MySQL 8.4 Release Notes
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:
The SQL nodes on all sources and replicas are the same.
All SQL nodes acting as sources and replicas are started with the system variable
log_replica_updatesenabled.
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
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:
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
variable log_replica_updates
enabled. Which mysqld processes require this
option is also shown in the preceding diagram.
Note
Using the log_replica_updates
system variable has no effect on servers not being run as
replicas.
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;
Note
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 25.7.4, “NDB Cluster Replication Schema and Tables”, for more
information.
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 15.7.7.35, “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”.