Tag Archives: mysql

MySQL HA

MySQL

This is very interesting topic from sysops’ point of view. Honestly – It’s not that common to find a well organised HA MySQL environment (I’m talking here about good failover solution in master – slave scenario).

I won’t write here details for the most known solutions / scenarios – I think it’s enough to point those that one can check if it fits his needs.

  1. Master – master replication. In this scenario we don’t need automated failover – when we have some outage of one of those masters then it’s no problem as second one is still working. It’s good to use some LB layer between application and MySQL hosts that makes the app always hit the working server.
  2. MySQL cluster. This is very comfortable scenario as MySQL cluster takes care of almost everything – this is “no single point of failure” solution. You can read about those here http://www.mysql.com/products/cluster/ and here http://dev.mysql.com/downloads/cluster/ –
  3. MySQL Galera cluster – for synchronous, active – active multi – master topology: http://www.codership.com/content/using-galera-cluster
  4. Failover – for now I know 2 solutions worth mentioning:
    1. Matsunobu Yoshinori created mysql-master-ha (working on MySQL servers 5.0, 5.1 and later) – this is very good tool, that handles automatic, manual and semi – manual failovers (even when slaves are in different relaylog positions). In addition You can use it when MySQL master host migration is needed. I recommend watching those slides: http://www.slideshare.net/matsunobu/automated-master-failover
    2. MySQL utilities – this is quite simple daemon which primary job is to perform automated failover when needed. You can read about it here: http://www.clusterdb.com/mysql/mysql-utilities-webinar-qa-replay-now-available/

MySQL statement based replication with triggers, events, procedures, functions and variables

Before starting statement based replication in MySQL database We have to be aware of some specific behaviors for this environment. This kind of replication (statement based) writes each query that modifies data to the Binary Log in order to replicate them on the slave or to use as a point-in-time recovery (PITR). Because of this kind of query logging We should be aware how MySQL replication engine behaves with some special queries like triggers, functions, procedures or events.

Functions

Function calls are logged directly to Binary Log, so If You forget to create on slave any function that is created on master – You will break your replication and probably You’ll see error like below:

When promoting slave to master no additional steps according to functions are required – everything is needed is having functions defined in both: master and slave.

Procedures

Procedure calls are not replicated as in functions – this is important to know. Only the queries inside the procedures get logged to the Binary Log, so You don’t have to create procedures on slaves.

In order to promote slave to master you should have procedures created on slave – so it is wise to have all the procedures created on both – master and slaves.

Events

Events created on master server get replicated to the slave with the DISABLE ON SLAVE option – that’s why those events are not reexecuted on every slave in our MySQL architecture and we have no duplicated and corrupted data. MySQL logs only queries from inside the event so only those queries are replicated via Binary Log.

In order to promote slave to master according to events we have to do some more job. I’ve created a simple event below (We create it on the master):

Now it’s replicated on slave via Binary Log – below I’ve placed replication entry for that event from Binary Log:

And how it looks like after replication on the slave:

So now with this knowledge a little procedure to promote slave to master using events:

  • Disabling event manager on slave with SET GLOBAL event_scheduler = OFF;
  • Enabling all the events with ALTER EVENT event_name ENABLE – We have to do this for each event, so writing a little script is very helpful here.
  • Enabling event manager with SET GLOBAL event_scheduler = ON;

In order to demote back the master to slave You should follow the previous procedure with a little change on ALTER EVENT – here You just need to DISABLE all the events (not ENABLE).

Triggers

In order to have triggers running properly on master and slaves You have to define them in both – master and slave servers. MySQL statement based replication replicates only the original query to the Binary Log – not the subsequent triggered statements.

When promoting slave to master no additional steps according to triggers are required – everything is needed is having triggers defined in both: master and slave.

Mixed triggers / procedures / functions calls

Let’s imagine that We have a trigger, that triggers a procedure which uses a function call. How will this behave in statement based replication?

  1. We should have trigger defined on both: master and slave
  2. We don’t have to have procedure defined on the slave – only on master is enough
  3. We should have function defined on both: master and slave

Despite of all – my advice is to keep function, triggers, procedures and events defined on all the servers (masters and slaves) – just to be sure, that We can always promote slave to master without any issues.

And one more thing before finishing this post. If You plan to start replication with just copying FRM, MYI, MYD and InnoDB files You should also dump any functions / triggers and stored procedures on master (or slave) and then import those on the new slave. You can do it (for every database) with:

And recreate those on the new box:

MySQL tunneling via SSH and error “channel: open failed: connect failed: Connection refused”

Lately I wrote a short article about MySQL tunneling via SSH in order to start safe MySQL replication. Afterwards I noticed some problems with creating a new SSH tunnel for MySQL connection on a quite different environment. After creating SSH tunnel and trying to connect via this tunnel to the SSH server I received SSH error on tunnel error-log:

or:

And below:

in the MySQL terminal.

First of all We have to make sure, that our tunnel is working properly, so We just kill the current tunnel and create new one without “-f” and “-N” options:

If everything is ok, then We can assume that tunnel is working fine. We can also try to create another tunnel to some other service on different target port and then just try if this other service is working via the tunnel – just to exclude any problems with SSH tunneling.

My problem was that MySQL was configured in the way it was blocking any connections outside localhost. It is default MySQL configuration – We can achieve it via my.cnf entries:

or:

So in order to make our MySQL accessible via our tunnel We have to comment out the skip-networking line and make sure that We are connecting to the correct IP addr in our tunnel. For example If we have in our my.cnf this line:

Then our tunnel should look like:

(notice that 127.0.0.1 in the above command).

If We would bind our MySQL to some other IP, like:

Then We should change our tunneling parameters:

After commenting out that skip-networking our security depends on IP address We are binding the MySQL to. If it’s local IP addres in DMZ, than there is no security breaches here. Unwise would be to bind to the WAN address and leave MySQL port opened without any SSL encryption or without filtering traffic by the client IP addr…