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22.6.10 NDB Cluster Replication: Multi-Master and Circular Replication
It is possible to use NDB Cluster in multi-master replication, including circular replication between a number of NDB 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 master for Cluster 2, Cluster 2 acts as the master for Cluster 3, and Cluster 3 acts as the master 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 masters and slaves are the same
All SQL nodes acting as replication masters and slaves are started using the
--log-slave-updates
option
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 masters and slaves.
It is also possible to set up circular replication in such a way that not all master SQL nodes are also slaves, as shown here:
In this case, different SQL nodes in each cluster are used as
replication masters and slaves. However, you must
not start any of the SQL nodes using
--log-slave-updates
. 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 slave NDB Cluster.
When setting up circular replication, it is possible to
initialize the slave cluster by using the management client
BACKUP
command on one NDB Cluster to create a
backup and then applying this backup on another NDB Cluster
using ndb_restore. However, this does not
automatically create binary logs on the second NDB Cluster
's SQL node acting as the replication slave. 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 SLAVE
.
This is a known issue which we intend to address in a future release.
Multi-master failover example. In this section, we discuss failover in a multi-master 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-master replication must act as both master and slave, and a given NDB Cluster might use different SQL nodes for different replication channels. Such a case is shown here:
MySQL servers acting as replication slaves must be run with the
--log-slave-updates
option. Which
mysqld processes require this option is also
shown in the preceding diagram.
Using the --log-slave-updates
option has no effect on servers not being run as replication
slaves.
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:
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 22.6.4, “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 MASTER
TO
statement:
- -> MASTER_HOST = 'serverC'
- -> MASTER_LOG_FILE='@file',
- -> MASTER_LOG_POS=@pos;
Once this has been done, you can issue a
START SLAVE
statement on MySQL
server F, and any missing updates originating from server B will
be replicated to server F.
The CHANGE MASTER 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. For more
information, see Section 13.4.2.1, “CHANGE MASTER TO Syntax”, and
Section 13.7.6.34, “SHOW SLAVE STATUS Syntax”. For information about how
this option intereacts with the
ndb_log_apply_status
variable,
see Section 22.6.8, “Implementing Failover with NDB Cluster Replication”.
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