IMS Connectivity in an On Demand Environment: A Practical Guide to IMS Connectivity

16 IMS Connectivity in an On Demand Environment: A Practical Guide to IMS Connectivity

Figure 2-7 Send-then-commit message flow with sync level=sync point

In the resource coordination, two-phase commit (2PC) protocol is used. The two-phase

commit protocol is a process involving the participants, the sync point manager, and the

resource managers to ensure that, for updates made to a set of resources by the third-party

application program, either all occur or none occurs. In simple terms, the application program

decides to commit its changes to some resources. This commit is made to the sync point

manager who then polls all of the resource managers as to the feasibility of the commit call.

This is the preparation phase, often called phase 1. Each resource manager votes

yes or no,

and when the sync point manager has gathered all the votes, phase 2 begins. If all votes are

to commit the changes, the phase 2 action is commit. Otherwise, phase 2 becomes a

backout. System failures, communication failures, resource manager failures, or application

failures are not barriers to the completion of the two-phase commit process.

The work done by various resource managers is called a unit of recovery (UOR) and spans

from one consistent point of the work to another consistent point, usually from one commit

point to another. It is the unit of recovery that is the object of the two-phase commit process.

The OTMA environment requires some additional code, because OTMA is not a resource

manager. The additional code needed is an OTMA client, which supplied by IBM or an

equivalent. The client indicates to IMS (in the OTMA message prefix) that this message is

part of a protected conversation; therefore, IMS and the client are participants in the

coordinated commit process as managed by RRS.

2.3.3 IMS commit mode 1 message processing

OTMA processing of commit mode 1 (send-then-commit) input messages can be confusing,

especially if program-to-program message switches are involved. Commit mode 1 (CM1)

input messages can produce commit mode 0 (CM0) IOPCB output messages, which can

require special processing by the OTMA client. CM1 input messages can also produce the

message DFS2082 RESPONSE MODE TRANSACTION TERMINATED WITHOUT REPLY

IMSOTMA Client Application

GU Call

...

ISRT to IOPCB

Sync Start

Sync End

Transaction

completes

Transaction

inserted to SMB

OTMA Prefix

DATA

DATA

TRAN

OTMA Prefix

TRAN DATA

MSGQ

DATA

Output sent -

Request ACK

Send

Receive

Receive

OTMA Prefix

Commit Confirmed

OTMA Prefix

ACK

Send

Commit confirmed

Transaction

committed if RRS

commit received