MySQL壓測--異步與半同步復制
1.背景知識
Asynchronous replication
the master writes events to its binary log and slaves request them when they are ready. There is no guarantee that any event will ever reach any slave.
--主庫只管把events寫入binlog中，不管從庫有沒有收到。
Fully synchronous replication
when a master commits a transaction, all slaves also will have committed the transaction before the master returns to the session that performed the transaction. The drawback of this is that there might be a lot of delay to complete a transaction.
--主庫提交一個事物，需要等待所有從庫先提交才能返回結果，執行這個事物。這樣會造成一個事物延時。
Semisynchronous replication
falls between asynchronous and fully synchronous replication. The master waits only until at least one slave has received and logged the events. It does not wait for all slaves to acknowledge receipt, and it requires only receipt, not that the events have been fully executed and committed on the slave side.
--介於異步復制和全復制之間，主庫僅僅只要等待至少一個從庫收到和記錄events。它不需要等待所有的從庫告訴它收到events,也不需要從庫執行和提交事物，從庫只是收到events就會告訴主庫，這樣主庫就可以提前提交事物了。
此外，半同步也分兩種，有參數rpl_semi_sync_master_wait_point控制，這裏我就不多做解釋了，我們使用默認設置after_sync,這種數據零丟失
AFTER_SYNC (the default): The master writes each transaction to its binary log and the slave, and syncs the binary log to disk. The master waits for slave acknowledgment of transaction receipt after the sync. Upon receiving acknowledgment, the master commits the transaction to the storage engine and returns a result to the client, which then can proceed.
AFTER_COMMIT: The master writes each transaction to its binary log and the slave, syncs the binary log, and commits the transaction to the storage engine. The master waits for slave acknowledgment of transaction receipt after the commit. Upon receiving acknowledgment, the master returns a result to the client, which then can proceed.
2.測試環境
| Role | Hostname | IP | CPU | Memory | MySQL Version |
| TPCC | sht-sgmhadoopcm-01 | 172.16.101.54 | 2Core | 8G | NO |
| master | sht-sgmhadoopdn-01 | 172.16.101.58 | 2Core | 6G | 5.7.21 |
| slave1 | sht-sgmhadoopdn-02 | 172.16.101.59 | 2Core | 6G | 5.7.21 |
| slave2 | sht-sgmhadoopdn-03 | 172.16.101.60 | 2Core | 6G | 5.7.21 |
一個master,slave1和slave2都是master的直接從庫。
分兩種情況測試：
（1）當slave1和slave2都是異步復制的時候
（2）當slave1是半同步復制，slave2是異步復制的時候
3.壓力測試
使用TPCC壓力測試軟件，比較TPS和QPS來判斷異步和半同步復制的性能差異到底有多大。
具體如何測試，可以參考之前的博客：MySQL壓測--TPCC安裝，測試
參考鏈接：
半同步復制安裝配置
https://dev.mysql.com/doc/refman/5.7/en/replication-semisync.html
MySQL壓測--異步與半同步復制