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Database Daily Ops Series: GTID Replication

 | November 10, 2016 |  Posted In: GTID, MySQL, Percona Managed Services

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GTID ReplicationThis post discusses ways of fixing broken GTID replication.

This blog series is all about the daily stories we have in Managed Services, dealing with customers’ environments (mostly when we need to quickly restore a service level within the SLA time).

One of the issues we encounter daily is replication using the GTID protocol. While there are a lot of blogs written about this subject, I would like to just highlight GTID replication operations, and the way you can deal with broken replication.

Most of the time we face way more complex scenarios then the one I’m about to present as an example, but the main goal of this blog is to quickly highlight the tools that can be used to fix issues to resume replication.

After reading this blog, you might ask yourself “Now, we know how to fix replication, but what about consistency?” The next blog will be entirely focused on that matter, data consistency!

Little less talk, little more action…

Replication is broken, and the SHOW SLAVE STATUS command output looks like below:


When a slave configured to replicate using the GTID protocol breaks, pay attention to the SHOW SLAVE STATUS command output. You will find the Retrieved_Gtid_Set and Executed_Gtid_Set in the listed columns. You can see that the last global transaction ID retrieved from the current master was not executed (it’s going to appear on the Retrieved_Gtid_Set but not on the Executed_Gtid_Set, following the GTID format).

That means that the slave has retrieved a transaction that, for some other reason, it couldn’t execute. That’s the global transaction ID you need if you want to inject a fake transaction and get replication resumed. The fake transaction you inject takes the place of the one that has an SQL that cannot be executed due to an error found in Last_Error from the SHOW SLAVE STATUS.

Let’s analyze it:

As shown above, we can see clearly that the transaction causing issues is global transaction ID number 4, coming from master under UUID 46fdb7ad-5852-11e6-92c9-0800274fb806. You can use the SHOW RELAYLOG EVENTS to try and verify that is the transaction’s query that is causing problems:


Before fixing and resuming the replication stream, we need to check why that INSERT query breaks replication. Let’s SELECT data and check the structure of table wb.t1:


It’s clear that something is wrong somewhere else than just the database. It’s time to fix and resume replication, and catch up with the master data. GTID replication data uses the below tools:

  • pt-slave-restart
  • mysqlslavetrx
  • inject a fake/empty transaction

pt-slave-restart

One of the easiest ways of resuming replication on slaves when replication is broken is using pt-slave-restart, which is part of Percona Toolkit. Once you find the above facts (mainly the master UUID of the problematic global transaction ID that broke slave replication), you can move forward using pt-slave-restart  with the GTID flag -master-uuid. This passes the master’s UUID and it skips all global transactions breaking replication on a specific slave server, as you can see below:


With the resources provided by pt-slave-restart, together with the above info, replication should resume. If you don’t have the Percona Toolkit package setup on your servers, make sure you follow these steps. It’s easier if you add the Percona Repository to your servers (you can use the Package Manager to install it for Debian-based and for RedHat-based systems).

mysqlslavetrx

To use mysqlslavetrx (which is part of MySQL Utilities developer by Oracle), I recommend you read the article written by Daniel Guzman, and install MySQL Utilities on your database servers. Using it to skip problematic transactions and inject fake ones is pretty straightforward as well .

So, find the below on the slave side:


Then use GTID_SUBTRACT as the first parameter, which you can find on Retrieved_Gtid_Set and the subset, which you can grab from the Executed_Gtid_Set. Use the UUID of the direct master in the function to find the global transaction IDs executed on slave:


Now we can use mysqlslavetrx to insert a fake transaction on the slave to resume replication, as below:

When you get back to the MySQL client on the slave, you’ll see that the retrieved and executed out of SHOW SLAVE STATUS will point that they are in the same position:


After, make sure you start the slave (mysqlslavetrx won’t start the replication as previous tool does).

Inject a Fake Transaction

Fake transactions are called empty transactions as well, but the fact is if a global transaction is affecting a slave, you must fake empty transactions that won’t affect data to resume replication and carry on processing the data streaming from the master aka replication! We need to know that it’s not going to affect future slaves, especially if this server becomes a new master after a failover/switchover process. You can get more information about Errant Transactions here and here.

Now, when you check retrieved and executed out of SHOW SLAVE STATUS, you can see the below:

It’s time to start slave (and be happy)!


Cheers!

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Wagner Bianchi

Bianchi, as he likes to be called, is a MySQL DBA for more than 10 years, has been working with many of the biggest companies in the world with focus centered in Data-Infrastructure, Performance, Scale-Out and HA. Before working at Percona, Bianchi worked at Splunk, Oracle and IBM.

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