Semi-sync Replication is a plugin available for mysql which allows you to create more durable replication topologies.  For instance you can ensure that in the event of a master crash that at least one of your replicas has all transaction currently written to the master so that when you promote, you know you're not missing any data.

That's a huge simplification.

What's the downside?  Write speed.  If a transaction on your master have to wait until a replica acknowledges it has that transaction, then there is going to be some delay.  Not only that, but your network latency between the two points matters a lot.  If you want greater durability, the cost is performance.

It's important to note that the master doesn't wait until the replica actually runs the transaction on the …

I have a customer who is considering Percona XtraDB Cluster (PXC) in a two colo WAN environment.  They wanted me to do a test comparing PXC against semi-synchronous replication to see how they stack up against each other.

Test Environment

The test environment included AWS EC2 nodes in US-East and US-West (Oregon).  The ping RTT latency between these nodes was right around 100ms.

All environments used Percona Server or Percona XtraDB Cluster server 5.5.24.  Innodb durability was disabled for all tests (innodb_flush_log_at_trx_commit=2).  All other settings were the same kept the same unless otherwise noted.

I tested against the following setups:

Control

The control …