IBM OS manual Protecting applications from one another

Protecting applications from one another

The transaction isolation function offers storage protection between application programs, ensuring that one application does not accidentally overwrite the storage of another.

Transaction isolation ensures that user-key programs1 execute in their own subspace, with appropriate access to any shared storage, or to CICS storage. Thus a user transaction is limited to its own view of the address space. In general, transaction isolation ensures that each user-key program is allocated a separate (unique) subspace, with appropriate access to any shared storage or to CICS storage. They do not have any access to the user-key task-lifetime storage of other tasks. Existing applications should run unmodi®ed provided they conform to transaction isolation requirements. However, a minority of applications may need special de®nition if they:

vIssue MVS macros directly

vModify CICS control blocks

vHave a legitimate need for one task to access or share another task's storage

Some existing transactions may share task-lifetime storage in various ways, which may prevent them running isolated from each other. To allow such transactions to continue running without requiring that they run in the base space (where they could corrupt CICS data or programs), a single common subspace is provided in which all such transactions can run. They are then isolated from the other transactions in the system that are running in their own subspaces, but are able to share each other's data within the common subspace.

You may have some transactions whose application programs access each other's storage in a valid way. One such case is when a task waits on one or more event control blocks (ECBs) that are later posted, by another task, either an MVS POST or `hand posting'. For example, a task can pass the address of a piece of its own storage to another task (via a temporary storage queue or some other method) and then WAIT for the other task to post an ECB to say that it has updated the storage. Clearly, if the original task is executing in a unique subspace, the posting task will fail when attempting the update and hence fail to post the ECB, unless the posting task is executing in CICS key. CICS therefore checks when a WAIT is issued that the ECB is in shared storage, and raises an INVREQ condition if it is not.

Storage for the timer-event control area on WAIT EVENT, and storage for event control blocks (ECBs) speci®ed on WAIT EXTERNAL and WAITCICS commands, must reside in shared storage.2

You can use the Transaction Affinities Utility to identify those transactions whose programs issue WAIT EVENT, WAIT EXTERNAL, or WAITCICS commands, or MVS POST macros.

1.User key de®nes both the storage and execution key for user application programs.

2.Shared storage is allocated from one of the user-key shared dynamic storage areas, below or above the 16MB boundary (SDSA or ESDSA).

Chapter 1. Introducing transaction affinities 7