Global Transaction ID

Описание

MariaDB replication in general works as follows (see Replication overview for more information):

On a master server, all updates to the database (DML and DDL) are written into the binary log as binlog events. A slave server connects to the master and reads the binlog events, then applies the events locally to replicate the same changes as done on the master. A server can be both a master and a slave at the same time, and it is thus possible for binlog events to replicated through multiple levels of servers.

A slave server keeps track of the position in the master's binlog of the last event applied on the slave. This allows the slave server to re-connect and resume from where it left off after replication has been temporarily stopped. It also allows a slave to disconnect, be cloned and then have the new slave resume replication from the same master.

Global transaction ID introduces a new event attached to each event group in the binlog. (An event group is a collection of events that are always applied as a unit. They are best thought of as a "transaction", though they also include non-transactional DML statements, as well as DDL). As an event group is replicated from master server to slave server, the global transaction ID is preserved. Since the ID is globally unique across the entire group of servers, this makes it easy to uniquely identify the same binlog events on different servers that replicate each other (this was not easily possible before MariaDB 10.0.2).

Преимущества

Using global transaction ID provides two main benefits:

1. Easy to change a slave server to connect to and replicate from a different master server.

The slave remembers the global transaction ID of the last event group applied from the old master. This makes it easy to know where to resume replication on the new master, since the global transaction IDs are known throughout the entire replication hierarchy. This is not the case when using old-style replication; in this case the slave knows only the specific file name and offset of the old master server of the last event applied. There is no simple way to guess from this the correct file name and offset on a new master.

2. The state of the slave is recorded in a crash-safe way.

The slave keeps track of its current position (the global transaction ID of the last transaction applied) in the mysql.gtid_slave_pos system table. If this table is using a transactional storage engine (such as InnoDB, which is the default), then updates to the state are done in the same transaction as the updates to the data. This makes the state crash-safe; if the slave server crashes, crash recovery on restart will make sure that the recorded replication position matches the changes that were actually replicated. This is not the case for old-style replication, where the state is recorded in a file relay-log.info, which is updated independently of the actual data changes and can easily get out of sync if the slave server crashes. (This works for DML to transactional tables; non-transactional tables and DDL in general are not crash-safe in MariaDB.)

Because of these two benefits, it is generally recommended to use global transaction ID for any replication setups based on MariaDB 10.0.2 or later. However, old-style replication continues to work as always, so there is no pressing need to change existing setups. Global transaction ID integrates smoothly with old-style replication, and the two can be used freely together in the same replication hierarchy. There is no special configuration needed of the server to start using global transaction ID. However, it must be explicitly set for a slave server with the appropriate CHANGE MASTER option; by default old-style replication is used by a replication slave, to maintain backwards compatibility.

Implementation

A global transaction ID, or GTID for short, consists of three numbers separated with dashes '-'. For example:

0-1-10

• The first number 0 is the domain ID, which is specific for global transaction ID (more on this below). It is a 32-bit unsigned integer.
• The second number is the server ID, the same as is also used in old-style replication. It is a 32-bit unsigned integer.
• The third number is the sequence number. This is a 64-bit unsigned integer that is monotonically increasing for each new event group logged into the binlog.

The server ID is set to the server ID of the server where the event group is first logged into the binlog. The sequence number is increased on a server for every event group logged. Since server IDs must be unique for every server, this makes the (server_id, sequence_number) pair, and hence the whole GTID, globally unique.

Using a 64-bit number provides ample range that there should be no risk of it overflowing in the foreseeable future. However, one should not artificially (by setting gtid_seq_no) inject a GTID with a very high sequence number close to the limit of 64-bit.

The domain ID

When events are replicated from a master server to a slave server, the events are always logged into the slave's binlog in the same order that they were read from the master's binlog. Thus, if there is only ever a single master server receiving (non-replication) updates at a time, then the binlog order will be identical on every server in the replication hierarchy.

This consistent binlog order is used by the slave to keep track of its current position in the replication. Basically, the slave remembers the GTID of the last event group replicated from the master. When reconnecting to a master, whether the same one or a new one, it sends this GTID position to the master, and the master starts sending events from the first event after the corresponding event group.

However, if user updates are done independently on multiple servers at the same time, then in general it is not possible for binlog order to be identical across all servers. This can happen when using multi-source replication, with multi-master ring topologies, or just if manual updates are done on a slave that is replicating from active master. If the binlog order is different on the new master from the order on the old master, then it is not sufficient for the slave to keep track of a single GTID to completely record the current state.

The domain ID, the first component of the GTID, is used to handle this.

In general, the binlog is not a single ordered stream. Rather, it consists of a number of different streams, each one identified by its own domain ID. Within each stream, GTIDs always have the same order in every server binlog. However, different streams can be interleaved in different ways on different servers.

A slave server then keeps track of its replication position by recording the last GTID applied within each replication stream. When connecting to a new master, the slave can start replication from a different point in the binlog for each domain ID.

For more details on using multi-master setups and multiple domain IDs, see Use with multi-source replication and other multi-master setups.

Simple replication setups only have a single master being updated by the application at any one time. In such setups, there is only a single replication stream needed. Then domain ID can be ignored, and left as the default of 0 on all servers.

Using global transaction IDs

From MariaDB 10.0.2, global transaction ID is enabled automatically. Each event group logged to the binlog receives a GTID event, as can be seen with mysqlbinlog or SHOW BINLOG EVENTS.

The slave automatically keeps track of the GTID of the last applied event group, as can be seen from the gtid_slave_pos variable:

SELECT @@GLOBAL.gtid_slave_pos
0-1-1

When a slave connects to a master, it can use either global transaction ID or old-style filename/offset to decide where in the master binlogs to start replicating from. To use global transaction ID, use the CHANGE MASTER master_use_gtid option:

CHANGE MASTER TO master_use_gtid = { slave_pos | current_pos | no }

A slave is configured to use GTID by CHANGE MASTER TO master_use_gtid=slave_pos. When the slave connects to the master, it will start replication at the position of the last GTID replicated to the slave, which can be seen in the variable @@gtid_slave_pos. Since GTIDs are the same across all replication servers, the slave can then be pointed to a different master, and the correct position will be determined automatically.

But suppose that we set up two servers A and B and let A be the master and B the slave. It runs for a while. Then at some point we take down A, and B becomes the new master. Then later we want to add A back, this time as a slave.

Since A was never a slave before, it does not have any prior replicated GTIDs, and @@gtid_slave_pos will be empty. To allow A to be added as a slave automatically, master_use_gtid=current_pos can be used. This will connect using the value of the variable @@gtid_current_pos instead of @@gtid_slave_pos, which takes into account also GTIDs written into the binlog when the server was a master.

Using master_use_gtid=current_pos is probably easiest, as there is then no need to consider whether a server was a master or a slave prior to using CHANGE MASTER. But care must be taken not to inject extra transactions into the binlog on the slave server that are not intended to be replicated to other servers. If such an extra transaction is the most recent when the slave starts, it will be used as the starting point of replication. This will probably fail because that transaction is not present on the master. To avoid local changes on a slave server to go into the binlog, set @@sql_log_bin to 0.

If it is undesirable that changes to the binlog on the slave affects the GTID replication position, then master_use_gtid=slave_pos should be used. Then the slave will always connect to the master at the position of the last replicated GTID. This may avoid some surprises for users that expect behaviour consistent with traditional replication, where the replication position is never changed by local changes done on a server.

When GTID strict mode is enabled (by setting @@GLOBAL.gtid_strict_mode to 1), it is normally best to use current_pos. In strict mode, extra transactions on the master are disallowed.

If a slave is configured with the binlog disabled, current_pos and slave_pos are equivalent.

Even when a slave is configured to connect with the old-style binlog filename and offset (CHANGE MASTER TO master_log_file=..., master_log_pos=...), it will still keep track of the current GTID position in @@GLOBAL.gtid_slave_pos. This means that an existing slave previously configured and running can be changed to connect with GTID (to the same or a new master) simply with:

CHANGE MASTER TO master_use_gtid = slave_pos

The slave remembers that master_use_gtid=slave_pos|master_pos was specified and will use it also for subsequent connects, until it is explicitly changed by specifying master_log_file/pos=... or master_use_gtid=no. The current value can be seen as the field Using_Gtid of SHOW SLAVE STATUS:

SHOW SLAVE STATUS\G
...
Using_Gtid: Slave_pos

The slave server internally uses the mysql.gtid_slave_pos table to store the GTID position (and so preserve the value of @@GLOBAL.gtid_slave_pos across server restarts). After upgrading a server to 10.0, it is necessary to run mysql_upgrade (as always) to get the table created.

In order to be crash-safe, this table must use a transactional storage engine such as InnoDB. When MariaDB is first installed (or upgraded to 10.0.2+) the table is created using the default storage engine - which itself defaults to InnoDB. If there is a need to change the storage engine for this table (to make it transactional on a system configured with MyISAM as the default storage engine, for example), use ALTER TABLE:

ALTER TABLE mysql.gtid_slave_pos ENGINE = InnoDB

The mysql.gtid_slave_pos table should not be modified in any other way. In particular, do not try to update the rows in the table to change the slave's idea of the current GTID position; instead use

SET GLOBAL gtid_slave_pos = '0-1-1'

Setting up a new slave server with global transaction ID

Setting up a new replication slave server with global transaction ID is not much different from setting up an old-style slave. The basic steps are:

1. Setup the new server and load it with the initial data.

2. Start the slave replicating from the appropriate point in the master's binlog.

Starting with an empty server

The simplest way for testing purposes is probably to setup a new, empty slave server and replicate all of the master's binlogs from the start (this is usually not feasible in a realistic production setup, as the initial binlog files will probably have been purged or take too long to apply).

The slave server is installed in the normal way. By default, the GTID position for a newly installed server is empty, which makes the slave replicate from the start of the master's binlogs. But if the slave was used for other purposes before, the initial position can be explicitly set to empty first:

SET GLOBAL gtid_slave_pos = "";

Next, point the slave to the master with CHANGE MASTER. Specify master_host etc. as usual. But instead of specifying master_log_file and master_log_pos manually, use master_use_gtid=current_pos (or slave_pos to have GTID do it automatically:

CHANGE MASTER TO master_host="127.0.0.1", master_port=3310, master_user="root", master_use_gtid=current_pos;
START SLAVE;

Setting up from backup

The normal way to set up a new replication slave is to restore a backup from an existing server (whether master or slave) as the new server, then point it to start replication from the appropriate position in the master's binlog.

It is important that the position at which replication is started corresponds exactly to the state of the data at the point in time that the backup was taken. Otherwise, the slave can end up with different data than the master because of transactions missing or duplicated.

Two common ways to take a backup are XtraBackup and mysqldump. Both of these can provide the current binlog position of the backup in a non-blocking way. Of course, if there are no writes to the server being backed up during the backup process, then a simple SHOW MASTER STATUS will give the correct position.

Once the current binlog position for the backup has been obtained, in the form of a binlog file name and offset, the corresponding GTID position can be obtained from BINLOG_GTID_POS() on the server that was backed up:

SELECT BINLOG_GTID_POS("master-bin.000001", 600);

From version 10.0.13, mysqldump does this automatically and includes the GTID position in the output when using --master-data or --dump-slave.

The new slave can now be started replicating by setting the correct @@gtid_slave_pos, issuing CHANGE MASTER to point to the master server, and starting the slave threads:

SET GLOBAL gtid_slave_pos = "0-1-2";
CHANGE MASTER TO master_host="127.0.0.1", master_port=3310, master_user="root", master_use_gtid=slave_pos;
START SLAVE;

(From version 10.0.13, mysqldump can automatically include these commands in the output if the --gtid option is used with --master-data or --dump-slave.)

This method is particularly useful when setting up a new slave from a backup of the master. Remember to ensure that the value of server_id for the new server is different from that of any other server (this is set in my.cnf).

If the backup was taken of an existing slave server, then it already has the correct GTID position stored in the table mysql.gtid_slave_pos (provided that the backup includes this table and is consistent with changes to other tables, of course). In this case, there is no need to explicitly look up the GTID position on the old server and set it on the new slave - it will be already correctly loaded from mysql.gtid_slave_pos. This however does not work if the backup was taken of the master - because then the current GTID position is contained in the binlog, not in mysql.gtid_slave_pos.

Switching an existing old-style slave to use GTID.

If there is already an existing slave running using old-style binlog filename/offset position, then this can be changed to use GTID directly. This can be useful for upgrades for example, or where there are already tools to setup new slaves using old-style binlog positions.

When a slave connects to a master using old-style binlog positions, and the master supports GTID (ie. is MariaDB 10.0.2 or later), then the slave automatically downloads the GTID position at connect and updates it during replication. Thus, once a slave has connected to the GTID-aware master at least once, it can be switched to using GTID without any other actions needed;

STOP SLAVE;
CHANGE MASTER TO master_host="127.0.0.1", master_port=3310, master_user="root", master_use_gtid=current_pos;
START SLAVE;

(A later version will probably add a way to setup the slave so that it will connect with old-style binlog file/offset the first time, and automatically switch to using GTID on subsequent connects.)

Changing a slave to replicate from a different master

Once replication is running with GTID (master_use_gtid=current_pos|slave_pos), the slave can be pointed to a new master simply by specifying in CHANGE MASTER the new master_host (and if required master_port, master_user, and master_password):

STOP SLAVE;
CHANGE MASTER TO master_host='127.0.0.1', master_port=3312;
START SLAVE;

The slave has a record of the GTID of the last applied transaction from the old master, and since GTIDs are identical across all servers in a replication hierarchy, the slave will just continue from the appropriate point in the new master's binlog.

It is important to understand how this change of masters work. The binlog is an ordered stream of events (or multiple streams, one per replication domain, (see Use with multi-source replication and other multi-master setups). Events within the stream are always applied in the same order on every slave that replicates it. The MariaDB GTID relies on this ordering, so that it is sufficient to remember just a single point within the stream. Since event order is the same on every server, switching to the point of the same GTID in the binlog of another server will give the same result.

This translates into some responsibility for the user. The MariaDB GTID replication is fully asynchronous, and fully flexible in how it can be configured. This makes it possible to use it in ways where the assumption that binlog sequence is the same on all servers is violated. In such cases, when changing master, GTID will still attempt to continue at the point of current GTID in the new binlog.

The most common way that binlog sequence gets different between servers is when the user/DBA does updates directly on a slave server (and these updates are written into the slaves binlog). This results in events in the slaves binlog that are not present on the master or any other slaves. This can be avoided by setting the session variable sql_log_bin false while doing such updates, so they do not go into the binlog.

It is normally best to avoid any differences in binlogs between servers. That being said, MariaDB replication is designed for maximum flexibility, and there can be valid reasons for introducing such differences from time to time. It this case, it just needs to be understood that the GTID position is a single point in each binlog stream (one per replication domain), and how this affects the users particular setup.

Differences can also occur when two masters are active at the same time in a replication hierarchy. This happens when using a multi-master ring. But it can also occur in a simple master-slave setup, during switch to a new master, if changes on the old master is not allowed to fully replicate to all slave servers before switching master. Normally, to switch master, first writes to the old master should be stopped, then one should wait for all changes to be replicated to the new master, and only then should writes begin on the new master. Deliberately using multiple active masters is also supported, this is described in the next section.

The GTID strict mode can be used to enforce identical binlogs across servers. When it is enabled, most actions that would cause differences are rejected with an error.

Use with multi-source replication and other multi-master setups

MariaDB global transaction ID supports having multiple masters active at the same time. Typically this happens with either multi-source replication or multi-master ring setups.

In such setups, each active master must be configured with its own distinct replication domain ID, gtid_domain_id. The binlog will then in effect consists of multiple independent streams, one per active master. Within one replication domain, binlog order is always the same on every server. But two different streams can be interleaved differently in different server binlogs.

The GTID position of a given slave is then not a single GTID. Rather, it becomes the GTID of the last event group applied for each value of domain ID, in effect the position reached in each binlog stream. When the slave connects to a master, it can continue from one stream in a different binlog position than another stream. Since order within one stream is consistent across all servers, this is sufficient to always be able to continue replication at the correct point in any new master server(s).

Domain IDs are assigned by the DBA, according to the need of the application. The default value of @@GLOBAL.gtid_domain_id is 0. This is appropriate for most replication setups, where only a single master is active at a time. The MariaDB server will never by itself introduce new domain_id values into the binlog.

When using multi-source replication, where a single slave connects to multiple masters at the same time, each such master should be configured with its own distinct domain ID.

Similarly, in a multi-master ring topology, where all master in the ring are updated by the application concurrently (with some mechanism to avoid conflicts), a distinct domain ID should be configured for each server (In a multi-master ring where the application is careful to only do updates on one master at a time, a single domain ID is sufficient).

Normally, a slave server should not receive direct updates (as this creates binlog differences compared to the master). Thus it does not matter what value of gtid_domain_id is set on a slave, though it may make sense to make it the same as the master (if not using multi-master) to make it easy to promote the slave as a new master. Of course, if a slave is itself an active master, as in a multi-master ring topology, the domain ID should be set according to the server's role as active master.

Note that domain ID and server ID are distinct concepts. It is possible to use a different domain ID on each server, but this is normally not desirable. It makes the current GTID position (@@global.gtid_slave_pos) more complicated to understand and work with, and loses the concept of a single ordered binlog stream across all servers. It is recommended only to configure as many domain IDs as there are master servers actively being updated by the application at the same time.

It is not an error in itself to configure domain IDs incorrectly (for example, not configuring them at all). For example, this will be typical in an upgrade scenario where a multi-master ring using 5.5 is upgraded to 10.0. The ring will continue to work as before even though everything is configured to use the default domain ID 0. It is even possible to use GTID for replication between the servers. However, care must be taken when switching a slave to a different master. If the binlog order between the old and the new master differs, then a single GTID position to start replication from in the new master's binlog may not be sufficient.

New syntax for global transaction ID

CHANGE MASTER

CHANGE MASTER has a new option, master_use_gtid=[current_pos|slave_pos|no]. When enabled (set to current_pos or slave_pos), the slave will connect to the master using the GTID position. When disabled (set to "no"), the old-style binlog filename/offset position is used to decide where to start replicating when connecting.

The value of master_use_gtid is saved across server restarts (in master.info). The current value can be seen as the field Using_Gtid in the output of SHOW SLAVE STATUS.

For a detailed look at the difference between the current_pos and slave_pos options, see Using global transaction ID's

START SLAVE UNTIL master_gtid_pos=xxx

When starting replication with START SLAVE, it is possible to request the slave to run only until a specific GTID position is reached. Once that position is reached, the slave will stop.

The syntax for this is:

START SLAVE UNTIL master_gtid_pos = <GTID position>

The slave will start replication from the current GTID position, run up to and including the event with the GTID specified, and then stop. Note that this stops both the IO thread and the SQL thread (unlike START SLAVE UNTIL MASTER_LOG_FILE/MASTER_LOG_POS, which stops only the SQL thread).

If multiple GTIDs are specified, then they must be with distinct replication domain ID, for example:

START SLAVE UNTIL master_gtid_pos = "1-11-100,2-21-50"

With multiple domains in the UNTIL condition, each domain runs only up to and including the specified position, so it is possible for different domains to stop at different places in the binlog (each domain will resume from the stopped position when the slave is started the next time).

Not specifying a replication domain at all in the UNTIL condition means that the domain is stopped immediately, nothing is replicated from that domain. In particular, specifying the empty string will stop the slave immediately.

When using START SLAVE UNTIL master_gtid_pos = XXX, if the UNTIL position is present in the master's binlog then it is permissible for the start position to be missing on the master. In this case, replication for the associated domains stop immediately.

Both slave threads must be already stopped when using UNTIL master_gtid_pos, otherwise an error occurs. It is also an error if the slave is not configured to use GTID (CHANGE MASTER TO master_use_gtid=current_pos|slave_pos). And both threads must be started at the same time, the IO_THREAD or SQL_THREAD options can not be used to start only one of them.

START SLAVE UNTIL master_gtid_pos=XXX is particularly useful for promoting a new master among a set of slaves when the old master goes away and slaves may have reached different positions in the old master's binlog. The new master needs to be ahead of all the other slaves to avoid losing events. This can be achieved by picking one server, say S1, and replicating any missing events from each other server S2, S3, ..., Sn:

    CHANGE MASTER TO master_host="S2";
    START SLAVE UNTIL master_gtid_pos = "<S2 GTID position>";
    ...
    CHANGE MASTER TO master_host="Sn";
    START SLAVE UNTIL master_gtid_pos = "<Sn GTID position>";

Once this is completed, S1 will have all events present on any of the servers. It can now be selected as the new master, and all the other servers set to replicate from it.

BINLOG_GTID_POS().

The BINLOG_GTID_POS() function takes as input an old-style binary log position in the form of a file name and a file offset. It looks up the position in the current binlog, and returns a string representation of the corresponding GTID position. If the position is not found in the current binlog, NULL is returned.

MASTER_GTID_WAIT

The MASTER_GTID_WAIT function was introduced in MariaDB 10.0.9. It is useful in replication for controlling master/slave synchronization, and blocks until the slave has read and applied all updates up to the specified position in the master log. See MASTER_GTID_WAIT for details.

System variables for global transaction ID

gtid_slave_pos

This variable is the GTID of the last event group replicated on a slave server, for each replication domain.

It can be set by the user to change the current replication position. This requires all slave threads to be stopped first. Note that the position is shared among all slave connections when using multi-source replication. To set position for two masters, one using replication domain 1 and another replication domain 2, set a GTID for both domains, for example:

SET GLOBAL gtid_slave_pos = "1-10-100,2-20-500";

The variable value is updated whenever an event group is replicated on a slave. The value of the variable is used as the starting point for replication when a slave is configured with CHANGE MASTER TO master_use_gtid=slave_pos.

If this variable is set to a position that is not more recent than the last GTID in the binary log on the server, a warning will be given. This helps protect the user from setting one value for @@gtid_slave_pos, then being surprised when a slave configured with CHANGE MASTER TO master_use_gtid=current_pos starts replicating at the more recent position found in the binary log. It also helps protect against the case where a server is rolled back to restart replication from an earlier point in time, but the user forgets to also use RESET MASTER to roll back the binary log, thus getting duplicate events in the binlog.

The mysql.gtid_slave_pos system table is used to store the contents of global.gtid_slave_pos and preserve it over restarts.

• Scope: Global
• Dynamic: Yes
• Data Type: string
• Introduced: MariaDB 10.0.3

gtid_binlog_pos

This variable is the GTID of the last event group written to the binary log, for each replication domain.

Note that when the binlog is empty (such as on a fresh install or after RESET MASTER), there are no event groups written in any replication domain, so in this case the value of @@gtid_binlog_pos will be the empty string.

The value is read-only, but it is updated whenever a DML or DDL statement is written to the binary log.

• Scope: Global
• Dynamic: Read-only
• Data Type: string
• Introduced: MariaDB 10.0.3

gtid_binlog_state

The variable gtid_binlog_state holds the internal state of the binlog. The state consists of the last GTID ever logged to the binary log for every combination of domain_id and server_id. This information is used by the master to determine whether a given GTID has been logged to the binlog in the past, even if it has later been deleted due to binlog purge. For each domain_id, the last entry in @@gtid_binlog_state is the last GTID logged into binlog, ie. this is the value that appears in @@gtid_binlog_pos.

Normally this internal state is not needed by users, as @@gtid_binlog_pos is more useful in most cases. The main usage of @@gtid_binlog_state is to restore the state of the binlog after RESET MASTER (or equivalently if the binlog files are lost). If the value of @@gtid_binlog_state is saved before RESET MASTER and restored afterwards, the master will retain information about past history, same as if PURGE BINARY LOGS had been used (of course the actual events in the binary logs are still deleted).

Note that to set the value of @@gtid_binlog_state, the binary log must be empty, that is it must not contain any GTID events and the previous value of @@gtid_binlog_state must be the empty string. If not, then RESET MASTER must be used first to erase the binary log first.

The value of @@gtid_binlog_state is preserved by the server across restarts by writing a file MASTER-BIN.state, where MASTER-BIN is the base name of the binlog set with the --log-bin option. This file is written at server shutdown, and re-read at next server start. (In case of a server crash, the data in the MASTER-BIN.state is not correct, and the server instead recovers the correct value during binlog crash recovery by scanning the binlog files and recording each GTID found).

For completeness, note that setting @@gtid_binlog_state internally executes a RESET MASTER. This is normally not noticeable as it can only be changed when the binlog is empty of GTID events. However, if executed e.g. immediately after upgrading to MariaDB 10, it is possible that the binlog is non-empty but without any GTID events, in which case all such events will be deleted, just as if RESET MASTER had been run.

• Scope: Global
• Dynamic: Yes
• Data Type: string
• Introduced: MariaDB 10.0.5

gtid_current_pos

This variable is the GTID of the last change to the database for each replication domain. Such changes can either be master events (ie. local changes made by user or application), or replicated events originating from another master server.

For each replication domain, if the server ID of the corresponding GTID in @@gtid_binlog_pos is equal to the servers own server_id, and the sequence number is higher than the corresponding GTID in @@gtid_slave_pos, then the GTID from @@gtid_binlog_pos will be used. Otherwise the GTID from @@gtid_slave_pos will be used for that domain.

Thus, @@gtid_current_pos contains the most recent GTID executed on the server, whether this was done as a master or as a slave. This value is used as the starting point of replication when a slave is configured with CHANGE MASTER TO master_use_gtid=current_pos.

The value is read-only, but it is updated whenever a DML or DDL statement is written to the binary log and/or replicated by a slave thread..

• Scope: Global
• Dynamic: Read-only
• Data Type: string
• Introduced: MariaDB 10.0.3

gtid_strict_mode

The GTID strict mode is an optional setting that can be used to help the DBA enforce a strict discipline about keeping binlogs identical across multiple servers replicating using global transaction ID.

When GTID strict mode is enabled, some additional errors are enabled for situations that could otherwise cause differences between binlogs on different servers in a replication hierarchy:

1. If a slave server tries to replicate a GTID with a sequence number lower than what is already in the binlog for that replication domain, the SQL thread stops with an error (this indicates an extra transaction in the slave binlog not present on the master).
2. Similarly, an attempt to manually binlog a GTID with a lower sequence number (by setting @@SESSION.gtid_seq_no) is rejected with an error.
3. If the slave tries to connect starting at a GTID that is missing in the master's binlog, this is an error in GTID strict mode even if a GTID exists with a higher sequence number (this indicates a GTID on the slave missing on the master). Note that this error is controlled by the setting of GTID strict mode on the connecting slave server.

GTID mode is off by default; this is needed to preserve backwards compatibility with existing replication setups (older versions of the server did not enforce any strict mode for binlog order). Global transaction ID is designed to work correctly even when strict mode is not enabled. However, with strict mode enforced, the semantics is simpler and thus easier to understand, because binlog order is always identical across servers and sequence numbers are always strictly increasing within each replication domain. This can also make automated scripting of large replication setups easier to implement correctly.

When GTID strict mode is enabled, the slave will stop with an error when a problem is encountered. This allows the DBA to become aware of the problem and take corrective actions to avoid similar issues in the future. One way to recover from such an error is to temporarily disable GTID strict mode on the offending slave, to be able to replicate past the problem point (perhaps using START SLAVE UNTIL master_gtid_pos=XXX).

• Scope: Global
• Dynamic: Yes
• Data Type: boolean
• Introduced: MariaDB 10.0.3

gtid_domain_id

• Description: This variable is used to decide which replication domain new GTIDs are logged in for a master server. See Use with multi-source replication and other multi-master setups for details. This variable can also be set on the session level by a user with the SUPER privilege. This is used by mysqlbinlog to preserve the domain ID of GTID events.
• Scope: Global, Session
• Dynamic: Yes
• Data Type: numeric (32-bit unsigned integer)
• Introduced: MariaDB 10.0.2

last_gtid

• Description: Holds the GTID that was assigned to the last transaction, or statement that was logged to the binary log. If the binary log is disabled, or if no transaction or statement was executed in the session yet, then the value is an empty string.
• Scope: Session
• Dynamic: Read-only
• Data Type: string
• Introduced: MariaDB 10.0.9

server_id

• Description: Server_id can be set on the session level to change which server_id value is logged in binlog events (both GTID and other events). This is used by mysqlbinlog to preserve the server ID of GTID events.
• Scope: Global, Session
• Dynamic: Yes
• Data Type: numeric (32-bit unsigned integer)

gtid_seq_no

• Description: gtid_seq_no can be set on the session level to change which sequence number is logged in the following GTID event. This is used by mysqlbinlog to preserve the sequence number of GTID events.
• Scope: Session
• Dynamic: Yes
• Data Type: numeric (64-bit unsigned integer)
• Introduced: MariaDB 10.0.2

gtid_ignore_duplicates

• Description: When set, different master connections in multi-source replication are allowed to receive and process event groups with the same GTID (when using GTID mode). Only one will be applied, any others will be ignored. Within a given replication domain, just the sequence number will be used to decide whether a given GTID has been already applied; this means it is the responsibility of the user to ensure that GTID sequence numbers are strictly increasing. With gtid_ignore_duplicates=OFF, a duplicate event based on domain id and sequence number, will be executed.
• Commandline: --gtid-ignore-duplicates=#
• Scope: Global
• Dynamic: Yes
• Data Type: boolean
• Default: OFF
• Introduced: MariaDB 10.0.10

GTID and MariaDB Galera Cluster

From MariaDB 10.1.4, wsrep GTID replication and binary logs GTID can stay in sync on the cluster node using the following wsrep variables, which enable asynchronous replication to failover on any of remaining Galera nodes.

• wsrep_gtid_domain_id
• wsrep_gtid_mode