Friday 16 November 2018

GTID implementation - Oracle vs MariaDB

Oracle MySQL has implemented GTID differently from MariaDB; this article walks through some of the key differences.  
Before we look at the details, let’s look at the benefits of using global transaction ID:

  • ·         Easy to switch a slave server to replicate from a different replication master server
  • ·         Makes it simple to check the consistency of master and slaves servers.
  • ·         Ignore transactions already executed on slaves
  • ·         Simplifies complex replication topology
  • ·         Permits replicas to be pointed to masters at different level of hierarchy

GTID format
A GTID is represented as 3 numbers separated with dashes (-). For example:  0-1-10.

The first number is the domain ID, the second number is the server ID and the third number is transaction id.

A GTID is represented as a pair of coordinates, separated by a colon character (:), as shown below:

The source_id represents the originating server; the master’s server_uuid is normally used for this purpose.


Two servers are already running as master and slave, using old-style log position based replication

There is no special configuration needed of the server to start using global transaction ID. it can be switched to using GTID without any other actions needed:

Online procedure works with 5.7.6+, it goes through series of steps:
·         On each server, set enforce_gtid_consistency to WARN
Let the servers run for a while, you must ensure that above step does not cause any warnings in MySQL error log.
·         On each server, set enforce_gtid_consistency to ON
·         On each server, set GTID_MODE to OFF_PERMISSIVE
·         Wait until all anonymous gets replicated:
·         On each server, execute:
·         SET @@GLOBAL.GTID_MODE = ON;
·         For more information, see online manual here

It is crash-safe without any other actions needed. The state of the slave is crash-safe; slave keeps track of its current positions in the mysql.gtid_slave_pos system table. Updates to the state are done in the same transaction as the updates to the data.
The state of the slave is crash-safe with following settings:
a)  --relay-log-info-repository is set to TABLE so that slave can keep track of its current position in the mysql.slave_relay_log_info system table. Updates to the relay log info log table are committed together with the transactions.
b)  To ensure automatic relay log recovery immediately following server start-up, the --relay-log-recovery must be enabled

Errant transactions
Any writes applied directly to the replication slave is known as Errant transactions, such transaction/writes do not exists on other slaves connected to the same master. Table below lists the way they are handled in MariaDB vs MySQL;

MariaDB may generate errors  when the slave restarts due to GTIDs being present on the slave but absent from the master, for example:

                Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Error: connecting slave requested to start from GTID 0-102-786684, which is not in the master's binlog'

You may discover errant transactions when performing maintenance operation which verify replication topology and reports presence of errant transactions. However the procedure to fix errant requires injecting empty transaction(s). It is crucial to avoid errant transactions in MySQL, as it can prevent failover!

Skipping transactions

set global sql_slave_skip_counter = 1

Still works and can be used moderately. However, it would not work when using parallel replication.
Not supported. If you need to skip transactions, use the value of the master's gtid_executed variable instead; see Injecting empty transactions, for more information.

Ignore transactions already executed
The default behaviour is to allow such transaction more than once on the slave. However, when gtid_stict_mode is enabled , error is raised and replication stops  e.g.
               Last_SQL_Error: An attempt was made to binlog GTID 0-101-786685 which would create an out-of-order sequen ce number with existing GTID 0-101-786685, and gtid strict mode is enabled.
Any attempt to execute a subsequent transaction with the same GTID is ignored by that server. No error is raised, and no statement in the transaction is executed.

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