A replica server creates several repositories of information to use for the replication process:
The 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 22.214.171.124, “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
mysql.slave_master_infotable or to a file.
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
mysql.slave_relay_log_infotable or to a file.
The connection metadata repository and applier metadata repository are collectively known as the replication metadata repositories. For information about these, see Section 126.96.36.199, “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
Section 16.3.2, “Handling an Unexpected Halt of a Replica”.