Distributed transaction - Final consistency scheme based on message compensation (local message table, message queue)

Preface

The final consistency scheme based on message compensation is an implementation of distributed transactions , It also includes the local message table 、 There are two ways of message queuing .

One 、 Local message table

The core idea is to split the distributed transaction into local transaction , Local message tables achieve final consistency .

• step 1 and 2, The system receives the user's order request , Write the order business data into the order table , At the same time, write the message data corresponding to the order into the local message table , The order table and the local message table are in the same database , Update the order and store the message as the same local transaction , Database transaction processing , Either they all succeed , Or they all failed .
• step 345, The order service sends a message to the message queue , The inventory service received a message , Perform inventory operations , Update inventory data
• step 6 and 7, Business processing results are returned , After the order service receives the result , Set the data in the local message table to the completion status or delete the data .
• step 8, Another scheduled task , Scan local message table regularly , See if there are any unfinished tasks , Try again if any .

Advantages and disadvantages

• Implement a simple , Low development cost
• The order table and the local message table are in the same database

Two 、 Message queues that support transactions

RMQ Support transaction messages , It is similar to the common database transaction process , The producer produces a transaction message , Wait for the local operation of the producer ok 了 , Will send to MQ Server Send submission confirmation .

under these circumstances , Transaction messages are initially thrown into the message queue and cannot be consumed , Only when the transaction message is committed , This information can be consumed by consumers .

Empathy , If rollback 了 ,RMQ The message will be deleted from the message queue .

1. Producers produce messages to MQ, Not submitted at this time , Call this message HalfMsg, Half message
2. MQ Received the message , Will give the producer a OK confirm
3. Producers perform local transactions
4. According to the result of local transaction execution, the MQ Submit commit perhaps rollback,MQ according to commit perhaps rollback Operate on the message accordingly , To consume or discard
5. If the producer submits commit perhaps rollback Submission timeout , That is, step 4 does not receive a request from the producer ,MQ The callback checks the message
6. MQ The callback checks the status of the local transaction

Reference resources

https://www.jianshu.com/p/e31d9ebed201