18.1.1.1 Primary-Secondary Replication

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Traditional MySQL Replication provides a simple Primary-Secondary approach to replication. There is a primary (master) and there is one or more secondaries (slaves). The primary executes transactions, commits them and then they are later (thus asynchronously) sent to the secondaries to be either re-executed (in statement-based replication) or applied (in row-based replication). It is a shared-nothing system, where all servers have a full copy of the data by default.

Figure 18.1 MySQL Asynchronous Replication

A transaction received by the master is executed, written to the binary log, then committed, and a response is sent to the client application. The record from the binary log is sent to the relay logs on Slave 1 and Slave 2 before the commit takes place on the master. On each of the slaves, the transaction is applied, written to the slave's binary log, and committed. The commit on the master and the commits on the slaves are all independent and asynchronous.

There is also semisynchronous replication, which adds one synchronization step to the protocol. This means that the Primary waits, at commit time, for the secondary to acknowledge that it has received the transaction. Only then does the Primary resume the commit operation.

Figure 18.2 MySQL Semisynchronous Replication

A transaction received by the master is executed and written to the binary log. The record from the binary log is sent to the relay logs on Slave 1 and Slave 2. The master then waits for an acknowledgement from the slaves. When both of the slaves have returned the acknowledgement, the master commits the transaction, and a response is sent to the client application. After each slave has returned its acknowlegement, it applies the transaction, writes it to the binary log, and commits it. The commit on the master depends on the acknowledgement from the slaves, but the commits on the slaves are independent from each other and from the commit on the master.

In the two pictures above, you can see a diagram of the classic asynchronous MySQL Replication protocol (and its semisynchronous variant as well). Diagonal arrows represent messages exchanged between servers or messages exchanged between servers and the client application.

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