I wrote about Galera about 1.5 years ago: State of the art: Galera – synchronous replication for InnoDB. It was about the 0.7 release, which was more like a proof-of-concept release (though Galera’s developers may not agree with that ) with some serious limitations (like using mysqldump for node propagation). The Galera team heard my suggestions and the new 0.8 release looks very promising. Well, it took 1.5 years to fix the limitations and come up with new features, but there is nothing to complain about it – a synchronous distributed transactional system is not an easy problem to solve, trust me.
So Galera 0.8 comes with many nice features:
Works with MySQL 5.1 and MariaDB 5.1. The latest is more interesting for us, as it is based on XtraDB. That means Galera supports the XtraDB storage engine.
Support for multi-threaded slaves.
Using custom scripts for node propagation.
RSYNCmethod comes with the Galera distribution, and it is quite easy to add support for Percona XtraBackup to propagate nodes.
Why MySQL/MariaDB + Galera 0.8 is interesting? It allows to solve following problems:
A Real High Availability solution for systems based on InnoDB/XtraDB. The recommended setup is 3 nodes and you can add / remove nodes almost transparently.
It is possible to use it in traditional master-slave setups, but with big difference – with
Galerawe have Semi-synchronous slaves. How it works on slaves: slaves just acknowledge reception of a network packet (not a transaction). Transaction is not guaranteed to be applied – what is guaranteed is that every node will do exactly the same thing with it. With parallel applying on slaves, the latency of round-trip transactions should be in an acceptable range.
It opens the possibility for active master – active master setups. You can write on both masters and do not worry about conflict resolution and get rid of those “slave is out of sync with master” pain-in-the-neck problems.
Combining all above, now we can setup distributed replication systems with masters in different data centers. This provides a HA solution for MySQL setups in the Cloud. E.g. with current state of MySQL EC2 setups suffer from lacking a good HA schema. With Galera, we can setup replication with the same availability and in different zones. Remember the recent and famous EC2 outage? Having masters in USA and Europe regions would solve this kind of problems.
Scaling Writes. From benchmarks provided by the Galera team we may see a good scaling of throughput writing to several nodes
As you may see, I am pretty excited that this solution is available for MySQL users as Free / Open Source Software.
“Where is the catch?”, you may ask. Yes, there are couple of points to consider:
Complexity of setup: I have been playing with Galera 0.8-pre for couple last weeks, and from my observation, the task to setup three node cluster is much more complex than an average MySQL master-slave setup.
Potential performance penalty: Although to have performance numbers I am going to run different benchmarks, I expect that for simple master-slave setups the response time and throughput will be affected (and not towards better side). But this is price to pay for synchronous not-getting-behind slaves. With additional nodes, the response time will only increase. And in multi-nodes setups, the performance of whole cluster will be defined by slowest server, so it will be recommended to have uniform servers across the cluster.
From my experiments with MariaDB/Galera 0.8 I have one serious feature request for the Galera team: provide the ability for incremental node provisioning.
By this I mean that if right now a node gets disconnected from the cluster, in order to join to the cluster again, it has to copy the whole data set again. But if it was disconnected only for a short period of time, we may want to copy only changes during this period. I believe that, with integration with Percona XtraBackup and its incremental backup features, it is possible to have incremental node provisioning.
To finalize this post, let me invite you to join me in testing MySQL/Galera 0.8, binaries are available from Launchpad.
The downside of MySQL/Galera is that it is based on standard InnoDB 5.1 instead of InnoDB-plugin. Standard InnoDB is seriously behind InnoDB-plugin in terms of features and performance.
Update (6-Jun-2011): It was my misunderstanding: Galera does work with InnoDB-plugin also.
You may try also MariaDB/Galera based on XtraDB, but it is only available in source code, and you may need system based on RedHat 6 or similar to have it compiled. There is also a helpful Wiki page with a bunch of information about Galera replication.