Addressing OLTP Solutions with CICS: The Transaction Server ... - Ibm

88 CICS for AIX as the Transaction Server
A TM assigns a data structure called a transaction identifier (XID). The XID lets
the TM track and coordinate all of the work associated with a global transaction.
Each RM maps the XID to the recoverable work it did for the transaction. For
global uniqueness, the XID should contain atomic action identifiers.
Activity among Functional Components: Activity among the three functional
components of the X/Open DTP model is dependent on the transaction operation.
Transaction Initiation: When an AP instructs its TM to start a global transaction,
the TM tells all appropriate RMs to associate the information about the global
transaction with any work the AP may request from them.
Some RMs are configured so that the TM does not inform them when a global
transaction starts. The RM contacts the TM to become associated with a global
transaction only after the AP calls it to request actual work. This is called
dynamic registration. If static registration is requested, each new transaction in
the process generates a call to the RM that tells it that it has joined a new
transaction or resumed an old one.
Transaction Commitment: When an AP instructs its TM to commit a transaction,
the TM and RMs use two-phase-commit presumed rollback to ensure that the
transaction′s updates are atomic.
In phase 1, the TM asks all RMs to prepare to commit (or prepare) their work. It
also asks whether the RM can guarantee its ability to commit the work it did on
behalf of a global transaction. If an RM can commit its work, it replies
affirmatively. A negative reply reports failure.
In phase 2, the TM directs all RMs either to commit or to roll back the work done
on behalf of a global transaction, as the case may be. All RMs commit or roll
back changes to shared resources and then return status to the TM.
When an AP calls its TM to commit a global transaction, the TM reports on
whether commitment or rollback was the outcome. This report is based on
reports the TM received (directly or through other TMs) from all involved RMs.
The XA specification contains two optimizations in the calling sequence between
the TM and RM. An RM can withdraw from further participation in a global
transaction during phase 1 if it was not asked to update shared resources (the
read-only optimization). A TM can use one-phase commit if it is dealing with
only one RM that is making changes to shared resources.
The XA specification discusses requirements for stable recording of transaction
data, including specifying when the TM and RMs are free to discard their
knowledge of the global transaction.
Transaction Rollback: The TM rolls back the global transaction if any RM
responds negatively to the phase 1 request or the AP directs the TM to roll back
the global transaction.
The TM effects phase 2 by telling RMs to roll back the transaction. The RMs
must not let any changes to shared resources become permanent.
Heuristic Transaction Completion: In certain, unusual, cases, the RM could
experience a long delay between phases 1 and 2 of the two-phase-commit
protocol. For example, the TM that issued the prepare-to-commit (phase 1)