In a Galera cluster, a node can log 2 kinds of transactions.
- Galera transactions : Ones which are replicated to other nodes within the cluster post-SST.
- Local transactions : Local transactions can be transactions involving non-innodb (except myisam when --wsrep-replicate-myisam is enabled) DMLs which are not relayed to other nodes.
Now, in a scenario where galera-transactions from one cluster are being
asynchronously replicated to an another cluster using the traditional
MariaDB replication, having following 2 additional features would come in
handy during setup and maintenance of such replication topologies.
- Separate domain_id for Galera-transaction :
An external 'listener' of an n-node galera cluster can essentially
see n (=local transactions) + 1 (=galera transactions) logical streams
of transactions. So, having all galera transactions logged using a separate
domain_id would make it easier to identify, filter (see #2) and replicate
those transactions to an external replication slave or a different cluster
This task introduces a new GTID domain ID (wsrep_gtid_domain_id) to be
used to tag all galera transactions. The local transactions, however,
will be tagged by gtid_domain_id as usual.
Now, since the Galera transactions can originate on any of the nodes,
it is important that all the cluster nodes have same GTID domain ID
(wsrep_gtid_domain_id). So, in order to enforce this, SST (Snapshot
State Transfer) scripts have been modified to automatically transfer
the value of donor's wsrep_gtid_domain_id to the joiner node during
SST process. This will make sure that all the nodes in the cluster
have same wsrep_gtid_domain_id and thus all the galera transactions
will essentially have same domain ID.
A new server system variable wsrep_gtid_mode has been introduced to
control this new behavior an this remain backward compatible. When OFF
(default), wsrep_gtid_domain_id is ignored and all the transactions will
be tagged based on gtid_domain_id. On the other hand, when wsrep_gtid_mode
is ON, wsrep_gtid_domain_id takes effect and will be used to mark all
New Server System Variables:
- Description: When wsrep_gtid_mode is set, this value is used as gtid_domain_id for galera transactions and also copied to the joiner nodes during state transfer. It is ignored, otherwise.
- Dynamic: Yes
- Data Type: Numeric (32-bit unsigned integer)
- Default : 0
- Description: Automatically update the (joiner) node's wsrep_gtid_domain_id value with that of donor's (received during state transfer) and use it in place of gtid_domain_id for all galera transactions. When OFF (default), wsrep_gtid_domain_id is simply ignored (backward compatibility).
- Scope: Global
- Dynamic: Yes
- Data Type: Boolean
- Default value : False
Changes in SST scripts:
- wsrep_sst_rsync :
The script has been modified to also transfer donor's wsrep_gtid_domain_id alongwith the state ID.
- wsrep_sst_xtrabackup (-v2) :
Similar to the rsync SST script, the xtrabackup scripts have been modified to also transfer donor's
wsrep_gtid_domain_id alongwith the state ID.
- wsrep_sst_mysqldump :
The script has been modified to set joiner's wsrep_gtid_domain_id with that of donor's only when Joiner's wsrep_gtid_mode=ON.
- Domain_id based replication filters (
Enable a replication slave to filter and apply transactions based on
transaction's domain_id. As described above, this would enable the
asynchronous replication of galera transaction outside the galera
A use case described here: