Glossary

ACB
See access control block.
Access control block (ACB)
The access control block that resides in the address space of an access method, such as ACF/VTAM®.
ACID properties
The term, coined by Haerder and Reuter [1983], and used by Jim Gray and Andreas Reuter to denote the properties of a transaction: 12
Atomicity
A transaction’s changes to the state (of resources) are atomic: either all happen or none happen.
Consistency
A transaction is a correct transformation of the state. The actions taken as a group do not violate any of the integrity constraints associated with the state.
Isolation
Even though transactions execute concurrently, they appear to be serialized. In other words, it appears to each transaction that any other transaction executed either before it, or after it.
Durability
After a transaction completes successfully (commits), its changes to the state survive failures.
Note:
In CICS®, the ACID properties apply to a unit of work (UOW). See also unit of work.
BWO
See backup-while-open.
backup-while-open (BWO)
A facility that allows a backup copy of a VSAM data set to be made while the data set remains open for update.

When you take a backup-while-open (BWO) copy of a data set, only the updates that are made after the BWO need to be recovered in the event of a disk failure. This considerably reduces the amount of forward recovery that is needed.

CICSplex
(1) A CICS complex--a set of interconnected CICS regions acting as resource managers, and combining to provide a set of coherent services for a customer’s business needs. In its simplest form, a CICSplex operates within a single MVS™ image. Within a Parallel Sysplex® environment, a CICSplex can be configured across all the MVS images in the sysplex.

The CICS regions in the CICSplex are generally linked through the CICS interregion communication (IRC) facility, using either the XM or IRC access method (between regions in the same MVS image), or the XCF/MRO access method (between regions in different MVS images).

(2) The largest set of CICS regions or systems to be manipulated by a single CICSPlex® SM entity.

CICSPlex System Manager (CICSPlex SM)
An IBM® CICS system-management product that provides a single-system image and a single point of control for one or more CICSplexes.
cloned CICS regions
CICS regions that are identical in every respect, except for their identifiers. This means that each clone has exactly the same capability. For example, all clones of an application-owning region can process the same transaction workload.
dirty read
A read request that does not involve any locking mechanism, and which may obtain invalid data--that is, data that has been updated, but is not yet committed, by another task. This could also apply to data that is about to be updated, and which will be invalid by the time the reading task has completed.

For example, if one CICS task rewrites an updated record, another CICS task that issues a read before the updating task has taken a syncpoint will receive the uncommitted record. This data could subsequently be backed out, if the updating task fails, and the read-only task would not be aware that it had received invalid data.

See also read integrity.

general log
A general purpose log stream used by CICS for any of the following:

Contrast with system log.

heuristic decision
A decision that enables a transaction manager to complete a failed in-doubt unit of work (UOW) that cannot wait for resynchronization after recovery from the failure.

Under the two-phase commit protocol, the loss of the coordinator (or loss of connectivity) that occurs while a UOW is in-doubt theoretically forces a participant in the UOW to wait forever for resynchronization. While a subordinate waits in doubt, resources remain locked and, in a CICS Transaction Server region, the failed UOW is shunted pending resolution.

Applying a heuristic decision provides an arbitrary solution for resolving a failed in-doubt UOW as an alternative to waiting for the return of the coordinator. In CICS, the heuristic decision can be made in advance by specifying in-doubt attributes on the transaction resource definition. These in-doubt attributes specify:

The heuristic decision can also be made by an operator when a failure occurs, and communicated to CICS using an API or operator command interface (such as CEMT SET UOW).

in-doubt
In CICS, the state at a particular point in a distributed UOW for which a two-phase commit syncpoint is in progress. The distributed UOW is said to be in-doubt when:

The UOW remains in-doubt until the coordinator issues either the commit or backout request as a result of responses received from all UOW participants. If the UOW is in the in-doubt state, and a failure occurs that causes loss of connectivity between a subordinate and its coordinator, it remains in-doubt until either:

Note:
In theory, a UOW can remain in-doubt forever if a UOW participant fails or loses connectivity with a coordinator, and is never recovered (for example, if a system fails and is not restarted). In practice, the in-doubt period is limited by attributes defined in the transaction resource definition associated with the UOW. After expiry of the specified in-doubt wait period, the recovery manager commits or backs out the UOW, based on the UOW’s in-doubt attributes.

For cases where data integrity is of paramount importance, CICS supports "wait forever", indicated by a WAITTIME of zero, in which case manual intervention is required to force a heuristic decision.

See also two-phase commit and heuristic decision.

log manager
A new domain in CICS Transaction Server for OS/390®, Version 1 Release 1, which replaces the CICS journal control management function of current CICS releases. The CICS log manager uses MVS system logger services to write CICS system logs, forward recovery logs, and user journals to log streams managed by the MVS system logger.
logical unit of work (LUW)
Old term used to describe a unit of work in earlier releases of CICS. The preferred term, adopted in CICS Transaction Server for OS/390, Version 1 Release 1, is unit of work (UOW). UOW is used as a keyword in a number of CICS interfaces in CICS TS. UOW also replaces LUW on two existing API commands (EXEC CICS ENQUEUE and EXEC CICS DEQUEUE), but the translator treats LUW as a synonym for UOW and correctly translates the command.

See unit of work.

LUW
See logical unit of work.
LUWID
Logical unit of work identifier. See UOW id.
orphan lock
An orphan lock is an RLS lock that is held by VSAM RLS but is unknown to any CICS region.

An RLS lock becomes an orphan lock if it is acquired from VSAM by a CICS region that fails before it can log it. A VSAM interface enables CICS, during an emergency restart, to detect the existence of these locks and release them.

Parallel Sysplex
An MVS sysplex where all the MVS images are linked through a coupling facility.
read integrity
An attribute of a read request, which ensures the integrity of the data passed to a program that issues a read-only request. CICS recognizes two forms of read integrity:
1. Consistent
A program is permitted to read only committed data--data that cannot be backed out after it has been passed to the program issuing the read request. Therefore, a consistent read request can succeed only when the data is free from all locks.
2. Repeatable
A program is permitted to issue multiple read-only requests, with repeatable read integrity, and be assured that none of the records passed can subsequently be changed until the end of the sequence of repeatable read requests. The sequence of repeatable read requests ends either when the transaction terminates, or when it takes a syncpoint, whichever is the earlier.

Contrast with dirty read.

recovery manager
A domain in CICS Transaction Server, the function of which is to ensure the integrity and consistency of recoverable resources. These recoverable resources, such as files and databases, can be within a single CICS region or distributed over interconnected systems. The recovery manager provides CICS unit of work (UOW) management in a distributed environment.
shunted
The status of a UOW that has failed at one of the following points:

If a UOW fails for one of these reasons, it becomes a candidate to be moved from the primary system log (DFHLOG) to the secondary system log (DFHSHUNT) at the next activity keypoint, pending recovery from the failure.

SMSVSAM
The name of the VSAM server that provides VSAM record-level sharing (RLS). See also VSAM RLS.
sphere
See VSAM sphere.
system log
A log stream maintained by CICS for back-out recovery purposes.

The system log is used by CICS to recover data to a consistent state following:

User transactions are allowed to write their own recovery records to the system log for use in an emergency restart, but the system log cannot be used for forward recovery log or autojournal records.

Contrast with general log.

system logger
A central logging facility provided by OS/390 (and also MVS/ESA SP 5.2). The MVS system logger provides an integrated MVS logging facility that can be used by system and subsystem components. For example, it is used by the CICS log manager.
two-phase commit
In CICS, the protocol observed when taking a syncpoint in a distributed UOW. At syncpoint, all updates to recoverable resources must be either committed or backed out. At this point, the coordinating recovery manager gives each subordinate participating in the UOW an opportunity to vote on whether its part of the UOW is in a consistent state and can be committed. If all participants vote "yes", the distributed UOW is committed. If any vote "no", all changes to the distributed UOW’s resources are backed out.

This is called the two-phase commit protocol, because there is first a "voting" phase (the prepare phase), which is followed by the actual commit phase. This can be summarized as follows:

1. Prepare
Coordinator invokes each UOW participant, asking each one if it is prepared to commit.
2. Commit
If all UOW participants acknowledge that they are prepared to commit (vote yes), the coordinator issues the commit request.

If only one UOW participant is not prepared to commit (votes no), the coordinator issues a back-out request to all.

unit of work (UOW)
A sequence of processing actions (database changes, for example) that must be completed before any of the individual actions performed by a transaction can be regarded as committed. After changes are committed (by successful completion of the UOW and recording of the syncpoint on the system log), they become durable, and are not backed out in the event of a subsequent failure of the task or system.

The beginning and end of the sequence may be marked by:

Thus a UOW is completed when a transaction takes a syncpoint, which occurs either when a transaction issues an explicit syncpoint request, or when CICS takes an implicit syncpoint at the end of the transaction. In the absence of user syncpoints explicitly taken within the transaction, the entire transaction is one UOW.

In earlier releases of CICS, this was referred to as a logical unit of work (LUW).

unshunting
The process of attaching a transaction to provide an environment under which to resume the processing of a shunted unit of work.
UOW
See unit-of-work.
UOW id
Unit-of-work identifier.

CICS uses two unit of work identifiers for two purposes, one short and one long:

Short UOW id
An 8-byte value that CICS passes to resource managers, such as DB2® and VSAM, for lock management purposes.
Long UOW id
A 27-byte value that CICS uses to identify a distributed UOW. This is built from a short UOW id prefixed by two 1-byte length fields and by the fully-qualified NETNAME of the CICS region.
VSAM RLS
VSAM record-level sharing, an access mode supported by DFSMS to allow multiple applications to share data sets, with data locking at the record level. Access to data sets is through an SMSVSAM server. See also SMSVSAM.
VSAM sphere
The collection of all the component data sets associated with a given VSAM base data set--the base, index, alternate indexes, and alternate index paths.
2-phase commit
See two-phase commit.

12.
Transaction Processing: Concepts and Techniques (1993)

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