2pc and 3pc of consistency agreement

In the distributed system, every machine node can know whether it succeeds or fails in the process of transaction operation , But we can't know the operation results of other nodes . So when a transaction needs to span multiple nodes , In order to guarantee the ACID Characteristics of , At this time, we need to introduce a " The coordinator " To uniformly schedule the logic of all nodes , These scheduled nodes are called " participants ". The coordinator is responsible for scheduling the execution logic of the participants , And finally decide whether these participants should really commit the transaction .

2PC

2PC, yes Two-Phase-Commit Abbreviation , That is, two-stage submission , It's computer networks, especially in the field of databases , A consistency algorithm designed to ensure atomicity and consistency in the transaction processing of all nodes under the distributed system architecture . Generally, two-phase submission is also considered as a consistency agreement , To ensure the consistency of distributed system data .

Stage 1 ： Submit transaction request

1. Business inquiry

The coordinator sends the transaction content to all participants , Ask if the transaction commit operation can be performed , And began to wait for the corresponding response from the participants .

2. Perform transactions

Each participant node performs transaction operations , And will Undo Log and Redo Log Information is logged in the transaction log .

3. Participants give feedback to the coordinator about the corresponding

If the participant successfully performs the transaction operation , Then give feedback to the coordinator YES, Indicates that the transaction can execute ; If the participant does not successfully execute the transaction , Then give it back to the coordinator NO, Indicates that the transaction cannot be executed .

To sum up, it is the voting and declaration process that the coordinator organizes each participant to conduct a transaction operation , Therefore, the first stage of two-stage submission is also called " Voting stage ", That is, each participant votes to indicate whether to continue the next transaction submission .

Stage two ： Execute transaction commit

In phase 2, the coordinator will decide whether to commit the transaction according to the feedback of each participant , The normal situation should include two situations .

• Execute transaction commit

Suppose the coordinator gets feedback from all submitters YES, Then a transaction commit will be executed .

1. Send submit request ： The coordinator sends... To all participant nodes commit request
2. Transaction submission ： The participants received commit After the request , The transaction submission operation will be formally executed , And release the transaction resources occupied during the whole transaction execution after the commit
3. Feedback results ： After the participant completes the transaction commit , Send... To the coordinator ack news
4. Complete the business ： The coordinator receives feedback from all participants ack After the news , Complete the business

• Interrupt the business

Suppose any participant gives feedback to the coordinator NO, Or after the wait timeout , The coordinator has not received feedback from all participants , Then the transaction will be interrupted .

1. Send rollback request ： The coordinator sends a rollback request to all participants
2. Transaction rollback ： After the participant receives the rollback request , Make use of Undo Log To perform rollback , So as to ensure atomicity , And release the resources occupied during the whole transaction execution after rollback
3. Feedback results ： After all participants are rolled back , Send... To the coordinator ack news
4. Interrupt the business ： The coordinator receives all participants ack After the news , Complete transaction interrupt

2PC Advantages and disadvantages

2PC The advantages are obvious ： The principle of simple 、 To achieve convenient .
Shortcomings are also easier to think of ： Synchronous blocking 、 A single point of the problem 、 Data inconsistency .

• Synchronous blocking

2PC An obvious problem is synchronization blocking , This will greatly limit the performance of distributed systems . In the two-phase commit process , All participants' transaction processing logic is blocked , That is to say, each participant cannot perform any other operation while waiting for the response of other participants .

• A single point of the problem

stay 2PC The role of the coordinator is crucial , Once the coordinator has problems, the whole process will not work , As a result, participants cannot complete transaction operations .

• Data inconsistency

When executing transaction submission in phase 2 , When the coordinator sends commit After the request , If there is a network failure, some participants will not receive commit request , Received at this time commit The requested participant will commit the transaction , Have not received commit The requested participant will not commit the transaction , Thus, the phenomenon of inconsistent data appears .

3PC

In the above content, we know 2PC Principle and implementation mechanism of , It is clearly pointed out that there may be synchronous blocking during operation 、 Single point problems and data inconsistency , So in 2PC On the basis of , Put forward 3PC, That is, the three-stage submission agreement .

Agreement that

3PC, yes Three-Phase Commit Abbreviation , Three stage submission , yes 2PC Improved version , It will submit the agreement in two stages " Submit transaction request " The process is split in two , Formed by CanCommit、PreCommit and do Commit Three phase transaction protocol .

Stage 1 ：CanCommit

1. Business inquiry

The coordinator sends a... With transaction content to all participants canCommit request , Ask if the transaction commit operation can be performed , And begin to wait for the corresponding .

2. Each participant feeds back the response of the transaction inquiry to the coordinator

Participants are receiving canCommit After the request , Under normal circumstances , If it thinks it can execute the transaction smoothly , Then feedback YES, Get ready , Otherwise feedback NO.

Stage two ：PreCommit

In stage two , The coordinator will decide whether to proceed with the transaction according to the feedback status of each participant PreCommit operation , Under normal circumstances, there are two possibilities .

• Perform transaction pre commit

Suppose the coordinator gets feedback from all participants that the status is YES, Then the pre commit operation will be performed

1. Send pre submit request

The coordinator sent... To all participants preCommit request , And enter Prepared Stage

2. Transaction pre commit

Participants receive preCommit After the request , Will perform transaction operations , And will Undo Log and Redo Log Information is logged in the transaction log .

3. Each participant feeds back the implementation results to the coordinator

If the participant successfully performs the transaction operation , Then it will be fed back to the coordinator ack Respond to , While waiting for the final implementation ：commit or abort

• Interrupt the business

Suppose any participant returns to the coordinator NO Response or wait timeout , Then the transaction will be interrupted

1. Send interrupt request
2. Interrupt the business

Stage three ：doCommit

In this phase, the real transaction commit , There are two possibilities

• Execute commit

1. Send submit request

Enter this stage , Suppose the coordinator is in normal working condition , And it receives... From all participants ack Respond to , So it will be from " Pre submission " The state changes to " Submit " state , And send it to all participants doCommit request

2. Transaction submission

Participant received doCommit After the request , It will actually execute the transaction submission operation , And release the resources occupied during the whole transaction execution after the completion of the commit

3. Feedback transaction submission results

After the participant completes the transaction commit , Send... To the coordinator ack news

4. Complete the business

The coordinator receives feedback from participants ack After message , Complete the business

• Interrupt the business

1. Send interrupt request

The coordinator sends... To all participants abort request

2. Transaction rollback

Participant received abort After the request , Will use in phase II undo log To perform transaction rollback , And release the resources occupied during transaction execution

3. Feedback transaction rollback result

After the participant completes the transaction rollback , Send... To the coordinator ack news

4. Interrupt the business

The coordinator receives feedback from all participants ack After the news , Interrupt the business

3PC Advantages and disadvantages

advantage ： Compared with the two-phase submission, the submission agreement , The biggest advantage of three-stage italics is that it reduces the blocking range of participants , And can continue to reach an agreement after a single point of failure .

shortcoming ： Three phase commit also introduces new problems in removing blocking colleagues , That is when participants receive preCommit After the news , If there's a network partition , At this time, the coordinator cannot communicate with the participants on the node , In this case, the change participant will still commit the transaction , This will inevitably lead to data inconsistency .