A replica server creates several repositories of information to use for the replication process:
The replica's relay log, which is written by the replication I/O thread, contains the transactions read from the replication source server's binary log. The transactions in the relay log are applied on the replica by the replication SQL thread. For information about the relay log, see Section 5.4.1, “The Relay Log”.
The replica's connection metadata repository contains information that the replication I/O thread needs to connect to the replication source server and retrieve transactions from the source's binary log. The connection metadata repository is written to the
The replica's applier metadata repository contains information that the replication SQL thread needs to read and apply transactions from the replica's relay log. The applier metadata repository is written to the
The replica's connection metadata repository and applier metadata repository are collectively known as the replication metadata repositories. For information about these, see Section 5.4.2, “Replication Metadata Repositories”.
Making replication resilient to unexpected halts.
mysql.slave_relay_log_info tables are created
using the transactional storage engine
InnoDB. Updates to the
replica's applier metadata repository table are committed
together with the transactions, meaning that the replica's
progress information recorded in that repository is always
consistent with what has been applied to the database, even in
the event of an unexpected server halt. For information on the
combination of settings on the replica that is most resilient to
unexpected halts, see
Section 3.2, “Handling an Unexpected Halt of a Replica”.