Q&A: High availability when using MySQL in the cloud

Percona MySQL webinar followup: Q&ALast week I hosted a webinar on using MySQL in the cloud for High Availability (HA) alongside 451 Research analyst Jason Stamper. You can watch the recording and also download the slides (free) here. Just click the “Register” button at the end of that page.

We had several excellent questions and we didn’t have time to get to several of them in the allotted time. I’m posting them here along with the answers. Feel free to ask follow-up questions in the comments below.

Q: Can the TokuDB engine be used in a PXC environment?

A: No, TokuDB cannot currently be used in a PXC environment, the only supported engine in Percona XtraDB Cluster 5.6 is InnoDB.

Q: With Galera replication (PXC), is balancing the load on each node?

A: No, you need to implement your own load balancing and HA layer between your clients and the Percona XtraDB Cluster server.  Examples mentioned in the webinar include HAProxy and F5 BigIP.

Q: What’s the best version of Percona XtraDB Cluster regarding InnoDB performance?

A: In general for best performance you should be using the latest release of Percona XtraDB Cluster 5.6, which is currently 5.6.24, released on June 3rd, 2015.

Q: Can I redirect my writes in Percona XtraDB Cluster to multiple nodes using the HAProxy? While trying with SysBench I can see write-only goes to first nodes in PXC while reads does goes to multiple nodes.

A: Yes you can configure HAProxy to distribute both reads and writes across all of your nodes in a Percona XtraDB Cluster environment. Perhaps SysBench created only one database connection for all writes, and so haproxy kept those confined to only one host. You may want to experiment with parallel_prepare.lua.

Q: What’s the optimal HA for small datasets (db is less than 10gb)?

A: The optimal HA deployment for small datasets would be dependent on your level of recovery required (tolerance for loss of transactions) and time that you can be in an unavailable state (seconds, minutes, hours?).  Unfortunately there isn’t a single answer to your question, however, if you are interested in further discussion on this point Percona would be happy to coordinate a time to speak.  Please feel free to contact me directly and we can continue the conversation at michael.coburn@percona.com.

 Q: Is there a concept of local master vs. remote master with PXC?

A: No there is no concept of local vs remote master.  All nodes in a Percona XtraDB Cluster can now be classified as Master, regardless of their proximity to the clients.

Q: Are there any concerns when considering AWS RDS or AURORA DB for MySQL HA in the Cloud?

A: Regarding AWS RDS, yes this a good option for MySQL HA in the Cloud.  I unfortunately haven’t worked with Aurora DB that much yet so I don’t have an opinion on it’s suitability for HA in the Cloud.

Q: We tried out PXC awhile back and it used to lock everything whenever any ddl was done. Has that changed?

A: We would have to look at the specifics of your environment, however, there have been numerous improvements in the 1½ years of development since Percona XtraDB Cluster went Generally Available (GA) on January 30th, 2014 in version 5.6.15.

Q: Is using the arbitrator a must?

A: No the arbitrator role via the garbd daemon is generally only used when operating in a minimal environment of two nodes that contain the data and you need a third node for quorum but don’t want to store the data a third time.

Q: Can we do a cluster across different zones?

A: Yes you can. However be aware that the latency incurred for all cluster certification operations will be impacted by the round trip time between nodes.

Q: Does PXC also support the MyISAM database?

A: No, Percona XtraDB Cluster does not support any storage engine other than InnoDB as of PXC 5.6.

Q: How do load balancers affect the throughput in a Galera-based setup given that the write would be limited by the slowest node?

A: Load balancers will introduce some measure of additional latency in the form of CPU time in the load balancer layer as it evaluates its own ruleset, and also in network time due to additional hop via load balancer.  Otherwise there should be no perceptible difference in the write throughput of a Percona XtraDB Cluster with and without a load balancer as it relates to the “slowest node” factor.

Q: Have you used MaxScale yet? If so, what are your thoughts?

A: Unfortunately I haven’t used MaxScale however Yves Trudeau, Percona Principal Architect, has recently written about MaxScale in this blog post.

Q: How do you configure timeout and maintain persistent connection to HAProxy?

A: I would encourage you to refer to the HAProxy Documentation.

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Using MySQL triggers and views in Amazon RDS

I recently had an opportunity to migrate a customer from a physical server into Amazon’s RDS environment. In this particular case the customers’ platform makes extensive use of MySQL triggers and views.  I came across two significant issues that prevented me from following Amazon’s documentation, which basically states “use mysqldump” but doesn’t call out a specific method of dealing with MySQL triggers and views.

Amazon Relational Database Service (Amazon RDS) is a great platform if you’re looking for complete hands-off management of your MySQL environment, but comes at a cost in the area of flexibility, i.e. you don’t have SUPER privilege and this brings up additional challenges.

  1. You need to ensure you set log_bin_trust_function_creators=1 ( by default this is off, 0).
  2. You need to clean up your mysqldump syntax.

#1 is easy, you simply make a configuration change within the Amazon RDS GUI on the node’s Parameter Group to set log_bin_trust_function_creators=1 and then a restart of your Amazon RDS node.  The restart is required since without the SUPER privilege you lose access to changing DYNAMIC variables on the fly.
#2 is a little more complex.  If you go with vanilla mysqldump (from say a 5.5 mysqldump binary) on a schema that has triggers and views, you will see error 1227, something like this:

ERROR 1227 (42000) at line 27311: Access denied; you need (at least one of) the SUPER privilege(s) for this operation

You’re seeing this message because MySQL in Amazon RDS doesn’t provide the SUPER privilege, and thus you cannot set up a trigger or view to run as a different user — only a user with SUPER can do that.

mysqldump will generate syntax for a trigger like this:

/*!50003 CREATE*/ /*!50017 DEFINER=`root`@`%`*/ /*!50003 TRIGGER `after_insert_lead` AFTER INSERT ON `leads` FOR EACH ROW BEGIN
UPDATE analytics.mapping SET id_lead = NEW.id_lead WHERE mc_email = NEW.email;
END */;;

and for a view like this:

/*!50013 DEFINER=`web`@`%` SQL SECURITY DEFINER */
/*!50001 VIEW `admin_user_view` AS SELECT ...

The problem is in the “DEFINER” lines.

Here’s one method that worked for me:

  1. Identify all the DEFINER lines in your schema. I found it helpful to dump out a –no-data and then weed through that to get a unique list of the DEFINER lines
  2. Create a sed line for each unique DEFINER line (see my example in a moment)
  3. Include this sed line in your dump/load script

Here’s what my sed matches looked like:

-e 's//*!50017 DEFINER=`root`@`localhost`*///'
-e 's//*!50017 DEFINER=`root`@`%`*///'
-e 's//*!50017 DEFINER=`web`@`%`*///'
-e 's//*!50017 DEFINER=`cron`@`%`*///'
-e 's//*!50013 DEFINER=`cron`@`%` SQL SECURITY DEFINER *///'
-e 's//*!50013 DEFINER=`root`@`localhost` SQL SECURITY DEFINER *///'
-e 's//*!50013 DEFINER=`root`@`%` SQL SECURITY DEFINER *///'
-e 's//*!50013 DEFINER=`web`@`%` SQL SECURITY DEFINER *///'

Note: the example above won’t directly work due to WordPress “helpfully” stripping my text… you need to escape the forward slashes and asterisks.

A big caveat: this method is akin to a brute force method of getting your data into Amazon RDS — you’ve lost the elegance & security of running your triggers and views as separate defined users within the database — they are all now going to run as the user you loaded them in as. If this is a show-stopper for you, contact Percona and I’d be happy to take on your case and develop a more comprehensive solution.  :)

Now all that’s left is to integrate this into your dump flow.  Something like this should work:

| sed
-e ... lots of lines
| mysql

I hope this helps someone!

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