have a list based on student ID number. But suppose we want the  information in this segment in an alphabetical list based on the student’s  names. To create this list using the StudeID field would be difficult. If we  create a secondary index based on the Name field, we can generate our listing  easily by processing sequentially on the new Name key field.

You can also use the secondary index to access segments individually. You do  this by referring to them specifically by the secondary index name (remember,  the secondary index is a database) in the segment search argument you write.

Finally, you can treat the secondary index as a database and use the  information stored in it. Using our StudeID secondary index, we can retrieve  just the contents of the field to which it points, giving us an alphabetical  list of student names.

When you want an application to perform processing by using a secondary  index, you must code the PROCSEQ parameter in the application program PCB. This  identifies the index database to the application program. If you want the  application program to use the database’s regular processing sequence, do not  include the PROCSEQ parameter. You can have the application program use the  regular processing sequence and the secondary index by including two PCBs in the  application program’s PSB. One PCB contains the PROCSEQ parameter and one does  not.

Secondary Index Segments

The segments used in setting up a secondary index are referred to as the  source, target, and pointer segments, depending on their function within the  index structure. When you set up a secondary index, you must define to IMS which  segments will act in which capacity.

Source Segment

The source segment in the primary database contains the field that is being  used as the pointer segment’s key field. The secondary index will be sequenced  on this field. In Figure 2-20, Enroll was the source segment.

Target Segment

The target segment in the primary database is the segment to which the index  points. It is the segment that is retrieved when the secondary index is used. In  many cases, but not always, the target segment is the same as the source  segment. In Figure 2-20, the Enroll segment is the target segment.

Pointer Segment

The pointer segment is the only type of segment that is stored in the  secondary index database. (Target and source segments are in the primary  database.)

Index Pointer Segment Layout

Figure 2-22 shows the index segment’s configuration.

Figure 2-22: Index pointer segment layout.

The layout of the index pointer segment is as follows:

 

Non-Unique PointerOptional. This 4-byte field is present only when the pointer segment  contains non-unique keys. It is used to chain together the KSDS and ESDS logical  records with the same key value. It is not part of the segment prefix. It is not  reflected in the prefix length of the generated DBD.
Segment CodeOptional. This 1-byte field appears only if DOS COMPAT was specified in the  index DBD. The prefix length in the generated DBD always assumes this field is  present, even though it may not be present.
Delete ByteRequired. This 1-byte field is always present. At load time it contains  X'00'.
Direct PointerOptional. This 4-byte field is present if the secondary index is pointing to  an HD-type database and direct pointing was selected. It contains the 4-byte RBA  of the target segment.
CONST                Optional. This 1-byte field contains a user-specified constant. It is  required for a shared secondary index. If present, it forms part of the VSAM  key. SRCH Required. It consists of one to five fields copied from the index  source segment. This field is the VSAM key. This is the data used to qualify a  call when accessing the target segment by using the secondary index.
SUBSEQOptional. It consists of one to five fields copied from the index source  segment. If present, it is concatenated with the search (SRCH) data to produce a  unique VSAM key.
DDATAOptional. It consists of one to five fields copied from the index source  segment. It is available only when processing the secondary index as a separate  database.
Symbolic PointerRequired if the primary database is HISAM. Optional for HD-type primary  databases. It is mutually exclusive with direct pointers. If present, it  contains the concatenated key of the target segment, which must be unique. The  symbolic pointer may be a separate field or may be imbedded in the SUBSEQ or  DDATA fields.
User DataOptional. It is inserted and maintained by the application program after the  secondary index has been created. There is no operand in the XDFLD macro to  define this area. It is the residual space left in the logical record after  space for all requested fields is satisfied.
PadOptional. This 1-byte field is not part of the segment data and is present  only if it is necessary to make the logical record length an even integer to  satisfy the VSAM requirement.

Secondary Index Implementation

Secondary indexes can accommodate non-unique keys. You can create a secondary  index based on a field that will not always be unique. In our previous example,  we discussed creating a secondary index based on student name. It is quite  possible for two students to have the same name, particularly if you are  indexing on the last name only. You would certainly have non-unique keys if you  created a secondary index based on the Major field. When this occurs, IMS  creates a SYNRBA field in the pointer segment of the secondary index. The SYNRBA  field contains a pointer that points to duplicate key segments. Duplicated key  segments are stored in a VSAM ESDS and are chained together by pointers of this  type. This section discusses secondary index implementation using non-unique or  unique keys.

Non-Unique Keys

To implement a secondary index with non-unique VSAM keys, two VSAM data sets  are required. The first (or only) occurrence of a non-unique key is placed in a  KSDS. Subsequent occurrences of that key are placed in an ESDS. Logical records  containing multiple occurrences of the same key are chained together. IMS  implements this chain by appending a 4-byte RBA pointer to the front of each  record.

Figure 2-23 shows the implementation of a secondary index with  non-unique
keys.

Figure 2-23: SecondaryiIndex implementation with non-unique  keys.

Unique Keys

When unique VSAM keys are present, no ESDS is required and no 4-byte pointer  is appended to the front of each logical record. Figure 2-24 shows the  implementation of a secondary index with unique keys.

Figure 2-24: Secondary index implementation with unique  keys.

Manuals have been written on the use of secondary indexes, and a complete  discussion of the subject is beyond the scope of this book. They are an  extremely powerful tool for use with IMS databases, however, and should be a  part of every IMS DBA's data management strategy.

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The Role of Logical Relationships

IMS provides the ability to create special types of segments that access data  in other segments through the use of pointers. You can create a segment type  that contains no data but points to a segment in the same or another database to  access the data stored there. Because the segment is not actually a physical  child of the segment to which it points, it is called a logical child. And  because the path established between the logical child and the segment to which  it points is not a physical relationship, it is called a logical  relationship.

The ability to create logical relationships among database segments is a  powerful feature of IMS. It lets you avoid the need to store redundant data, and  it lets you create new database hierarchies without creating new databases. By  establishing a logical relationship, you can create a hierarchy that does not  actually exist in storage but can be processed as if it does. You can relate  segments that exist in separate hierarchies within the same database or in  separate databases.

Note: Logical relationships cannot be used with Fast Path DEDB or MSDB  databases.

To illustrate the concept of logical relationships, let's look again at the  example College database. Suppose we decide that, as part of the College  database, we want a segment under Dept that lists the classrooms being used for  each course. We could create a new segment, Rooms, as a dependent. But suppose  we discover that the physical descriptions we need are already contained in a  segment of another database. The data we need is included in the Units segment  of the Buildings database. It would be redundant to repeat the same data in our  College database. By doing so, we would be vulnerable to the problems associated  with duplicate data:

      • It requires additional maintenance to keep the duplicate sets of data up to  date and synchronized.

      • It requires extra DASD to store the duplicate data. We can achieve the  effect of a new Rooms segment in the College database by creating a logical data  structure that ties the College and Building databases together. To do this, we  create a logical DBD that defines the structure we want. In the College  database, where we want our Rooms segment, we create what is called a logical  child segment. It points to the Units segment in the Buildings database. The  Units segment, where the data is physically stored, is referred to as the  logical parent. In our logical child segment, Rooms, we simply store pointers  that point to the appropriate occurrences of the Units segment, the logical  parent.

The logical relationship we have just established gives us the logical  structure shown in Figure 2-25.

Figure 2-25: Logical structure based on college and  buildings databases.

Figure 2-25 is an example of a unidirectional logical relationship. It is  called unidirectional because we can go in only one direction, from the College  database to the Units segment. You can use the Rooms segment of the College  database to find information on classroom space available for courses, but you  could not use any segment of the Buildings database to find information on what  courses are being taught in specific rooms. You can, however, create logical  relationships that will allow you to access data in both directions. In addition  to unidirectional relationships, you can create two types of bidirectional  relationships-physically paired and virtually paired.

Physical Pairing

Suppose that we want to be able to access data in the College database as if  the data were in a segment of the Buildings database. When we access the Units  segment of the Buildings database, we want to know which courses will be taught  in each room. We can do this by storing the Rooms logical child segment in the  Buildings database and pointing to the Course segment in the College database.  As a room is assigned to a course, the Rooms logical child will be updated in  both databases. We will have a bidirectional path that lets us determine what  room a course will be taught in and what courses will be taught in any room. The  Rooms segment is physically stored in both databases and the segments are said  to be physically paired. IMS automatically updates both logical child segments  when a logical parent segment occurrence is added.

Virtual Pairing

In the discussion of physical pairing, we discovered how to link to databases  bidirectionally by using two logical child segments, one in each database. By  using virtual pairing, we can do the same thing without creating a logical child  in the Buildings database. Instead, we can add another pointer in the Rooms  logical child in the College database to give us a two-way path between the two  databases. This gives us the effect of two logical child segments, although we  only have one. The logical child that is stored in the College database is  called the real logical child. The logical child that is created by the second  pointer in the real logical child is called the virtual logical child. The  relationship we have created between the two database segments is called  bidirectional virtual pairing.

Intersection Data

When two segments are logically related, it is possible to create data that  is relevant only to the relationship. For example, in our College database, the  number of seats available in the room where a course will be taught is a unique  combination of the course and the room assigned for it. This information could  be useful in determining if the enrollment for a course has exceeded the number  of seats available. This type of data is called intersection data. It is  meaningful only in the context of the two logically related segments.  Intersection data is stored in the logical child segment. There are two types of  intersection data:

      • fixed intersection data (FID)

      • variable intersection data (VID)

Fixed Intersection Data

Fixed intersection data is any data stored in the logical child. When you are  using direct pointing, FID is the only data in the logical child segment. In  symbolic pointing, the FID is stored in the data portion of the segment after  the LPCK.

Variable Intersection Data

Variable intersection data is used when you have several occurrences of  intersection data for the same logical relationship. It is stored as a dependent  of the logical child, and there can be as many occurrences per logical child as  needed.

Use of Pointers in Logical Relationships

Pointers used in logical relationships fall into two categories-direct and  symbolic. It is possible to implement logical relationships using both types.  The major differences are that a direct pointer is a true pointer. With a direct  pointer, the logical child segment contains the address of the logical parent  segment to which it points. With a symbolic pointer, the logical child segment  contains the key of the logical parent segment to which it points. Four types of  pointers can be specified in logical relationships:

      • logical parent (LP)

      • logical child (LC)

      • logical twin (LT)

      • physical parent (PP)

Logical Parent Pointers

LP pointers point from the logical child to the logical parent. When pointing  into HDAM and HIDAM databases, an LP pointer can be a direct pointer or a  symbolic pointer. It must be a symbolic pointer if pointing into a HISAM  database; however, direct LP pointers can exist in a HISAM database as long as  they are not pointing into a HISAM database. Because pointers are not required  between segments in a HISAM database, the only time direct pointers are used in  them is to maintain a logical relationship with an HD database.

A symbolic LP pointer consists of the logical parent's concatenated key  (LPCK). A symbolic LP pointer can point into a HISAM database or HD database. It  is stored in the first part of the logical child segment.

In HISAM or HIDAM databases, IMS uses symbolic pointers to access the  database's index and (from there) to locate the logical parent segment. In HDAM,  the randomizing module changes the symbolic pointer into a block and RAP address  to find the logical parent.

Logical Child Pointers

Logical child pointers are used only for logical relationships that use  virtual pairing. With virtual pairing, only one logical child exists on DASD,  and it contains a pointer to a logical parent. The logical parent points to the  logical child segment. Two types of logical child pointers can be used:

      • logical child first (LCF)

      • a combination of LCF and logical child last (LCL)  pointers

Because LCF and LCL pointers are direct pointers, the segment they are  pointing to must be in an HD database. The logical parent (the segment pointing  to the logical child) must be in a HISAM or HD database. If the parent is in a  HISAM database, the logical child must use a symbolic pointer to point to the  parent.

Logical Twin Pointer

Like logical child pointers, logical twin pointers are used only for logical  relationships that use virtual pairing. Logical twins are multiple occurrences  of logical child segments that point to the same occurrence of a logical parent  segment. Two types of logical twin pointers can be used:

      • logical twin forward (LTF)

      • a combination of LTF and logical twin backward (LTB) pointers LTF pointers  point from a logical twin to the logical twin stored after it. LTB pointers,  used only with LTF pointers, point from a logical twin to the logical twin  stored before it. Logical twin pointers function similarly to the physical twin  pointers described in "Physical Twin Forward and Backward  Pointers."

LTF and LTB pointers are direct pointers, containing the 4-byte address of  the segment to which they point, and can exist only in HD databases.

Physical Parent Pointer

Physical parent (PP) pointers point from a segment to its physical parent.  IMS generates PP pointers automatically for HD databases involved in logical  relationships. They are put in the prefix of all logical child and logical  parent segments and in the prefix of all segments on which a logical child or  logical parent is dependent in its physical database. Having a PP pointer in  each segment in the chain from logical segment to physical parent creates a path  back to the root segment on which the segment is dependent. This allows access  to any of the segments in the chain in forward or backward order.

As with direct pointers, PP pointers contain the 4-byte address of the  segment to which they point.