Thank you to all of you who attended my webinar last week about Global Transaction IDs (GTIDs), which were introduced in MySQL 5.6 to make the reconfiguration of replication straightforward. If you missed my webinar, you can still listen to the recording and download the sides (free). We had a lot of questions during the webinar, so let me try to answer them here. Please let me know in the comments if additional clarification is needed.
Q: Does GTID provide any benefit to master-master replication? If yes, how?
Q: Is ACTIVE ACTIVE MASTER MASTER successful in MySQL with GTID?
A: GTIDs don’t change the nature of MySQL replication: for instance it is still asynchronous and if you write on both masters in a master-master setup, there is still no write conflict detection mechanism. The main benefit of GTIDs is that any change of the replication topology is easy because you no longer need to run complex calculations to find the right binlog positions when connecting a slave to a new master.
So master-master replication can be configured with GTIDs, it does not provide a lot of benefits compared to position-based replication as you will never change the replication topology.
And having a setup where both masters receive writes is still not recommended with GTIDs.
Q: Will GTIDs work well with master:standby master? How quick would the failover be?
A: Yes, GTIDs works well with this kind of setup (which is one master and one slave). GTIDs do not provide failover, you will have to use an external tool. Speed of failover then depends on the tool you will use.
Q: For already set up MASTER-MASTER/MASTER-SLAVE Replication, after getting GTID set up, we need to rebuild replication again using AUTO POS=1, correct?
A: Yes, using MASTER_AUTO_POSITION=1 is necessary to indicate that you will use GTID replication. So you will have to run: STOP SLAVE; CHANGE MASTER TO … MASTER_AUTO_POSITION = 1; START SLAVE;
Q: Application having tables from different Engines(InnoDB and MyISAM), how that will handled in GTID?
A: Transactions using both MyISAM and InnoDB tables are not allowed, please refer to the documentation
Q: In a master-slave replication topology (with GTID enabled), how does slave get data from the master if the master’s binary logs are purged given that AUTO_POSITION=1 is used as part of the change master command?
A: This will break replication with error 1236.
Q: Whats the value of show slave status who determines if there is a lag on the slave?
A: This is Seconds_Behind_Master. It’s not always reliable though. For instance if you have a replication setup like A -> B -> C, Seconds_Behind_Master on C will shop the lag relatively to B, not A.
Q: What is the value of saving the history of previous master’s GTIDs executed in the show slave status -> Executed_Gtid_Set?
A: The new replication protocol makes sure that when the slave connects to its master, it sends the range of GTIDs it has already executed. Then the master sends back all other transactions. That’s why Executed_Gtid_Set contains the history of all executed transactions.
Q: We use DB Master and Slave VIPs on our servers, can the mysqlfailover tool also switch the VIP to the new master? Is it scriptable on the event of a failover?
A: Yes you can use extension points to add you own custom scripts with mysqlfailover. See the documentation for –exec-before and –exec-after.
Q: How does mysqlfailover handle brief network instability between the Master and Slaves?
A: mysqlfailover only triggers failover when it suspects the master is no longer alive. So network instability between the master and its slaves won’t affect it for master crash detection. However it can prevent the tool from reconfiguring replication correctly during failover/switchover if one or several slaves are not reachable.
Q: Does Facebook use MySQL with GTID? if yes, which module or all together for everything?
A: I can’t speak for Facebook, but this talk at the MySQL Conference this year suggests that they’re using GTIDs in production. They have added custom code to make GTIDs easier to use.
Q: is GTID_SUBSET function part of MySQL utilities? or we should set a script to regularly detect it? is GTID_SUBSET beneficial in case of an ACTIVE ACTIVE MASTER MASTER setup?
Q: Can you please confirm how to get gtid set in order to use gtid functions?
A: GTID_SUBSET() is a built-in function in MySQL 5.6, you don’t need to install MySQL Utilities to use it. It can be used to easily know whether Executed_Gtid_Set on a given server is a subset of Executed_Gtid_Set on another server, so it can be beneficial to use it in any replication topology.
Q: What is difference between HOLE and BUGS?
A: Holes are not allowed in MySQL 5.6 implementation of GTIDs. So if you see a hole in a GTID sequence, you’re hitting a bug!
Q: Using MySQL utilities, we can set-up replication also using a python script with just one command. Does it automatically takes dump from master to slave and starts replication? If yes so a 300 GB data directory, will it run as background if executed using shell script? Or it just starts the replicationn from current position and won’t take the dump?
A: I think you are talking about mysqlreplicate. This tool only runs CHANGE MASTER TO for you so it doesn’t take a backup of any kind.
Q: Is it possible to use mysqlfailover script at any node(like slave) any time to know which is its MASTER and other SLAVE options also? If no, is this available by some other means?
A: You should probably use mysqlrplshow instead.
Q: As told during limitations of MySQL Utilities on automatic failover, so how can I achieve AUTOMATIC failover if I want this as primary option?
A: The node running mysqlfailover is not highly available so if it is down you lose the ability of doing automatic failover. There are several options if you want to achieve automatic failover: carefully monitor the monitoring node or use solutions like Percona Replication Manager which relies on Pacemaker or Percona XtraDB Cluster which relies on Galera replication.
Q: mysqlrpadmin failover/switchover: Can we execute this command on slave or any other monitoring node?
A: Yes, as long as mysqlrpladmin is installed on a server and if it can connect to the database servers, the command can be executed from anywhere.
Q: If we set slaves to read_only…is that recommended to eliminate the errant transaction?
A: Yes, it is recommended. However it doesn’t prevent users with the SUPER privilege from accidentally writing on a slave.
Q: Can this errant transaction issue be prevented with (active-passive) master-master replication?
A: Master-master replication will make sure that any transaction written on one server will automatically end up being written on the other slave. So it is like avoiding errant transactions. However writing on both masters is not recommended as you can have write conflicts.
Q: Is there any practical advantage of using GTID with MHA?
A: GTIDs provide no way to perform failover, they only simplify how you can reconfigure replication. So using MHA to leverage GTIDs makes sense.
Q: What is diifference GUID and GTID? When do we have to use GUID and GTID?
A: I’m not sure I correctly understand the question. A GTID is made of a source id and a transaction id. The source id is the master’s server_uuid, which is a GUID that is automatically generated when MySQL is started for the first time.
Q: How to check the slave database tables and record? we have to sync data from master to salve database or automatically will be happen?
A: You can use pt-table-checksum and pt-table-sync from Percona Toolkit.
Thanks again for attending the webinar! You can replay it, download the slide – and also access Percona’s vast library of other MySQL webinar recordings here.