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Errant Transactions: Major Hurdle for GTID-Based Failover in MySQL 5.6

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I have previously written about the new replication protocol that comes with GTIDs in MySQL 5.6. Because of this new replication protocol, you can inadvertently create errant transactions that may turn any failover to a nightmare. Let’s see the problems and the potential solutions.

In short

  • Errant transactions may cause all kinds of data corruption/replication errors when failing over.
  • Detection of errant transactions can be done with the GTID_SUBSET() and GTID_SUBTRACT() functions.
  • If you find an errant transaction on one server, commit an empty transaction with the GTID of the errant one on all other servers.
  • If you are using a tool to perform the failover for you, make sure it can detect errant transactions. At the time of writing, only mysqlfailover and mysqlrpladmin from MySQL Utilities can do that.

What are errant transactions?

Simply stated, they are transactions executed directly on a slave. Thus they only exist on a specific slave. This could result from a mistake (the application wrote to a slave instead of writing to the master) or this could be by design (you need additional tables for reports).

Why can they create problems that did not exist before GTIDs?

Errant transactions have been existing forever. However because of the new replication protocol for GTID-based replication, they can have a significant impact on all servers if a slave holding an errant transaction is promoted as the new master.

Compare what happens in this master-slave setup, first with position-based replication and then with GTID-based replication. A is the master, B is the slave:

# POSITION-BASED REPLICATION
# Creating an errant transaction on B
mysql> create database mydb;
 
# Make B the master, and A the slave
 
# What are the databases on A now?
mysql> show databases like 'mydb';
Empty set (0.01 sec)

As expected, the mydb database is not created on A.

# GTID-BASED REPLICATION
# Creating an errant transaction on B
mysql> create database mydb;
 
# Make B the master, and A the slave
 
# What are the databases on A now?
mysql> show databases like 'mydb';
+-----------------+
| Database (mydb) |
+-----------------+
| mydb            |
+-----------------+

mydb has been recreated on A because of the new replication protocol: when A connects to B, they exchange their own set of executed GTIDs and the master (B) sends any missing transaction. Here it is the create database statement.

As you can see, the main issue with errant transactions is that when failing over you may execute transactions ‘coming from nowhere’ that can silently corrupt your data or break replication.

How to detect them?

If the master is running, it is quite easy with the GTID_SUBSET() function. As all writes should go to the master, the GTIDs executed on any slave should always be a subset of the GTIDs executed on the master. For instance:

# Master
mysql> show master status\G
*************************** 1. row ***************************
             File: mysql-bin.000017
         Position: 376
     Binlog_Do_DB:
Binlog_Ignore_DB:
Executed_Gtid_Set: 8e349184-bc14-11e3-8d4c-0800272864ba:1-30,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-7
 
# Slave
mysql> show slave status\G
[...]
Executed_Gtid_Set: 8e349184-bc14-11e3-8d4c-0800272864ba:1-29,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-9
 
# Now, let's compare the 2 sets
mysql> > select gtid_subset('8e349184-bc14-11e3-8d4c-0800272864ba:1-29,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-9','8e349184-bc14-11e3-8d4c-0800272864ba:1-30,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-7') as slave_is_subset;
+-----------------+
| slave_is_subset |
+-----------------+
|               0 |
+-----------------+

Hum, it looks like the slave has executed more transactions than the master, this indicates that the slave has executed at least 1 errant transaction. Could we know the GTID of these transactions? Sure, let’s use GTID_SUBTRACT():

select gtid_subtract('8e349184-bc14-11e3-8d4c-0800272864ba:1-29,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-9','8e349184-bc14-11e3-8d4c-0800272864ba:1-30,
8e3648e4-bc14-11e3-8d4c-0800272864ba:1-7') as errant_transactions;
+------------------------------------------+
| errant_transactions                      |
+------------------------------------------+
| 8e3648e4-bc14-11e3-8d4c-0800272864ba:8-9 |
+------------------------------------------+

This means that the slave has 2 errant transactions.

Now, how can we check errant transactions if the master is not running (like master has crashed, and we want to fail over to one of the slaves)? In this case, we will have to follow these steps:

  • Check all slaves to see if they have executed transactions that are not found on any other slave: this is the list of potential errant transactions.
  • Discard all transactions originating from the master: now you have the list of errant transactions of each slave

Some of you may wonder how you can know which transactions come from the master as it is not available: SHOW SLAVE STATUS gives you the master’s UUID which is used in the GTIDs of all transactions coming from the master.

How to get rid of them?

This is pretty easy, but it can be tedious if you have many slaves: just inject an empty transaction on all the other servers with the GTID of the errant transaction.

For instance, if you have 3 servers, A (the master), B (slave with an errant transaction: XXX:3), and C (slave with 2 errant transactions: YYY:18-19), you will have to inject the following empty transactions in pseudo-code:

# A
- Inject empty trx(XXX:3)
- Inject empty trx(YYY:18)
- Inject empty trx(YYY:19)
 
# B
- Inject empty trx(YYY:18)
- Inject empty trx(YYY:19)
 
# C
- Inject empty trx(XXX:3)

Conclusion

If you want to switch to GTID-based replication, make sure to check errant transactions before any planned or unplanned replication topology change. And be specifically careful if you use a tool that reconfigures replication for you: at the time of writing, only mysqlrpladmin and mysqlfailover from MySQL Utilities can warn you if you are trying to perform an unsafe topology change.


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Published at DZone with permission of Peter Zaitsev, DZone MVB. See the original article here.

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