= data_fabric

DataFabric provides flexible database connection switching for ActiveRecord.

We needed two features to scale our mysql database: application-level sharding and 
master/slave replication.  Sharding is the process of splitting a dataset across many 
independent databases.  This often happens based on geographical region (e.g. 
craigslist) or category (e.g. ebay).  Replication provides a near-real-time copy 
of a database which can be used for fault tolerance and to reduce load on the master 
node.  Combined, you get a scalable database solution which does not require huge 
hardware to scale to huge volumes.  Or: DPAYEIOB - don't put all your eggs in one 
basket.  :-)

== Installation

  gem install data_fabric

then in config/environment.rb, add a config.gem entry:

  Rails::Initializer.run do |config|
    ...
    config.gem 'data_fabric'


== How does it work?

You describe the topology for your database infrastructure in your model(s).  
Different models can use different topologies.

  class MyHugeVolumeOfDataModel < ActiveRecord::Base
    data_fabric :replicated => true, :shard_by => :city
  end

There are four supported modes of operation, depending on the options given to the 
data_fabric method.  The plugin will look for connections in your 
config/database.yml with the following convention:

No connection topology:
#{environment} - this is the default, as with ActiveRecord, e.g. "production"

  data_fabric :replicated => true
#{environment}_#{role} - no sharding, just replication, where role is "master" or "slave", e.g. "production_master"

  data_fabric :shard_by => :city
#{group}_#{shard}_#{environment} - sharding, no replication, e.g. "city_austin_production"

  data_fabric :replicated => true, :shard_by => :city
#{group}_#{shard}_#{environment}_#{role} - sharding with replication, e.g. "city_austin_production_master"


When marked as replicated, all write and transactional operations for the model 
go to the master, whereas read operations go to the slave.

Since sharding is an application-level concern, your application must set the shard
to use based on the current request or environment.  The current shard
is set on a thread local variable.  For example, you can set the shard in an 
ActionController around_filter based on the user as follows:

  class ApplicationController < ActionController::Base
    around_filter :select_shard

    private
    def select_shard(&block)
      DataFabric.activate_shard(:city => @current_user.city, &block)
    end
  end


== Warnings

* Sharded models should never be placed in the session store or you will get "Shard not set" errors when the session is persisted.
* DataFabric does not support running with ActiveRecord's allow_concurrency = true in AR 2.0 and 2.1.  allow_concurrency is gone in AR 2.2.
* DataFabric does not work in development mode with cache_classes = false.  You will get an error in ActiveRecord's clear_reloadable_connections! method.

== Testing and Bug Reports

If you think you've found a problem with data_fabric, please use the example application to reproduce the bug and send me the diff.  The example application is a stock Rails 2.1 application which uses data_fabric in the expected fashion.


== Thanks to...

Rick Olsen of Rails Core:: for the Masochism plugin, which showed me how to bend AR's connection handling to my will
Bradley Taylor of RailsMachine:: for the advice to shard while at acts_as_conference
FiveRuns:: for paying me to develop this code and allowing its release


== Author

Mike Perham <mperham@gmail.com>
http://mikeperham.com


== LICENSE:
 
(The FiveRuns License)
 
Copyright (c) 2008 FiveRuns Corporation
 
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'Software'), to deal in the Software without restriction, including
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permit persons to whom the Software is furnished to do so, subject to
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The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
 
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