What is gtid

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 The replica remembers the global transaction ID of the last event group applied from the old primary.

This makes it easy to know where to resume replication on the new primary, 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 replica knows only the specific file name and offset of the old primary server of the last event applied. There is no simple way to guess from this the correct file name and offset on a new primary. The replica keeps track of its current position the global transaction ID of the last transaction applied in the mysql.

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 replica 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. Because of these two benefits, it is generally recommended to use global transaction ID for any replication setups based on MariaDB 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. For example:. 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. Using a bit number provides ample range that there should be no risk of it overflowing in the foreseeable future. When events are replicated from a primary server to a replica server, the events are always logged into the replica's binlog in the same order that they were read from the primary's binlog.

Thus, if there is only ever a single primary 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 replica to keep track of its current position in the replication. Basically, the replica remembers the GTID of the last event group replicated from the primary. When reconnecting to a primary, whether the same one or a new one, it sends this GTID position to the primary, and the primary 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-primary ring topologies, or just if manual updates are done on a replica that is replicating from active primary. If the binlog order is different on the new primary from the order on the old primary, then it is not sufficient for the replica to keep track of a single GTID to completely record the current state.

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 replica server then keeps track of its replication position by recording the last GTID applied within each replication stream.

When connecting to a new primary, the replica can start replication from a different point in the binlog for each domain ID. For more details on using multi-primary setups and multiple domain IDs, see Use with multi-source replication and other multi-primary setups.

Simple replication setups only have a single primary 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.

Global transaction ID is enabled automatically. Since GTIDs are the same across all replication servers, the replica can then be pointed to a different primary, and the correct position will be determined automatically. But suppose that we set up two servers A and B and let A be the primary and B the replica.

It runs for a while. Then at some point we take down A, and B becomes the new primary. Then later we want to add A back, this time as a replica. But care must be taken not to inject extra transactions into the binlog on the replica server that are not intended to be replicated to other servers.

If such an extra transaction is the most recent when the replica starts, it will be used as the starting point of replication. This will probably fail because that transaction is not present on the primary. Create a new account. Log In. Know what is GTID? Got another good explanation for GTID?

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