In order for replication to be resilient to unexpected halts of the server (sometimes described as crash-safe) it must be possible for the slave to recover its state before halting. This section describes the impact of an unexpected halt of a slave during replication and how to configure a slave for the best chance of recovery to continue replication.

After an unexpected halt of a slave, upon restart the slave's SQL thread must recover which transactions have been executed already. The information required for recovery is stored in the slave's relay log info log. From MySQL 8.0, this log is created by default as an InnoDB table named mysql.slave_relay_log_info (with the system variable relay_log_info_repository set to the default of TABLE). By using this transactional storage engine the information is always recoverable upon restart.

Updates to the relay log info log table are committed together with the transactions, meaning that the slave's progress information recorded in that log is always consistent with what has been applied to the database, even in the event of an unexpected server halt. Previously, this information was stored by default in a file in the data directory that was updated after the transaction had been applied. This held the risk of losing synchrony with the master depending at which stage of processing a transaction the slave halted at, or even corruption of the file itself. The setting relay_log_info_repository = FILE is now deprecated, and will be removed in a future release. For further information on the slave logs, see Section 17.2.4, “Replication Relay and Status Logs”.

When the relay log info log is stored in the mysql.slave_relay_log_info table, DML transactions and also atomic DDL make the following three updates together, atomically:

In all other cases, including DDL statements that are not fully atomic, and exempted storage engines that do not support atomic DDL, the mysql.slave_relay_log_info table might be missing updates associated with replicated data if the server halts unexpectedly. Restoring updates in this case is a manual process. For details on atomic DDL support in MySQL 8.0, and the resulting behavior for the replication of certain statements, see Section 13.1.1, “Atomic Data Definition Statement Support”.

Exactly how a replication slave recovers from an unexpected halt is influenced by the chosen method of replication, whether the slave is single-threaded or multithreaded, the setting of variables such as relay_log_recovery, and whether features such as MASTER_AUTO_POSITION are being used.

The following table shows the impact of these different factors on how a single-threaded slave recovers from an unexpected halt.

As the table shows, when using a single-threaded slave the following configurations are most resilient to unexpected halts:

The following table shows the impact of these different factors on how a multithreaded slave recovers from an unexpected halt.

As the table shows, when using a multithreaded slave the following configurations are most resilient to unexpected halts:

It is important to note the impact of sync_relay_log=1, which requires a write of to the relay log per transaction. Although this setting is the most resilient to an unexpected halt, with at most one unwritten transaction being lost, it also has the potential to greatly increase the load on storage. Without sync_relay_log=1, the effect of an unexpected halt depends on how the relay log is handled by the operating system. Also note that when relay_log_recovery=0, the next time the slave is started after an unexpected halt the relay log is processed as part of recovery. After this process completes, the relay log is deleted.

An unexpected halt of a multithreaded replication slave using the recommended file position based replication configuration above may result in a relay log with transaction inconsistencies (gaps in the sequence of transactions) caused by the unexpected halt. See Section 17.4.1.33, “Replication and Transaction Inconsistencies”. If the relay log recovery process encounters such transaction inconsistencies they are filled and the recovery process continues automatically.

When you are using multi-source replication and relay_log_recovery=1, after restarting due to an unexpected halt all replication channels go through the relay log recovery process. Any inconsistencies found in the relay log due to an unexpected halt of a multithreaded slave are filled.