Multi-Source Replication

Multi-source replication means that one server has many primaries from which it replicates.

New Syntax

You specify which primary connection you want to work with by either specifying the connection name in the command or setting default_master_connection to the connection you want to work with.

The connection name may include any characters and should be less than 64 characters. Connection names are compared without regard to case (case insensitive). You should preferably keep the connection name short as it will be used as a suffix for relay logs and primary info index files.

The new syntax introduced to handle many connections:

• CHANGE MASTER ['connection_name'] TO .... This creates or modifies a connection to a primary.
• FLUSH RELAY LOGS ['connection_name']
• MASTER_POS_WAIT(....,['connection_name'])
• RESET SLAVE ['connection_name'] [ALL]. This is used to reset a replica's replication position or to remove a replica permanently.
• SHOW RELAYLOG ['connection_name'] EVENTS
• SHOW SLAVE ['connection_name'] STATUS
• SHOW ALL SLAVES STATUS
• START SLAVE ['connection_name'...]]
• START ALL SLAVES ...
• STOP SLAVE ['connection_name'] ...
• STOP ALL SLAVES ...

The original old-style connection is an empty string ''. You don't have to use this connection if you don't want to.

You create new primary connections with CHANGE MASTER.
You delete the connection permanently with RESET SLAVE 'connection_name' ALL.

Replication Variables for Multi-Source

The new replication variable default_master_connection specifies which connection will be used for commands and variables if you don't specify a connection. By default this is '' (the default connection name).

The following replication variables are local for the connection. (In other words, they show the value for the @@default_master_connection connection). We are working on making all the important ones local for the connection.

You can access all of the above variables with either SESSION or GLOBAL.

Note that in contrast to MySQL, all variables always show the correct active value!

Example:

set @@default_master_connection='';
show status like 'Slave_running';
set @@default_master_connection='other_connection';
show status like 'Slave_running';

If @@default_master_connection contains a non existing name, you will get a warning.

All other primary-related variables are global and affect either only the '' connections or all connections. For example, Slave_retried_transactions now shows the total number of retried transactions over all replicas.

If you need to set gtid_slave_pos you need to set this for all primaries at the same time.

New status variables:

SHOW ALL SLAVES STATUS has the following new columns:

New Files

The basic principle of the new files used by multi source replication is that they have the same name as the original relay log files suffixed with connection_name before the extension. The main exception is the file that holds all connection is named as the normal master-info-file with a multi- prefix.

When you are using multi source, the following new files are created:

When creating the file, the connection name is converted to lower case and all special characters in the connection name are converted, the same way as MySQL table names are converted. This is done to make the file name portable across different systems.

Hint:

Instead of specifying names for mysqld with --relay-log, --relay-log-index, --general-log-file, --slow-query-log-file, --log-bin and --log-bin-index, you can just specify --log-basename and all the other variables are set with this as a prefix.

Other Things

• All error messages from a replica with a connection name, that are written to the error log, are prefixed with Master 'connection_name':. This makes it easy to see from where an error originated.
• Errors ER_MASTER_INFO and WARN_NO_MASTER_INFO now includes connection_name.
• There is no conflict resolution. The assumption is that there are no conflicts in data between the different primaries.
• All executed commands are stored in the normal binary log (nothing new here).
• If the server variable log_warnings > 1 then you will get some information in the log about how the multi-master-info file is updated (mainly for debugging).
• The output of SHOW ALL SLAVES STATUS has one more column than SHOW SLAVE STATUS, since it includes the connection_name column.
• RESET SLAVE now deletes all relay-log files.

replicate-... Variables

• One can set the values for the replicate-... variables from the command line or in my.cnf for a given connection by prefixing the variable with the connection name.
• If one doesn't use any connection name prefix for a replicate.. variable, then the value will be used as the default value for all connections that don't have a value set for this variable.

Example:

mysqld --main_connection.replicate_do_db=main_database --replicate_do_db=other_database

The have sets the replicate_do_db variable to main_database for the connection named main_connection. All other connections will use the value other_database.

One can also use this syntax to set replicate-rewrite-db for a given connection.

Typical Use Cases

• You are partitioning your data over many primaries and would like to get it all together on one machine to do analytical queries on all data.
• You have many databases spread over many MariaDB/MySQL servers and would like to have all of them on one machine as an extra backup.
• In a Galera cluster the default replication filter rules like replicate-do-db do not apply to replication connections, but also to Galera write set applier threads. By using a named multi-primary replication connection instead, even when replicating from just one primary into the cluster, the primary-replica replication rules can be kept separate from the Galera intra-node replication traffic.

Limitations

• Each active connection will create 2 threads (as is normal for MariaDB replication).
• You should ensure that all primaries have different server-id's. If you don't do this, you will get into trouble if you try to replicate from the multi-source replica back to your primaries.
• One can change max_relay_log_size for any active connection, but new connections will always use the server startup value for max_relay_log_size, which can't be changed at runtime.
• Option innodb-recovery-update-relay-log (xtradb feature to store and restore relay log position for replicas) only works for the default connection ''. As this option is not really safe and can easily cause loss of data if you use storage engines other than InnoDB, we don't recommend this option be used.
• slave_net_timeout affects all connections. We don't check anymore if it's less than Slave_heartbeat_period, as this doesn't make sense in a multi-source setup.

Incompatibilities with MariaDB/MySQL 5.5

• max_relay_log_size is now (almost) a normal variable and not automatically changed if max_binlog_size is changed. To keep things compatible with old config files, we set it to max_binlog_size at startup if its value is 0.
• You can now access replication variables that depend on the active connection with either GLOBAL or SESSION.
• We only write information about relay log positions for recovery if innodb-recovery-update-relay-log is set.
• Slave_retried_transactions now shows the total count of retried transactions over all replicas.
• The status variable Com_slave_start is replaced with Com_start_slave.
• The status variable Com_slave_stop is replaced with Com_stop_slave.
• FLUSH RELAY LOGS are not replicated anymore. This is not safe as connection names may be different on the replica.

See Also

• Using multi-source with global transaction id
• The work in MariaDB is based on the project description at MDEV-253.
• The original code base comes from Taobao, developed by Peng Lixun. A big thanks to them for this important feature!