A global transaction identifier (GTID) is a unique identifier created and associated with each transaction committed on the server of origin (the master). 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 master, and replicated transactions, which are reproduced on a slave. When a client transaction is committed on the master, 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 slave, or is filtered out on the slave. 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 master can be applied no more than once on the slave, 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 will block. 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
source_id identifies the
originating server. Normally, the server's
server_uuid is used for this
transaction_id is a
sequence number determined by the order in which the transaction
was committed on this server; 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
10. It is not possible for a transaction to
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
This format is used to represent GTIDs in the output of statements
SHOW SLAVE STATUS as well
as in the binary log. They can also be seen when viewing the log
file with mysqlbinlog
in the output from
As written in the output of statements such as
SHOW MASTER STATUS or
SLAVE STATUS, a sequence of GTIDs originating from the
same server may be collapsed into a single expression, as shown
The example just shown represents the first through fifth
transactions originating on the MySQL Server whose
This format is also used to supply the argument required by the
START SLAVE options
A GTID set is a set of global transaction identifiers which is represented as shown here:
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)
GTID sets are used in the MySQL Server in several ways. For
example, the values stored by the
gtid_purged system variables
are represented as GTID sets. In addition, the functions
GTID sets as input. When GTID sets are returned from server
variables, UUIDs are in alphabetical order and numeric intervals
are merged and in ascending order.
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.
mysql.gtid_executed table is created (if
it does not already exist) when the MySQL Server is installed or
upgraded, using a
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) )
As with other MySQL system tables, do not attempt to create or modify this table yourself.
mysql.gtid_executed table enables a slave
to use GTIDs when binary logging is disabled on the slave, and
it enables retention of the GTID history when the binary logs
have been lost.
GTIDs are stored in the
table only when
point at which GTIDs are stored depends on whether binary
logging is enabled or disabled:
If binary logging is disabled (
OFF), or if
log_slave_updatesis disabled, the server stores the GTID belonging to each transaction together with the transaction in the table. In addition, the table is compressed periodically at a user-configurable rate; see mysql.gtid_executed Table Compression, for more information. This situation can only apply on a replication slave where binary logging or slave update logging is disabled. It does not apply on a replication master, because on a master, binary logging must be enabled for replication to take place.
If binary logging is enabled (
ON), 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 the
mysql.gtid_executedtable. This situation applies on a replication master, or a replication slave where binary logging is enabled.
In the event of the server stopping unexpectedly, the set of GTIDs from the current binary log is not saved in the
mysql.gtid_executedtable. In this case, these GTIDs are added to the table and to the set of GTIDs in the
gtid_executedsystem variable during recovery.
When binary logging is enabled, the
mysql.gtid_executedtable does not provide a complete record of the GTIDs for all executed transactions. That information is provided by the global value of the
mysql.gtid_executed table is reset by
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 sequence,
similar to what is shown here:
mysql> SELECT * FROM mysql.gtid_executed; +--------------------------------------+----------------+--------------+ | 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 | ...
Considerable space can be saved if this table is compressed 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 | ...
When GTIDs are enabled, the server performs this type of
compression on the
periodically. You can control the number of transactions that
are allowed to elapse before the table is compressed, and thus
the compression rate, by setting the
system variable. This variable's default value is 1000;
this means that, by default, compression of the table is
performed after each 1000 transactions. Setting
to 0 prevents the compression from being performed at all;
however, you should be prepared for a potentially large increase
in the amount of disk space that may be required by the
gtid_executed table if you do this.
When binary logging is enabled, the value of
is not used and the
mysql.gtid_executed table is compressed on
each binary log rotation.
Compression of the
is performed by a dedicated foreground thread named
thread/sql/compress_gtid_table. This thread
is not listed in the output of
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
normally sleeps until
transactions have been executed, then wakes up to perform
compression of the
as described previously. It then sleeps until another
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.