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:

Figure 25.14 NDB Cluster Circular Replication with All Sources As Replicas

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:

Figure 25.15 NDB Cluster Circular Replication Where Not All Sources Are Replicas

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 system variable log_replica_updates 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 experimental.

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 START 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:

Figure 25.16 NDB Cluster Multi-Source Replication With 3 Sources

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:

Figure 25.17 NDB Cluster Multi-Source Replication, With MySQL Servers

MySQL servers acting as replicas must be run with the system variable log_replica_updates 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;

Copy over the values for @file and @pos manually from server C to server F (or have your application perform the equivalent). Then, on server F, execute the following CHANGE REPLICATION SOURCE TO 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 START REPLICA statement on MySQL server F; this causes any missing updates originating from server B to be replicated to server F.

The CHANGE REPLICATION SOURCE TO statement also supports an IGNORE_SERVER_IDS option which takes a comma-separated list of server IDs and causes events originating from the corresponding servers to be ignored. See the documentation for this statement for more information, as well as Section 15.7.7.36, “SHOW REPLICA STATUS Statement”. For information about how this option interacts with the ndb_log_apply_status variable, see Section 25.7.8, “Implementing Failover with NDB Cluster Replication”.