IBM SC34-7012-01 manual Resource recovery in CICS, Units of work, Shunted units of work

Chapter 2. Resource recovery in CICS

Before you begin to plan and implement resource recovery in CICS, you should understand the concepts involved, including units of work, logging and journaling.

Units of work

When resources are being changed, there comes a point when the changes are complete and do not need backout if a failure occurs later. The period between the start of a particular set of changes and the point at which they are complete is called a unit of work (UOW). The unit of work is a fundamental concept of all CICS backout mechanisms.

From the application designer's point of view, a UOW is a sequence of actions that needs to be complete before any of the individual actions can be regarded as complete. To ensure data integrity, a unit of work must be atomic, consistent, isolated, and durable.

The CICS recovery manager operates with units of work. If a transaction that consists of multiple UOWs fails, or the CICS region fails, committed UOWs are not backed out.

A unit of work can be in one of the following states:

vActive (in-flight)

vShunted following a failure of some kind

vIndoubt pending the decision of the unit of work coordinator.

vCompleted and no longer of interest to the recovery manager

Shunted units of work

A shunted unit of work is one awaiting resolution of an indoubt failure, a commit failure, or a backout failure. The CICS recovery manager attempts to complete a shunted unit of work when the failure that caused it to be shunted has been resolved.

A unit of work can be unshunted and then shunted again (in theory, any number of times). For example, a unit of work could go through the following stages:

1.A unit of work fails indoubt and is shunted.

2.After resynchronization, CICS finds that the decision is to back out the indoubt unit of work.

3.Recovery manager unshunts the unit of work to perform backout.

4.If backout fails, it is shunted again.

5.Recovery manager unshunts the unit of work to retry the backout.

6.Steps 4 and 5 can occur several times until the backout succeeds.

These situations can persist for some time, depending on how long it takes to resolve the cause of the failure. Because it is undesirable for transaction resources to be held up for too long, CICS attempts to release as many resources as possible while a unit of work is shunted. This is generally achieved by abending the user task to which the unit of work belongs, resulting in the release of the following:

vTerminals

vUser programs

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