SwitchPoint

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Switching database connection between readonly one and writable one.

Installation

Add this line to your application's Gemfile:

gem 'switch_point'

And then execute:

$ bundle

Or install it yourself as:

$ gem install switch_point

Usage

See spec/models.

auto_writable

auto_writable is disabled by default.

When auto_writable is enabled, destructive queries is sent to writable connection even in readonly mode. But it does NOT work well on transactions.

Suppose after_save callback is set to User model. When User.create is called, it proceeds as follows.

  1. BEGIN TRANSACTION is sent to READONLY connection.
  2. switch_point switches the connection to WRITABLE.
  3. INSERT statement is sent to WRITABLE connection.
  4. switch_point reset the connection to READONLY.
  5. after_save callback is called.
    • At this point, the connection is READONLY and in a transaction.
  6. COMMIT TRANSACTION is sent to READONLY connection.

Internals

There's a proxy which holds two connections: readonly one and writable one. A proxy has a thread-local state indicating the current mode: readonly or writable.

Each ActiveRecord model refers to a proxy. ActiveRecord::Base.connection is hooked and delegated to the referred proxy.

When the writable connection is requested to execute destructive query, the readonly connection clears its query cache.

switch_point

Special case: ActiveRecord::Base.connection

Basically, each connection managed by a proxy isn't shared between proxies. But there's one exception: ActiveRecord::Base.

If :writable key is omitted (e.g., Nanika1 model in spec/models), it uses ActiveRecord::Base.connection as writable one. When ActiveRecord::Base.connection is requested to execute destructive query, all readonly connections managed by a proxy which uses ActiveRecord::Base.connection as a writable connection clear query cache.

Contributing

  1. Fork it ( https://github.com/eagletmt/switch_point/fork )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request