November 22, 2014

Simple MySQL Master HA with mysqlnd_ms

I had the pleasure of presenting to the PHP Users Group Philippines a few days ago about mysqlnd_ms. The mysqlnd plugin, MySQL Master Slave, is a transparent layer on top of mysqlnd extension. This allows you to do read-write splitting and slave reads load balancing without needing to change anything from your application. But do you know you can also achieve a form of high availability with this plugin? I shared 2 forms on my presentation, using async MySQL replication either in master-slave configuration or master-master configuration, while the second form is having an all primary cluster where you can write to all nodes.

This first part is to demonstrate how you can achieve a simple HA solution using the first form. First, all the sample code here can be found on my GitHub repository. So, to use the mysqlnd_ms plugin, it uses an additional external configuration file in JSON format. This configuration file, will define your master and slave nodes, failover properties and any filters (connection selection method) you want to dictate how the algorithm will provide you the connection.

Let’s start with the mysqlnd_ms configuration I used, mysqlnd_ms_ms.ini :

Here, I have two applications defined, one called “primary” and another called “standby”, I have not defined any slaves for simplicity. The two MySQL instances running on port 33001 and 33002 are in master-master configuration.

This is the custom INI file I used for the tests, master-slave.ini . The first line simply enables the plugin for use. The second line, mysqlnd_ms.disable_rw_split instructs the plugin that I should only send all queries to the master because I only have masters for this test.

As for the PHP script, the full copy can be found here, as it is a bit lengthy I will just explain the logic on what it does.

  1. To start the test, it bootstraps the test table via DROP and then CREATE queries.
  2. It then enters a for loop where it will execute an INSERT followed by a SELECT to validate the newly inserted row and additional information like the current active server id and the connection id.
  3. For every iteration of the loop, a new mysqli object is created to simulate non-persistent connections to the database server.
  4. To create the new connection, a call to the function  connect_mysql  is made which returns a mysqli object when successful. An important thing to remember here is that mysqlnd_ms uses lazy connections by default, this means that when the mysqli object is created, it is not really connected yet to the server. One has to issue a statement like  'SELECT 1'  to start the connection manually or call mysqli::real_connect . Not even  mysqli::ping  does not work without the former, I’ve opened this bug.
  5. After the mysqli object is returned, the INSERT statement will trigger mysqlnd_ms to actually establish the connection and then execute the statement. This is where the good part is, if the connection cannot be made, the query_write_mysql function will know and will re-request the connection from connect_mysql, this time within the connect_mysql function, connection to the primary will be retried at least 10 times if the type of error from the previous failure is something related to a connection like error numbers  2002  and 2003 . If the connection cannot be established after 10 retries, the application creates a sentinel file as  /tmp/PRIMARY_HAS_FAILED  and will retry the connection to the secondary (slave or passive-master).

Here is an example run, my primary has a server id or 101 while my standby is 102:

This is not the perfect setup and there are a number of limitations, however it tells us that if you have a simple HA requirement like if you’re not running a very critical application but still do not want to be waken up at night but rather deal with issues in the morning, this might just fit. So here are some more notes:

    • If you have a master-slave configuration, you just basically shot your primary (master) in the foot during the failover. You may need to rebuild its data in the morning.
    • If instead you have master-master, you might just be able to bring the primary master back online, get it caught up in replication and then delete  /tmp/PRIMARY_HAS_FAILED  file to switch your application back to it.
    • The use of  /tmp/PRIMARY_HAS_FAILED  sentinel file is rudimentary, its not the only way. You should consider sending notifications to yourself when failover happens because this method requires human intervention to put back the primary master back in rotation.

The same effect can be achieved with a little more coding, but you can already take advantage of the plugin with less.

I’ve also tested the plugin on the second form where you can write to multiple masters using Percona XtraDB Cluster. I’ve found a few interesting issues there so stay tuned.

About Jervin Real

Jervin is a member of Percona's Rapid Response Consulting team. When you come to Percona for consulting, chances are he'll be greeting you first. His primary role is to make sure customer issues are handled efficiently and professionally. Jervin joined Percona in April 2010.

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