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17.3.11 Semisynchronous Replication
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In addition to the built-in asynchronous replication, MySQL 8.0 supports an interface to semisynchronous replication that is implemented by plugins. This section discusses what semisynchronous replication is and how it works. The following sections cover the administrative interface to semisynchronous replication and how to install, configure, and monitor it.
MySQL replication by default is asynchronous. The master writes events to its binary log but does not know whether or when a slave has retrieved and processed them. With asynchronous replication, if the master crashes, transactions that it has committed might not have been transmitted to any slave. Consequently, failover from master to slave in this case may result in failover to a server that is missing transactions relative to the master.
Semisynchronous replication can be used as an alternative to asynchronous replication:
A slave indicates whether it is semisynchronous-capable when it connects to the master.
If semisynchronous replication is enabled on the master side and there is at least one semisynchronous slave, a thread that performs a transaction commit on the master blocks and waits until at least one semisynchronous slave acknowledges that it has received all events for the transaction, or until a timeout occurs.
The slave acknowledges receipt of a transaction's events only after the events have been written to its relay log and flushed to disk.
If a timeout occurs without any slave having acknowledged the transaction, the master reverts to asynchronous replication. When at least one semisynchronous slave catches up, the master returns to semisynchronous replication.
Semisynchronous replication must be enabled on both the master and slave sides. If semisynchronous replication is disabled on the master, or enabled on the master but on no slaves, the master uses asynchronous replication.
While the master is blocking (waiting for acknowledgment from a slave), it does not return to the session that performed the transaction. When the block ends, the master returns to the session, which then can proceed to execute other statements. At this point, the transaction has committed on the master side, and receipt of its events has been acknowledged by at least one slave.
The number of slave acknowledgments the master must receive per
transaction before proceeding is configurable using the
rpl_semi_sync_master_wait_for_slave_count
system variable. The default value is 1.
Blocking also occurs after rollbacks that are written to the binary log, which occurs when a transaction that modifies nontransactional tables is rolled back. The rolled-back transaction is logged even though it has no effect for transactional tables because the modifications to the nontransactional tables cannot be rolled back and must be sent to slaves.
For statements that do not occur in transactional context (that
is, when no transaction has been started with
START
TRANSACTION or
SET autocommit =
0), autocommit is enabled and each statement commits
implicitly. With semisynchronous replication, the master blocks
for each such statement, just as it does for explicit transaction
commits.
To understand what the “semi” in “semisynchronous replication” means, compare it with asynchronous and fully synchronous replication:
With 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.
With 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.
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.
Compared to asynchronous replication, semisynchronous replication
provides improved data integrity because when a commit returns
successfully, it is known that the data exists in at least two
places. Until a semisynchronous master receives acknowledgment
from the number of slaves configured by
rpl_semi_sync_master_wait_for_slave_count,
the transaction is on hold and not committed.
Semisynchronous replication also places a rate limit on busy sessions by constraining the speed at which binary log events can be sent from master to slave. When one user is too busy, this will slow it down, which is useful in some deployment situations.
Semisynchronous replication does have some performance impact because commits are slower due to the need to wait for slaves. This is the tradeoff for increased data integrity. The amount of slowdown is at least the TCP/IP roundtrip time to send the commit to the slave and wait for the acknowledgment of receipt by the slave. This means that semisynchronous replication works best for close servers communicating over fast networks, and worst for distant servers communicating over slow networks.
The
rpl_semi_sync_master_wait_point
system variable controls the point at which a semisynchronous
replication master waits for slave acknowledgment of transaction
receipt before returning a status to the client that committed the
transaction. These values are permitted:
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.
The replication characteristics of these settings differ as follows:
With
AFTER_SYNC, all clients see the committed transaction at the same time: After it has been acknowledged by the slave and committed to the storage engine on the master. Thus, all clients see the same data on the master.In the event of master failure, all transactions committed on the master have been replicated to the slave (saved to its relay log). A crash of the master and failover to the slave is lossless because the slave is up to date.
With
AFTER_COMMIT, the client issuing the transaction gets a return status only after the server commits to the storage engine and receives slave acknowledgment. After the commit and before slave acknowledgment, other clients can see the committed transaction before the committing client.If something goes wrong such that the slave does not process the transaction, then in the event of a master crash and failover to the slave, it is possible that such clients will see a loss of data relative to what they saw on the master.
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