Two stage submission and three stage submission

Two stage submission and three-stage submission

Two-phase commit

In the last article, we talked about two-stage submission 2pc, Two phase submission still has many disadvantages , It may cause data inconsistency due to network problems , For example, only some participants can receive the request to submit a transaction sent by the coordinator due to network problems , In this way, some participants commit transactions while others fail to commit, resulting in data inconsistency , The second problem is if the coordinator hangs up , The participant has neither committed nor rolled back , The third problem is that the services of participating nodes are blocked when executing at the same time , Can't provide external services

Three stage commit

So what is three-phase submission , Can three-stage submission solve these problems ？

The three-stage submission is divided into CanCommit Stage ,PreCommit Phase and doCommit Stage . The first stage is to check whether its state meets the conditions of transaction operation , Generally, it is possible to obtain the lock of this distributed transaction first , The second stage is to start the transaction , But didn't submit , And return the result to the coordinator , The third stage is the commit or rollback of transactions .

This is the general content of the three-stage submission , So does the three-phase submission solve the problems caused by the two-phase submission ？ Three stage submission participants also have a timeout mechanism , stay preCommit When , If the coordinator's message is not received, the interrupt transaction will be executed , stay doCommit If the coordinator's message is not received in the phase, the transaction will be committed , The problem of data consistency and the coordinator's single point of failure still exist

summary

There will be some problems in two-phase submission and three-phase submission , Compared with the two-phase submission, the three-phase submission has more participants' timeout mechanism , But the remaining problems have not been solved , For example, the consistency of data and the single point of failure of the coordinator .

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