Inconsistencies in the sequence of transactions that have been executed from the relay log can occur depending on your replication configuration. This section explains how to avoid inconsistencies and solve any problems they cause.
The following types of inconsistencies can exist:
Half-applied transactions. A transaction which updates non-transactional tables has applied some but not all of its changes.
Gaps. A gap in the externalized transaction set appears when, given an ordered sequence of transactions, a transaction that is later in the sequence is applied before some other transaction that is prior in the sequence. Gaps can only appear when using a multithreaded replica.
To avoid gaps occurring on a multithreaded replica, set
replica_preserve_commit_order=ON(from MySQL 8.0.26) or
slave_preserve_commit_order=ON(before MySQL 8.0.26). From MySQL 8.0.27, this setting is the default, because all replicas are multithreaded by default from that release.
Up to and including MySQL 8.0.18, preserving the commit order requires that binary logging (
log_bin) and replica update logging (
log_slave_updates) are also enabled, which are the default settings from MySQL 8.0. From MySQL 8.0.19, binary logging and replica update logging are not required on the replica to set
slave_preserve_commit_order=ON, and can be disabled if wanted.
In all releases, setting
replica_parallel_type(from MySQL 8.0.26) or
slave_parallel_type(before MySQL 8.0.26) is set to
LOGICAL_CLOCK. From MySQL 8.0.27 (but not for earlier releases), this is the default setting.
In some specific situations, as listed in the description for
slave_preserve_commit_order=ONcannot preserve commit order on the replica, so in these cases gaps might still appear in the sequence of transactions that have been executed from the replica's relay log.
slave_preserve_commit_order=ONdoes not prevent source binary log position lag.
Source binary log position lag. Even in the absence of gaps, it is possible that transactions after
Exec_master_log_poshave been applied. That is, all transactions up to point
Nhave been applied, and no transactions after
Nhave been applied, but
Exec_master_log_poshas a value smaller than
N. In this situation,
Exec_master_log_posis a “low-water mark” of the transactions applied, and lags behind the position of the most recently applied transaction. This can only happen on multithreaded replicas. Enabling
slave_preserve_commit_orderdoes not prevent source binary log position lag.
The following scenarios are relevant to the existence of half-applied transactions, gaps, and source binary log position lag:
While replication threads are running, there may be gaps and half-applied transactions.
mysqld shuts down. Both clean and unclean shutdown abort ongoing transactions and may leave gaps and half-applied transactions.
KILLof replication threads (the SQL thread when using a single-threaded replica, the coordinator thread when using a multithreaded replica). This aborts ongoing transactions and may leave gaps and half-applied transactions.
Error in applier threads. This may leave gaps. If the error is in a mixed transaction, that transaction is half-applied. When using a multithreaded replica, workers which have not received an error complete their queues, so it may take time to stop all threads.
STOP REPLICAwhen using a multithreaded replica. After issuing
STOP REPLICA, the replica waits for any gaps to be filled and then updates
Exec_master_log_pos. This ensures it never leaves gaps or source binary log position lag, unless any of the cases above applies, in other words, before
STOP REPLICAcompletes, either an error happens, or another thread issues
KILL, or the server restarts. In these cases,
STOP REPLICAreturns successfully.
If the last transaction in the relay log is only half-received and the multithreaded replica's coordinator thread has started to schedule the transaction to a worker, then
STOP REPLICAwaits up to 60 seconds for the transaction to be received. After this timeout, the coordinator gives up and aborts the transaction. If the transaction is mixed, it may be left half-completed.
STOP REPLICAwhen the ongoing transaction updates transactional tables only, in which case it is rolled back and
STOP REPLICAstops immediately. If the ongoing transaction is mixed,
STOP REPLICAwaits up to 60 seconds for the transaction to complete. After this timeout, it aborts the transaction, so it may be left half-completed.
The global setting for the system variable
MySQL 8.0.26) or
MySQL 8.0.26) is unrelated to the process of stopping the
replication threads. It only makes the client that issues
REPLICA return to the client, but the replication
threads continue to try to stop.
If a replication channel has gaps, it has the following consequences:
The replica database is in a state that may never have existed on the source.
SHOW REPLICA STATUSis only a “low-water mark”. In other words, transactions appearing before the position are guaranteed to have committed, but transactions after the position may have committed or not.
If mysqldump is used with
--dump-slave, it does not record the existence of gaps; thus it prints
CHANGE REPLICATION SOURCE TO|
CHANGE MASTER TOwith
RELAY_LOG_POSset to the “low-water mark” position in
After applying the dump on another server, and starting the replication threads, transactions appearing after the position are replicated again. Note that this is harmless if GTIDs are enabled (however, in that case it is not recommended to use
If a replication channel has source binary log position lag but no gaps, cases 2 to 5 above apply, but case 1 does not.
The source binary log position information is persisted in
binary format in the internal table
[SQL_THREAD] always consults this information so that
it applies only the correct transactions. This remains true even
slave_parallel_workers has been
changed to 0 before
REPLICA, and even if
REPLICA is used with
UNTIL SQL_AFTER_MTS_GAPS only applies as many
transactions as needed in order to fill in the gaps. If
REPLICA is used with
that tell it to stop before it has consumed all the gaps, then
it leaves remaining gaps.
REPLICA removes the relay logs and resets the
replication position. Thus issuing
REPLICA on a multithreaded replica with gaps means
the replica loses any information about the gaps, without
correcting the gaps. In this situation, if binary log position
based replication is in use, the recovery process fails.
When GTID-based replication is in use
SOURCE_AUTO_POSITION is set for the
replication channel using the
REPLICATION SOURCE TO statement, the old relay logs
are not required for the recovery process. Instead, the replica
can use GTID auto-positioning to calculate what transactions it
is missing compared to the source. From MySQL 8.0.26, the
process used for binary log position based replication to
resolve gaps on a multithreaded replica is skipped entirely when
GTID-based replication is in use. When the process is skipped, a
UNTIL SQL_AFTER_MTS_GAPS statement behaves
differently, and does not attempt to check for gaps in the
sequence of transactions. You can also issue
CHANGE REPLICATION SOURCE TO
statements, which are not permitted on a non-GTID replica where
there are gaps.