Yesterday I upgraded our running elasticsearch cluster on a site which serves a few million search requests a day, with zero downtime. I've been asked to describe the process, hence this blogpost.
To make it more complicated, the cluster was running elasticsearch version 0.17.8 (released 6 Oct 2011) and I upgraded it to the latest 0.19.10. There have been 21 releases between those two versions, with a lot of functional changes, so I needed to be ready to roll back if necessary.
- elasticsearch
We run elasticsearch on two biggish boxes: 16 cores plus 32GB of RAM. All indices have 1 replica, so all data is stored on both boxes (about 45GB of data). The primary data for our main indices is also stored in our database. We have a few other indices whose data is stored only in elasticsearch, but are updated once daily only. Finally, we store our sessions in elasticsearch, but active sessions are cached in memcached.