2.3.1 GTID Format and Storage

A global transaction identifier (GTID) is a unique identifier created and associated with each transaction committed on the server of origin (the source). This identifier is unique not only to the server on which it originated, but is unique across all servers in a given replication topology.

GTID assignment distinguishes between client transactions, which are committed on the source, and replicated transactions, which are reproduced on a replica. When a client transaction is committed on the source, it is assigned a new GTID, provided that the transaction was written to the binary log. Client transactions are guaranteed to have monotonically increasing GTIDs without gaps between the generated numbers. If a client transaction is not written to the binary log (for example, because the transaction was filtered out, or the transaction was read-only), it is not assigned a GTID on the server of origin.

Replicated transactions retain the same GTID that was assigned to the transaction on the server of origin. The GTID is present before the replicated transaction begins to execute, and is persisted even if the replicated transaction is not written to the binary log on the replica, or is filtered out on the replica. The MySQL system table mysql.gtid_executed is used to preserve the assigned GTIDs of all the transactions applied on a MySQL server, except those that are stored in a currently active binary log file.

The auto-skip function for GTIDs means that a transaction committed on the source can be applied no more than once on the replica, which helps to guarantee consistency. Once a transaction with a given GTID has been committed on a given server, 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.

If a transaction with a given GTID has started to execute on a server, but has not yet committed or rolled back, any attempt to start a concurrent transaction on the server with the same GTID blocks. The server neither begins to execute the concurrent transaction nor returns control to the client. Once the first attempt at the transaction commits or rolls back, concurrent sessions that were blocking on the same GTID may proceed. If the first attempt rolled back, one concurrent session proceeds to attempt the transaction, and any other concurrent sessions that were blocking on the same GTID remain blocked. If the first attempt committed, all the concurrent sessions stop being blocked, and auto-skip all the statements of the transaction.

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

GTID = source_id:transaction_id

The source_id identifies the originating server. Normally, the source's server_uuid is used for this purpose. The transaction_id is a sequence number determined by the order in which the transaction was committed on the source. For example, the first transaction to be committed has 1 as its transaction_id, and the tenth transaction to be committed on the same originating server is assigned a transaction_id of 10. It is not possible for a transaction to have 0 as a sequence number in a GTID. For example, the twenty-third transaction to be committed originally on the server with the UUID 3E11FA47-71CA-11E1-9E33-C80AA9429562 has this GTID:

3E11FA47-71CA-11E1-9E33-C80AA9429562:23

The upper limit for sequence numbers for GTIDs on a server instance is the number of non-negative values for a signed 64-bit integer (2 to the power of 63 minus 1, or 9,223,372,036,854,775,807). If the server runs out of GTIDs, it takes the action specified by binlog_error_action. From MySQL 8.0.23, a warning message is issued when the server instance is approaching the limit.

The GTID for a transaction is shown in the output from mysqlbinlog, and it is used to identify an individual transaction in the Performance Schema replication status tables, for example, replication_applier_status_by_worker. The value stored by the gtid_next system variable (@@GLOBAL.gtid_next) is a single GTID.

GTID Sets

A GTID set is a set comprising one or more single GTIDs or ranges of GTIDs. GTID sets are used in a MySQL server in several ways. For example, the values stored by the gtid_executed and gtid_purged system variables are GTID sets. The START REPLICA (or before MySQL 8.0.22, START SLAVE) clauses UNTIL SQL_BEFORE_GTIDS and UNTIL SQL_AFTER_GTIDS can be used to make a replica process transactions only up to the first GTID in a GTID set, or stop after the last GTID in a GTID set. The built-in functions GTID_SUBSET() and GTID_SUBTRACT() require GTID sets as input.

A range of GTIDs originating from the same server can be collapsed into a single expression, as shown here:

3E11FA47-71CA-11E1-9E33-C80AA9429562:1-5

The above example represents the first through fifth transactions originating on the MySQL server whose server_uuid is 3E11FA47-71CA-11E1-9E33-C80AA9429562. Multiple single GTIDs or ranges of GTIDs originating from the same server can also be included in a single expression, with the GTIDs or ranges separated by colons, as in the following example:

3E11FA47-71CA-11E1-9E33-C80AA9429562:1-3:11:47-49

A GTID set can include any combination of single GTIDs and ranges of GTIDs, and it can include GTIDs originating from different servers. This example shows the GTID set stored in the gtid_executed system variable (@@GLOBAL.gtid_executed) of a replica that has applied transactions from more than one source:

2174B383-5441-11E8-B90A-C80AA9429562:1-3, 24DA167-0C0C-11E8-8442-00059A3C7B00:1-19

When GTID sets are returned from server variables, UUIDs are in alphabetical order, and numeric intervals are merged and in ascending order.

The syntax for a GTID set is as follows:

gtid_set:
    uuid_set [, uuid_set] ...
    | ''

uuid_set:
    uuid:interval[:interval]...

uuid:
    hhhhhhhh-hhhh-hhhh-hhhh-hhhhhhhhhhhh

h:
    [0-9|A-F]

interval:
    n[-n]

    (n >= 1)

mysql.gtid_executed Table

GTIDs are stored in a table named gtid_executed, in the mysql database. A row in this table contains, for each GTID or set of GTIDs that it represents, the UUID of the originating server, and the starting and ending transaction IDs of the set; for a row referencing only a single GTID, these last two values are the same.

The mysql.gtid_executed table is created (if it does not already exist) when MySQL Server is installed or upgraded, using a CREATE TABLE statement similar to that shown here:

CREATE TABLE gtid_executed (
    source_uuid CHAR(36) NOT NULL,
    interval_start BIGINT(20) NOT NULL,
    interval_end BIGINT(20) NOT NULL,
    PRIMARY KEY (source_uuid, interval_start)
)

Warning

As with other MySQL system tables, do not attempt to create or modify this table yourself.

The mysql.gtid_executed table is provided for internal use by the MySQL server. It enables a replica to use GTIDs when binary logging is disabled on the replica, and it enables retention of the GTID state when the binary logs have been lost. Note that the mysql.gtid_executed table is cleared if you issue RESET MASTER.

GTIDs are stored in the mysql.gtid_executed table only when gtid_mode is ON or ON_PERMISSIVE. If binary logging is disabled (log_bin is OFF), or if log_replica_updates or log_slave_updates is disabled, the server stores the GTID belonging to each transaction together with the transaction in the buffer when the transaction is committed, and the background thread adds the contents of the buffer periodically as one or more entries to the mysql.gtid_executed table. In addition, the table is compressed periodically at a user-configurable rate, as described in mysql.gtid_executed Table Compression.

If binary logging is enabled (log_bin is ON), from MySQL 8.0.17 for the InnoDB storage engine only, the server updates the mysql.gtid_executed table in the same way as when binary logging or replica update logging is disabled, storing the GTID for each transaction at transaction commit time. However, in releases before MySQL 8.0.17, and for other storage engines, the server only updates the mysql.gtid_executed table when the binary log is rotated or the server is shut down. At these times, the server writes GTIDs for all transactions that were written into the previous binary log into the mysql.gtid_executed table. This situation applies on a source prior to MySQL 8.0.17, or on a replica prior to MySQL 8.0.17 where binary logging is enabled, or with storage engines other than InnoDB, it has the following consequences:

• In the event of the server stopping unexpectedly, the set of GTIDs from the current binary log file is not saved in the mysql.gtid_executed table. These GTIDs are added to the table from the binary log file during recovery so that replication can continue. The exception to this is if you disable binary logging when the server is restarted (using --skip-log-bin or --disable-log-bin). In that case, the server cannot access the binary log file to recover the GTIDs, so replication cannot be started.

• The mysql.gtid_executed table does not hold a complete record of the GTIDs for all executed transactions. That information is provided by the global value of the gtid_executed system variable. In releases before MySQL 8.0.17 and with storage engines other than InnoDB, always use @@GLOBAL.gtid_executed, which is updated after every commit, to represent the GTID state for the MySQL server, instead of querying the mysql.gtid_executed table.

The MySQL server can write to the mysql.gtid_executed table even when the server is in read only or super read only mode. In releases before MySQL 8.0.17, this ensures that the binary log file can still be rotated in these modes. If the mysql.gtid_executed table cannot be accessed for writes, and the binary log file is rotated for any reason other than reaching the maximum file size (max_binlog_size), the current binary log file continues to be used. An error message is returned to the client that requested the rotation, and a warning is logged on the server. If the mysql.gtid_executed table cannot be accessed for writes and max_binlog_size is reached, the server responds according to its binlog_error_action setting. If IGNORE_ERROR is set, an error is logged on the server and binary logging is halted, or if ABORT_SERVER is set, the server shuts down.

mysql.gtid_executed Table Compression

Over the course of time, the mysql.gtid_executed table can become filled with many rows referring to individual GTIDs that originate on the same server, and whose transaction IDs make up a range, similar to what is shown here:

+--------------------------------------+----------------+--------------+
| source_uuid                          | interval_start | interval_end |
|--------------------------------------+----------------+--------------|
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 37             | 37           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 38             | 38           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 39             | 39           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 40             | 40           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 41             | 41           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 42             | 42           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 43             | 43           |
...

To save space, the MySQL server can compress the mysql.gtid_executed table periodically by replacing each such set of rows with a single row that spans the entire interval of transaction identifiers, like this:

+--------------------------------------+----------------+--------------+
| source_uuid                          | interval_start | interval_end |
|--------------------------------------+----------------+--------------|
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 37             | 43           |
...

The server can carry out compression using a dedicated foreground thread named thread/sql/compress_gtid_table. This thread is not listed in the output of SHOW PROCESSLIST, but it can be viewed as a row in the threads table, as shown here:

mysql> SELECT * FROM performance_schema.threads WHERE NAME LIKE '%gtid%'\G
*************************** 1. row ***************************
          THREAD_ID: 26
               NAME: thread/sql/compress_gtid_table
               TYPE: FOREGROUND
     PROCESSLIST_ID: 1
   PROCESSLIST_USER: NULL
   PROCESSLIST_HOST: NULL
     PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Daemon
   PROCESSLIST_TIME: 1509
  PROCESSLIST_STATE: Suspending
   PROCESSLIST_INFO: NULL
   PARENT_THREAD_ID: 1
               ROLE: NULL
       INSTRUMENTED: YES
            HISTORY: YES
    CONNECTION_TYPE: NULL
       THREAD_OS_ID: 18677

When binary logging is enabled on the server, this compression method is not used, and instead the mysql.gtid_executed table is compressed on each binary log rotation. However, when binary logging is disabled on the server, the thread/sql/compress_gtid_table thread sleeps until a specified number of transactions have been executed, then wakes up to perform compression of the mysql.gtid_executed table. It then sleeps until the same number of transactions have taken place, then wakes up to perform the compression again, repeating this loop indefinitely. The number of transactions that elapse before the table is compressed, and thus the compression rate, is controlled by the value of the gtid_executed_compression_period system variable. Setting that value to 0 means that the thread never wakes up, meaning that this explicit compression method is not used. Instead, compression occurs implicitly as required.

From MySQL 8.0.17, InnoDB transactions are written to the mysql.gtid_executed table by a separate process to non-InnoDB transactions. This process is controlled by a different thread, innodb/clone_gtid_thread. This GTID persister thread collects GTIDs in groups, flushes them to the mysql.gtid_executed table, then compresses the table. If the server has a mix of InnoDB transactions and non-InnoDB transactions, which are written to the mysql.gtid_executed table individually, the compression carried out by the compress_gtid_table thread interferes with the work of the GTID persister thread and can slow it significantly. For this reason, from that release it is recommended that you set gtid_executed_compression_period to 0, so that the compress_gtid_table thread is never activated.

From MySQL 8.0.23, the gtid_executed_compression_period default value is 0, and both InnoDB and non-InnoDB transactions are written to the mysql.gtid_executed table by the GTID persister thread.

For releases before MySQL 8.0.17, the default value of 1000 for gtid_executed_compression_period can be used, meaning that compression of the table is performed after each 1000 transactions, or you can choose an alternative value. In those releases, if you set a value of 0 and binary logging is disabled, explicit compression is not performed on the mysql.gtid_executed table, and you should be prepared for a potentially large increase in the amount of disk space that may be required by the table if you do this.

When a server instance is started, if gtid_executed_compression_period is set to a nonzero value and the thread/sql/compress_gtid_table thread is launched, in most server configurations, explicit compression is performed for the mysql.gtid_executed table. In releases before MySQL 8.0.17 when binary logging is enabled, compression is triggered by the fact of the binary log being rotated at startup. In releases from MySQL 8.0.20, compression is triggered by the thread launch. In the intervening releases, compression does not take place at startup.