In the event that the primary Cluster replication process fails, it is possible to switch over to the secondary replication channel. The following procedure describes the steps required to accomplish this.
Obtain the time of the most recent global checkpoint (GCP).
That is, you need to determine the most recent epoch from the
ndb_apply_status table on the slave
cluster, which can be found using the following query:
In MySQL Cluster NDB 7.3, you can use the following query to
obtain the needed records from the master's
@file:=SUBSTRING_INDEX(next_file, '/', -1),->
WHERE epoch = @latest->
ORDER BY epoch ASC LIMIT 1;
These are the records saved on the master since the failure of
the primary replication channel. We have employed a user
@latest here to represent the
value obtained in Step 1. Of course, it is not possible for
one mysqld instance to access user
variables set on another server instance directly. These
values must be “plugged in” to the second query
manually or in application code.
Now it is possible to synchronize the secondary channel by running the following query on the secondary slave server:
CHANGE MASTER TO->
Again we have employed user variables (in this case
represent the values obtained in Step 2 and applied in Step 3;
in practice these values must be inserted manually or using
application code that can access both of the servers involved.
@file is a string value such as
and so must be quoted when used in SQL or application code.
However, the value represented by
must not be quoted. Although MySQL
normally attempts to convert strings to numbers, this case
is an exception.
You can now initiate replication on the secondary channel by issuing the appropriate command on the secondary slave mysqld:
Once the secondary replication channel is active, you can investigate the failure of the primary and effect repairs. The precise actions required to do this will depend upon the reasons for which the primary channel failed.
The secondary replication channel is to be started only if and when the primary replication channel has failed. Running multiple replication channels simultaneously can result in unwanted duplicate records being created on the replication slaves.
If the failure is limited to a single server, it should (in
theory) be possible to replicate from
S', or from
however, this has not yet been tested.