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MySQL Replication  /  ...  /  Provisioning a Multi-Source Replica for GTID-Based Replication

2.5.2 Provisioning a Multi-Source Replica for GTID-Based Replication

If the sources in the multi-source replication topology have existing data, it can save time to provision the replica with the relevant data before starting replication. In a multi-source replication topology, cloning or copying of the data directory cannot be used to provision the replica with data from all of the sources, and you might also want to replicate only specific databases from each source. The best strategy for provisioning such a replica is therefore to use mysqldump to create an appropriate dump file on each source, then use the mysql client to import the dump file on the replica.

If you are using GTID-based replication, you need to pay attention to the SET @@GLOBAL.gtid_purged statement that mysqldump places in the dump output. This statement transfers the GTIDs for the transactions executed on the source to the replica, and the replica requires this information. However, for any case more complex than provisioning one new, empty replica from one source, you need to check what effect the statement has in the version of MySQL used by the replica, and handle the statement accordingly. The following guidance summarizes suitable actions, but for more details, see the mysqldump documentation.

The behavior of the SET @@GLOBAL.gtid_purged statement written by mysqldump is different in releases from MySQL 8.0 compared to MySQL 5.6 and 5.7. In MySQL 5.6 and 5.7, the statement replaces the value of gtid_purged on the replica, and also in those releases that value can only be changed when the replica's record of transactions with GTIDs (the gtid_executed set) is empty. In a multi-source replication topology, you must therefore remove the SET @@GLOBAL.gtid_purged statement from the dump output before replaying the dump files, because you cannot apply a second or subsequent dump file including this statement. Also note that for MySQL 5.6 and 5.7, this limitation means all the dump files from the sources must be applied in a single operation on a replica with an empty gtid_executed set. You can clear a replica's GTID execution history by issuing RESET MASTER on the replica, but if you have other, wanted transactions with GTIDs on the replica, choose an alternative method of provisioning from those described in Section 2.3.5, “Using GTIDs for Failover and Scaleout”.

From MySQL 8.0, the SET @@GLOBAL.gtid_purged statement adds the GTID set from the dump file to the existing gtid_purged set on the replica. The statement can therefore potentially be left in the dump output when you replay the dump files on the replica, and the dump files can be replayed at different times. However, it is important to note that the value that is included by mysqldump for the SET @@GLOBAL.gtid_purged statement includes the GTIDs of all transactions in the gtid_executed set on the source, even those that changed suppressed parts of the database, or other databases on the server that were not included in a partial dump. If you replay a second or subsequent dump file on the replica that contains any of the same GTIDs (for example, another partial dump from the same source, or a dump from another source that has overlapping transactions), any SET @@GLOBAL.gtid_purged statement in the second dump file fails, and must therefore be removed from the dump output.

For sources from MySQL 8.0.17, as an alternative to removing the SET @@GLOBAL.gtid_purged statement, you may set mysqldump's --set-gtid-purged option to COMMENTED to include the statement but commented out, so that it is not actioned when you load the dump file. If you are provisioning the replica with two partial dumps from the same source, and the GTID set in the second dump is the same as the first (so no new transactions have been executed on the source in between the dumps), you can set mysqldump's --set-gtid-purged option to OFF when you output the second dump file, to omit the statement.

In the following provisioning example, we assume that the SET @@GLOBAL.gtid_purged statement cannot be left in the dump output, and must be removed from the files and handled manually. We also assume that there are no wanted transactions with GTIDs on the replica before provisioning starts.

  1. To create dump files for a database named db1 on source1 and a database named db2 on source2, run mysqldump for source1 as follows:

    mysqldump -u<user> -p<password> --single-transaction --triggers --routines --set-gtid-purged=ON --databases db1 > dumpM1.sql

    Then run mysqldump for source2 as follows:

    mysqldump -u<user> -p<password> --single-transaction --triggers --routines --set-gtid-purged=ON --databases db2 > dumpM2.sql
  2. Record the gtid_purged value that mysqldump added to each of the dump files. For example, for dump files created on MySQL 5.6 or 5.7, you can extract the value like this:

    cat dumpM1.sql | grep GTID_PURGED | cut -f2 -d'=' | cut -f2 -d$'\''
    cat dumpM2.sql | grep GTID_PURGED | cut -f2 -d'=' | cut -f2 -d$'\''

    From MySQL 8.0, where the format has changed, you can extract the value like this:

    cat dumpM1.sql | grep GTID_PURGED | perl -p0 -e 's#/\*.*?\*/##sg' | cut -f2 -d'=' | cut -f2 -d$'\''
    cat dumpM2.sql | grep GTID_PURGED | perl -p0 -e 's#/\*.*?\*/##sg' | cut -f2 -d'=' | cut -f2 -d$'\''

    The result in each case should be a GTID set, for example:

    source1:   2174B383-5441-11E8-B90A-C80AA9429562:1-1029
    source2:   224DA167-0C0C-11E8-8442-00059A3C7B00:1-2695
  3. Remove the line from each dump file that contains the SET @@GLOBAL.gtid_purged statement. For example:

    sed '/GTID_PURGED/d' dumpM1.sql > dumpM1_nopurge.sql
    sed '/GTID_PURGED/d' dumpM2.sql > dumpM2_nopurge.sql
  4. Use the mysql client to import each edited dump file into the replica. For example:

    mysql -u<user> -p<password> < dumpM1_nopurge.sql
    mysql -u<user> -p<password> < dumpM2_nopurge.sql
  5. On the replica, issue RESET MASTER to clear the GTID execution history (assuming, as explained above, that all the dump files have been imported and that there are no wanted transactions with GTIDs on the replica). Then issue a SET @@GLOBAL.gtid_purged statement to set the gtid_purged value to the union of all the GTID sets from all the dump files, as you recorded in Step 2. For example:

    mysql> RESET MASTER;
    mysql> SET @@GLOBAL.gtid_purged = "2174B383-5441-11E8-B90A-C80AA9429562:1-1029, 224DA167-0C0C-11E8-8442-00059A3C7B00:1-2695";

    If there are, or might be, overlapping transactions between the GTID sets in the dump files, you can use the stored functions described in Section 2.3.8, “Stored Function Examples to Manipulate GTIDs” to check this beforehand and to calculate the union of all the GTID sets.