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. :-)
gem install data_fabric
then in config/environment.rb, add a config.gem entry:
Rails::Initializer.run do |config| ... config.gem 'data_fabric'
or add data_fabric to your Gemfile:
data_fabric has been tested with Rails 2.3.10 and 3.0.7.
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) .(:city => @current_user.city, &block) end end
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 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 example23 or example30 application to reproduce the bug and send me the diff.
- 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
Copyright © 2010 Mike Perham
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