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= Fast Sessions =
`FastSessions` is a sessions class for `ActiveRecord` sessions store created to work fast
(really fast). It uses some techniques which are not so widely known in developers' community
and only when they cause huge problems, performance consultants are trying to help with them.
==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 [http://www.scribd.com 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), 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.
==Controversial Decisions==
Many plugin users would never think about one problem we've introduced when removed that
auto-increment primary key, so I'd like to describe it here. The problem is following.
`InnoDB` groups all data in tables by primary key. This means that when we create
auto-increment primary key and insert records to a table, our sessions records are
grouped together and saved sequentially on the disk. But if we'll make pretty random
value (like crc32 of a random session id) a primary key, then every session record will
be inserted in its own place and it will generate some random I/O which is not so good
for I/O bound servers.
So, we decided to let the user choose what primary key to use in his deployment of our
plugin, so if you're going to use this module with MySQL 5.1.22+, then you'dlike to set
CGI::Session::ActiveRecordStore::FastSessions.use_auto_increment = true
because it will provide you with consecutive data inserts in InnoDB. Another cases when
you'd like to use it is when your MySQL server is I/O bound now and you do not want to
add random I/O because of randomized primary key.
==Working With Old AR Sessions Table==
If you do not like to loose old sessions created with default AR sessions plugin, you could set
CGI::Session::ActiveRecordStore::FastSessions.fallback_to_old_table = true
and then all session reads will fall back to old sessions table if some session_id was not
found in default fast sessions table. Old sessions table name could be set using
CGI::Session::ActiveRecordStore::FastSessions.old_table_name
variable.
==Installation==
*** for Rails 3 ***
1) Install the Gem via Gemfile:
include the gem in your Gemfile
gem 'fast_sessions', :git => "git@github.com:gsusmonzon/fast_sessions.git"
run 'bundle install' to update your gems
2) Enable `ActiveRecord` session store in your `config/environment.rb` file:
Rails::Initializer.run do |config|
......
config.action_controller.session_store = :active_record_store
......
end
or in your config/initializers/session_store.rb
YourApplicationName::Application.config.session_store :active_record_store
3) Create migration for your new sessions table:
rails generate fast_session_migration AddFastSessions
4) Open your newly created migration and change `table_name` and `use_auto_increment`
parameters of the plugin (if you want to).
5) Run your migration:
rake db:migrate
6) Start your application and try to perform some actions which would definitely
save some data to your session. Then check your `fast_sessions` table (if you did
not renamed it) for a records.
*** for Rails 2 ***
This plugin installation is pretty simple and described in a few steps below:
1) Install this plugin sources in your `vendor/plugins` directory (it could be
./script/plugin install, or piston import command - it is up to you) from our
SVN reposipory. For example:
./script/plugin install git://git@github.com:gsusmonzon/fast_sessions.git
2) Enable `ActiveRecord` session store in your `config/environment.rb` file:
Rails::Initializer.run do |config|
......
config.action_controller.session_store = :active_record_store
......
end
3) Create migration for your new sessions table:
./script/generate fast_session_migration AddFastSessions
4) Open your newly created migration and change `table_name` and `use_auto_increment`
parameters of the plugin (if you want to).
5) Run your migration:
rake db:migrate
6) Start your application and try to perform some actions which would definitely
save some data to your session. Then check your `fast_sessions` table (if you did
not renamed it) for a records.
==Downloading==
Original version of this plugin could be found on the project's site:
http://code.google.com/p/rails-fast-sessions/
or in SVN repository:
http://rails-fast-sessions.googlecode.com/svn/trunk/
==Author==
This plugin has been created by Alexey Kovyrin. Development is sponsored
by [http://www.scribd.com Scribd.com].
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