Besides the two-phase commit protocol, traditional transaction processing systems employ an additional protocol, often referred to as the synchronization protocol. With the original ACID properties, Durability is important when state changes need to be available despite failures. Applications interact with a persistence store of some kind, such as a database, and this interaction can impose a significant overhead, because disk access is much slower to access than main computer memory.
One solution to the problem disk access time is to cache the state in main memory and only operate on the cache for the duration of a transaction. Unfortunately, this solution needs a way to flush the state back to the persistent store before the transaction terminates, or risk losing the full ACID properties. This is what the synchronization protocol does, with Synchronization Participants.
Synchronizations are informed that a transaction is about to commit. At that point, they can flush cached state, which might be used to improve performance of an application, to a durable representation prior to the transaction committing. The synchronizations are then informed about when the transaction completes and its completion state.
Procedure 17.1. The "Four Phase Protocol" Created By Synchronizations
Synchronizations essentially turn the two-phase commit protocol into a four-phase protocol:
Before the transaction starts the two-phase commit, all registered Synchronizations are informed. Any failure at this point will cause the transaction to roll back.
Steps 2 and 3
The coordinator then conducts the normal two-phase commit protocol.
Once the transaction has terminated, all registered Synchronizations are informed. However, this is a courtesy invocation because any failures at this stage are ignored: the transaction has terminated so there’s nothing to affect.
The synchronization protocol does not have the same failure requirements as the traditional two-phase commit protocol. For example, Synchronization participants do not need the ability to recover in the event of failures, because any failure before the two-phase commit protocol completes cause the transaction to roll back, and failures after it completes have no effect on the data which the Synchronization participants are responsible for.