Thursday, July 21, 2011


Back in February I wrote an article titled A Small Fix For mysql-agent. Since then we did a few more fixes to the agent and included a Bytes Behind Master (or BBM) chart. For those who can't wait to get their hands on the code, here's the current version: MySQL SNMP agent RPM. For those who'd like to learn about it's capabilities and issues, keep reading.

What to Expect From this Version

The article I quoted above pretty much describes the main differences with the original project, but we went further with the changes while still relying on Masterzen's code for the data collection piece.

The first big change is that we transformed Masterzen's code into a Perl module, this way we can easily plug in a new version without having to do massive editing to ours.

The 2nd change is that we added the code to calculate how many bytes behind is a slave, which should be cross checked always with seconds behind master to get replication's full picture. When a slave is just a few bytes behind, the script calculates the difference straight out of the SHOW SLAVE STATUS information. If the SQL thread is executing statements that are in a binary log file older than the one being updated by the I/O thread, then the script logs into the master to collect the sizes of the previous binary logs and make an accurate calculation of the delta.

For this change we hit another bug in CentOS 5 SNMP agent, by which 64bit counters were being truncated. The solution is to upgrade to CentOS 6 (not anytime soon, but that's another story) or a work around. We decided for the latter and display a variable flagging this value roll over. This is not needed for non-CentOS 5 platforms as far as we know.

By now I expect that many of you would have a question in your mind:

Why Not Branch / Fork?

Why provide an RPM instead of creating a branch/fork in the original project? There are many reasons, but I'll limit myself to a couple. I trust that before you write an enraged comment you'll keep in mind that this is a personal perception, which might be in disagreement with yours.

This code is different enough from the original that creating a branch to the original project would be too complicated to maintain. For example: we are using a completely different SNMP protocol and created a module out of the original code. We don't have the resources to follow behind all of Masterzen's possible patches and I wouldn't expect him to adopt my changes.

If we would've created a fork (a new project derived from the original), I believe at this point, it would divert the attention from the original project or others like PalominoDB's Nagios plugin.

What's Next

We plan to continue maintaining this RPM driven by our specific needs and keep sharing the results this way. If at some point we see it fit to drive the merge into another project or create a new fork of an existing one, we'll do it.

I will be presenting the project at OSCON next week. If you're going to be around, please come to my talk: Monitoring MySQL through SNMP and we can discuss issues like: why use pass_persist, why not use information schema instead of the current method, why not include your personal MySQL instrumentation pet peeve, I'd be glad to sit down with you and personally chat about it.

In the meantime, enjoy, provide feedback and I hope to get to know you at OSCON next Thursday.