The Problem

Original ActiveRecord sessions st ore is slow. It is fine for some low traffic sites, but it is too slow to use it for something larger. First of all, it is slow because ActiveRecord itself is slow. It is powerful ORM framework, but it is obviously overkill for such a simple task as a sessions management.

There are other possible solutions like cookie session store (session size is limited, no sensible data in sessions), memcached (no persistence + harder to implement high-available solutions).

That is why SqlSession store was created. It works with mysql directly with database APIs and works much faster than original AR session store. But sometimes it is still slow too because:

• it creates/updates session on each hit – every bot or occasional web serfer create session records in your database so you end up with thousands of thousands of useless records in your sessions table while 99% of hits do not require any session updates.
• it uses 32-characters string as a key for sessions records – all RDBMS work with string keys MUCH slower than with integers, so it would be much better to use integers, but we have these long session IDs and all session stores use them as a table 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

As a solution for mentioned problems FastSessions Rails plugin was born.

So, first of all, we removed id field from the sessions table, so we do not need to use auto-increment locks. Next step was to make lookups faster and we’ve used a following technique: instead of using (session_id) as a lookup key, we’ve started using (CRC32(session_id), session_id) – two-columns key which really helps MySQL to find sessions faster because key cardinality is higher (so, mysql is able to find a record earlier w/o checking a lots of index rows). We’ve benchmarked this approach and it shows 10-15% performance gain on large sessions tables.