The relay log, like the binary log, consists of a set of numbered files containing events that describe database changes, and an index file that contains the names of all used relay log files. The default location for relay log files is the data directory.
The term “relay log file” generally denotes an individual numbered file containing database events. The term “relay log” collectively denotes the set of numbered relay log files plus the index file.
Relay log files have the same format as binary log files and can be read using mysqlbinlog (see mysqlbinlog — Utility for Processing Binary Log Files). If binary log transaction compression (available as of MySQL 8.0.20) is in use, transaction payloads written to the relay log are compressed in the same way as for the binary log. For more information on binary log transaction compression, see Binary Log Transaction Compression.
For the default replication channel, relay log file names have
the default form
host_name is the name of the
replica server host and
nnnnnn is a
sequence number. Successive relay log files are created using
successive sequence numbers, beginning with
000001. For non-default replication channels,
the default base name is
channel is the name of the
replication channel recorded in the relay log.
The replica uses an index file to track the relay log files
currently in use. The default relay log index file name is
for the default channel, and
for non-default replication channels.
The default relay log file and relay log index file names and
locations can be overridden with, respectively, the
variables (see Section 2.6, “Replication and Binary Logging Options and Variables”).
If a replica uses the default host-based relay log file names,
changing a replica's host name after replication has been set up
can cause replication to fail with the errors Failed
to open the relay log and Could not find
target log during relay log initialization. This is
a known issue (see Bug #2122). If you anticipate that a
replica's host name might change in the future (for example, if
networking is set up on the replica such that its host name can
be modified using DHCP), you can avoid this issue entirely by
variables to specify relay log file names explicitly when you
initially set up the replica. This causes the names to be
independent of server host name changes.
If you encounter the issue after replication has already begun, one way to work around it is to stop the replica server, prepend the contents of the old relay log index file to the new one, and then restart the replica. On a Unix system, this can be done as shown here:
$> cat new_relay_log_name.index >> old_relay_log_name.index $> mv old_relay_log_name.index new_relay_log_name.index
A replica server creates a new relay log file under the following conditions:
Each time the replication I/O (receiver) thread starts.
When the logs are flushed (for example, with
FLUSH LOGSor mysqladmin flush-logs).
When the size of the current relay log file becomes too large, which is determined as follows:
The replication SQL (applier) thread automatically deletes each
relay log file after it has executed all events in the file and
no longer needs it. There is no explicit mechanism for deleting
relay logs because the replication SQL thread takes care of
doing so. However,
rotates relay logs, which influences when the replication SQL
thread deletes them.