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 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).
By default, relay log file names have the form
in the data directory, where
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
replica uses an index file to track the relay log files
currently in use. The default relay log index file name is
in the data directory.
The default relay log file and relay log index file names can be
overridden with, respectively, the
variables (see Section 2.4, “Replication and Binary Logging Options and Variables”).
If a replica uses the default host-based relay log file names,
changing the replica server'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 using the
variables to specify relay log file names explicitly when you
initially set up the replica. This will make the names
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:
shell> cat new_relay_log_name.index >> old_relay_log_name.index shell> 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 I/O 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,” determined as follows:
The SQL 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 SQL thread takes care of doing so. However,
FLUSH LOGS rotates relay logs,
which influences when the SQL thread deletes them.