Plugins - FastSessions - Agile Web Development

Plugins - FastSessions

The Problem
Original ActiveRecord sessions store is slow. It is fine for some low traffic blogs, but it is too slow to use it on some big/large/huge sites. First of all, it is slow because ActiveRecord is slow. It is powerful ORM framework, but it is overkill for such simple task as a sessions management.
That is why people created SqlSession store. It works with mysql directly with database APIs and works much faster than original AR session store. But it is still slow because:
it creates/updates session on each hit – even dumb bots crawling your sites create thousands of thousands of useless records in your sessions table, 99% of hits do not require any session updates!
it uses 32-char string as a key for sessions records – all databases work with string keys MUCH slower that with integers keys, so it would be much better to use integers, but we have so long session ids and all session stores use these session ids as a key.
it uses auto_increment primary key, which causes table-level locks in InnoDB for all MySQL versions prior to 5.1.21. These table-level locks with unnecessary inserts cause really weird problems for large sites.
The Solution
FastSessions plugin was born as a hack created for Scribd.com (large RoR-based web project), which was suffering from InnoDB auto-increment table-level locks on sessions table.
So, first of all, we removed id field from the table. Next step was to make lookups faster and we’ve used a following technique: instead of using (session_id) as a lookup key, we started using (CRC32, session_id) – two-columns key which really helps MySQL to find sessions faster because almost all lookups use crc32 field only to find needed record.
And last, but most powerful change we’ve tried to make was to not create database records for empty sessions and to not save sessions data back to database if this data has not been changed during current request processing.
All of these changes were implemented and you can use them automatically after a simple plugin installation.