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MySQL 8.0 Reference Manual  /  ...  /  GTIDs and Group Replication

18.4.1Ā GTIDs and Group Replication

Group Replication uses GTIDs (global transaction identifiers) to track exactly which transactions have been committed on every server instance. The settings gtid_mode=ON and enforce_gtid_consistency=ON are required on all group members. Incoming transactions from clients are assigned a GTID by the group member that receives them. Any replicated transactions that are received by group members on asynchronous replication channels from source servers outside the group retain the GTIDs that they have when they arrive on the group member.

The GTIDs that are assigned to incoming transactions from clients use the group name specified by the group_replication_group_name system variable as the UUID part of the identifier, rather than the server UUID of the individual group member that received the transaction. All the transactions received directly by the group can therefore be identified and are grouped together in GTID sets, and it does not matter which member originally received them. Each group member has a block of consecutive GTIDs reserved for its use, and when these are consumed it reserves more. The group_replication_gtid_assignment_block_size system variable sets the size of the blocks, with a default of 1 million GTIDs in each block.

View change events (View_change_log_event), which are generated by the group itself when a new member joins, are given GTIDs when they are recorded in the binary log. By default, the GTIDs for these events also use the group name specified by the group_replication_group_name system variable as the UUID part of the identifier. From MySQL 8.0.26, you can set the Group Replication system variable group_replication_view_change_uuid to use an alternative UUID in the GTIDs for view change events, so that they are easy to distinguish from transactions received by the group from clients. This can be useful if your setup allows for failover between groups, and you need to identify and discard transactions that were specific to the backup group. The alternative UUID must be different from the server UUIDs of the members. It must also be different from any UUIDs in the GTIDs applied to anonymous transactions using the ASSIGN_GTIDS_TO_ANONYMOUS_TRANSACTIONS option of the CHANGE REPLICATION SOURCE TO statement.