The two-phase commit protocol

The two-phase sync point protocol consists of two requests issued by the sync point manager to each of the resource managers involved in the UOR. Each of the UOR states, in-flight and in-doubt, define what happens to the UOR in the event of a thread failure.

Phase 1

The sync point manager asks all participants if they are ready to commit a UOR.

Phase 2

The sync point manager tells each participant to commit or abort based on the response to the request issued in phase 1.

A UOR has two states: in-flight and in-doubt. The UOR is in an in-flight state from its creation time until the time IMS DB logs the phase 1 end (point C in the following two tables). The UOR is in an in-doubt state from (point C) until IMS DB logs phase 2 (point D in the following two tables).

The in-doubt state for a single-phase sync point request is a momentary state between points C and D in Table 1.

The in-flight and in-doubt states are important because they define what happens to the UOR in the event of a thread failure. If a thread fails while its IMS DB UOR is in-flight the UOR database changes are backed out. If a thread fails when its IMS DB UOR is in-doubt, during single-phase commit, the UOR database changes are kept for an individual thread abend, but are not kept for a system abend. If a thread fails when its IMS DB UOR is in-doubt during two-phase commit, the database changes are kept.

Thread failure refers to either of these cases:

• Individual thread abends.
• System abends: IMS DB failure, CCTL failure, ODBA application failure, or z/OS® failure (which abends all threads).

The following code shows the system events that occur when CCTL is used for single-phase sync point processing.

Time →
–––A–––B––––––C–––D–––E––––

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Table 1. CCTL single-phase sync point processing

Points In Time	System Events
A	CCTL phase 1 send
B	IMS DB phase 1 receive
C	IMS DB log phase 1 end
D	IMS DB log phase 2
E	CCTL phase 2 receive

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The following table shows the system events that occur when CCTL is used for two-phase sync point processing.

Time →
–––A–––B–––––C–––D–––E–––––––––––F–––G––––H––––––J–––K––––––––

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Table 2. CCTL two-phase sync point processing

Points in time	System events
A	CCTL phase 1 send
B	IMS DB phase 1 receive
C	IMS DB log phase 1 end
D	IMS DB phase 1 respond
E	CCTL phase 1 receive
F	CCTL phase 2 send
G	IMS DB phase 2 receive
H	IMS DB log phase 2
J	IMS DB phase 2 respond
K	CCTL phase 2 receive

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The following figure shows the system events that occur when two-phase sync point processing is completed using ODBA.

Figure 1. ODBA two-phase sync point processing