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MySQL 5.7 Reference Manual

16.1.3.1 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 tablemysql.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

Thesource_id identifies the originating server. Normally, the source'sserver_uuid is used for this purpose. Thetransaction_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 has1 as itstransaction_id, and the tenth transaction to be committed on the same originating server is assigned atransaction_id of10. It is not possible for a transaction to have0 as a sequence number in a GTID. For example, the twenty-third transaction to be committed originally on the server with the UUID3E11FA47-71CA-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 bybinlog_error_action.

The GTID for a transaction is shown in the output frommysqlbinlog, 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 thegtid_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 thegtid_executed andgtid_purged system variables are GTID sets. TheSTART SLAVE clausesUNTIL SQL_BEFORE_GTIDS andUNTIL 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 functionsGTID_SUBSET() andGTID_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 whoseserver_uuid is3E11FA47-71CA-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 thegtid_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-hhhhhhhhhhhhh:    [0-9|A-F]interval:n[-n]    (n >= 1)

mysql.gtid_executed Table

GTIDs are stored in a table namedgtid_executed, in themysql 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.

Themysql.gtid_executed table is created (if it does not already exist) when MySQL Server is installed or upgraded, using aCREATE 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.

Themysql.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 themysql.gtid_executed table is cleared if you issueRESET MASTER.

GTIDs are stored in themysql.gtid_executed table only whengtid_mode isON orON_PERMISSIVE. The point at which GTIDs are stored depends on whether binary logging is enabled or disabled:

If binary logging is disabled (log_bin isOFF), or iflog_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 themysql.gtid_executed table. In addition, the table is compressed periodically at a user-configurable rate; seemysql.gtid_executed Table Compression, for more information. This situation can only apply on a replica where binary logging or replica update logging is disabled. It does not apply on a replication source server, because on the source, binary logging must be enabled for replication to take place.
If binary logging is enabled (log_bin isON), whenever the binary log is rotated or the server is shut down, the server writes GTIDs for all transactions that were written into the previous binary log into themysql.gtid_executed table. This situation applies on a replication source server, or a replica where binary logging is enabled.
In the event of the server stopping unexpectedly, the set of GTIDs from the current binary log file is not saved in themysql.gtid_executed table. These GTIDs are added to the table from the binary log file during recovery. The exception to this is if binary logging is not enabled when the server is restarted. In this situation, the server cannot access the binary log file to recover the GTIDs, so replication cannot be started.
When binary logging is enabled, themysql.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 thegtid_executed system variable. Always use@@GLOBAL.gtid_executed, which is updated after every commit, to represent the GTID state for the MySQL server, and do not query themysql.gtid_executed table.

mysql.gtid_executed Table Compression

Over the course of time, themysql.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 compresses themysql.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           |...

You can control the number of transactions that are allowed to elapse before the table is compressed, and thus the compression rate, by setting thegtid_executed_compression_period system variable. This variable's default value is 1000, meaning that by default, compression of the table is performed after each 1000 transactions. Settinggtid_executed_compression_period to 0 prevents the compression from being performed at all, and you should be prepared for a potentially large increase in the amount of disk space that may be required by thegtid_executed table if you do this.

Note

When binary logging is enabled, the value ofgtid_executed_compression_period isnot used and themysql.gtid_executed table is compressed on each binary log rotation.

Compression of themysql.gtid_executed table is performed by a dedicated foreground thread namedthread/sql/compress_gtid_table. This thread is not listed in the output ofSHOW PROCESSLIST, but it can be viewed as a row in thethreads 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: NULLPROCESSLIST_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

Thethread/sql/compress_gtid_table thread normally sleeps untilgtid_executed_compression_period transactions have been executed, then wakes up to perform compression of themysql.gtid_executed table as described previously. It then sleeps until anothergtid_executed_compression_period transactions have taken place, then wakes up to perform the compression again, repeating this loop indefinitely. Setting this value to 0 when binary logging is disabled means that the thread always sleeps and never wakes up, meaning that this explicit compression method is not used. Instead, compression occurs implicitly as required.

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