It is possible to use NDB Cluster for bidirectional replication between two clusters, as well as for circular replication between any number of clusters.
Circular replication example. In the next few paragraphs we consider the example of a replication setup involving three NDB Clusters numbered 1, 2, and 3, in which Cluster 1 acts as the replication source for Cluster 2, Cluster 2 acts as the source for Cluster 3, and Cluster 3 acts as the source for Cluster 1. Each cluster has two SQL nodes, with SQL nodes A and B belonging to Cluster 1, SQL nodes C and D belonging to Cluster 2, and SQL nodes E and F belonging to Cluster 3.
Circular replication using these clusters is supported as long as the following conditions are met:
This type of circular replication setup is shown in the following diagram:
In this scenario, SQL node A in Cluster 1 replicates to SQL node C in Cluster 2; SQL node C replicates to SQL node E in Cluster 3; SQL node E replicates to SQL node A. In other words, the replication line (indicated by the curved arrows in the diagram) directly connects all SQL nodes used as replication sources and replicas.
It is also possible to set up circular replication in such a way that not all source SQL nodes are also replicas, as shown here:
In this case, different SQL nodes in each cluster are used as
replication sources and replicas. You must
not start any of the SQL nodes with the
log_replica_updates (from MySQL
(before MySQL 8.0.26) enabled. This type of circular replication
scheme for NDB Cluster, in which the line of replication (again
indicated by the curved arrows in the diagram) is discontinuous,
should be possible, but it should be noted that it has not yet
been thoroughly tested and must therefore still be considered
Using NDB-native backup and restore to initialize a replica cluster.
When setting up circular replication, it is possible to
initialize the replica cluster by using the management client
START BACKUP command on one
NDB Cluster to create a backup and then applying this backup on
another NDB Cluster using ndb_restore. This
does not automatically create binary logs on the second NDB
Cluster's SQL node acting as the replica; in order to cause
the binary logs to be created, you must issue a
SHOW TABLES statement on that SQL
node; this should be done prior to running
REPLICA. This is a known issue.
Multi-source failover example. In this section, we discuss failover in a multi-source NDB Cluster replication setup with three NDB Clusters having server IDs 1, 2, and 3. In this scenario, Cluster 1 replicates to Clusters 2 and 3; Cluster 2 also replicates to Cluster 3. This relationship is shown here:
In other words, data replicates from Cluster 1 to Cluster 3 through 2 different routes: directly, and by way of Cluster 2.
Not all MySQL servers taking part in multi-source replication must act as both source and replica, and a given NDB Cluster might use different SQL nodes for different replication channels. Such a case is shown here:
MySQL servers acting as replicas must be run with the system
(from MySQL 8.0.26) or
log_slave_updates (before MySQL
8.0.26) enabled. Which mysqld processes require
this option is also shown in the preceding diagram.
The need for failover arises when one of the replicating clusters goes down. In this example, we consider the case where Cluster 1 is lost to service, and so Cluster 3 loses 2 sources of updates from Cluster 1. Because replication between NDB Clusters is asynchronous, there is no guarantee that Cluster 3's updates originating directly from Cluster 1 are more recent than those received through Cluster 2. You can handle this by ensuring that Cluster 3 catches up to Cluster 2 with regard to updates from Cluster 1. In terms of MySQL servers, this means that you need to replicate any outstanding updates from MySQL server C to server F.
On server C, perform the following queries:
mysqlC> SELECT @latest:=MAX(epoch) -> FROM mysql.ndb_apply_status -> WHERE server_id=1; mysqlC> SELECT -> @file:=SUBSTRING_INDEX(File, '/', -1), -> @pos:=Position -> FROM mysql.ndb_binlog_index -> WHERE orig_epoch >= @latest -> AND orig_server_id = 1 -> ORDER BY epoch ASC LIMIT 1;
You can improve the performance of this query, and thus likely
speed up failover times significantly, by adding the appropriate
index to the
ndb_binlog_index table. See
Section 7.4, “NDB Cluster Replication Schema and Tables”, for more
Copy over the values for
@pos manually from server C to server F
(or have your application perform the equivalent). Then, on server
F, execute the following
SOURCE TO statement (MySQL 8.0.23 and later) or
CHANGE MASTER TO statement (prior
to MySQL 8.0.23):
mysqlF> CHANGE MASTER TO -> MASTER_HOST = 'serverC' -> MASTER_LOG_FILE='@file', -> MASTER_LOG_POS=@pos;
Beginning with MySQL 8.0.23, you can also use the following statement:
mysqlF> CHANGE REPLICATION SOURCE TO -> SOURCE_HOST = 'serverC' -> SOURCE_LOG_FILE='@file', -> SOURCE_LOG_POS=@pos;
Once this has been done, you can issue a
REPLICA statement on MySQL server F; this causes any
missing updates originating from server B to be replicated to
CHANGE REPLICATION SOURCE TO |
CHANGE MASTER TO statement also
IGNORE_SERVER_IDS option which
takes a comma-separated list of server IDs and causes events
originating from the corresponding servers to be ignored. For more
information, see CHANGE MASTER TO Statement, and
SHOW SLAVE | REPLICA STATUS Statement. For information about how
this option intereacts with the
see Section 7.8, “Implementing Failover with NDB Cluster Replication”.