Module: Sequel::Plugins::ConcurrentEagerLoading

Defined in:
lib/sequel/plugins/concurrent_eager_loading.rb

Overview

The concurrent_eager_loading plugin allows for eager loading multiple associations concurrently in separate threads. You must load the async_thread_pool Database extension into the Database object the model class uses in order for this plugin to work.

By default in Sequel, eager loading happens in a serial manner. If you have code such as:

Album.eager(:artist, :genre, :tracks)

Sequel will load the albums, then the artists for the albums, then the genres for the albums, then the tracks for the albums.

With the concurrent_eager_loading plugin, you can use the eager_load_concurrently method to allow for concurrent eager loading:

Album.eager_load_concurrently.eager(:artist, :genre, :tracks)

This will load the albums, first, since it needs to load the albums to know which artists, genres, and tracks to eagerly load. However, it will load the artists, genres, and tracks for the albums concurrently in separate threads. This can significantly improve performance, especially if there is significant latency between the application and the database. Note that using separate threads is only used in the case where there are multiple associations to eagerly load. With only a single association to eagerly load, there is no reason to use a separate thread, since it would not improve performance.

If you want to make concurrent eager loading the default, you can load the plugin with the :always option. In this case, all eager loads will be concurrent. If you want to force a non-concurrent eager load, you can use eager_load_serially:

Album.eager_load_serially.eager(:artist, :genre, :tracks)

Note that making concurrent eager loading the default is probably a bad idea if you are eager loading inside transactions and want the eager load to reflect changes made inside the transaction, unless you plan to use eager_load_serially for such cases. See the async_thread_pool Database extension documentation for more general caveats regarding its use.

The default eager loaders for all of the association types that ship with Sequel support safe concurrent eager loading. However, if you are specifying a custom :eager_loader for an association, it may not work safely unless it it modified to support concurrent eager loading. Taking this example from the Advanced Associations guide

Album.many_to_one :artist, :eager_loader=>(proc do |eo_opts|
  eo_opts[:rows].each{|album| album.associations[:artist] = nil}
  id_map = eo_opts[:id_map]
  Artist.where(:id=>id_map.keys).all do |artist|
    if albums = id_map[artist.id]
      albums.each do |album|
        album.associations[:artist] = artist
      end
    end
  end
end)

This would not support concurrent eager loading safely. To support safe concurrent eager loading, you need to make sure you are not modifying the associations for objects concurrently by separate threads. This is implemented using a mutex, which you can access via eo_opts[:mutex]. To keep things simple, you can use Sequel.synchronize_with to only use this mutex if it is available. You want to use the mutex around the code that initializes the associations (usually to nil or []), and also around the code that sets the associatied objects appropriately after they have been retreived. You do not want to use the mutex around the code that loads the objects, since that will prevent concurrent loading. So after the changes, the custom eager loader would look like this:

Album.many_to_one :artist, :eager_loader=>(proc do |eo_opts|
  Sequel.synchronize_with(eo[:mutex]) do
    eo_opts[:rows].each{|album| album.associations[:artist] = nil}
  end
  id_map = eo_opts[:id_map]
  rows = Artist.where(:id=>id_map.keys).all
  Sequel.synchronize_with(eo[:mutex]) do
    rows.each do |artist|
      if albums = id_map[artist.id]
        albums.each do |album|
          album.associations[:artist] = artist
        end
      end
    end
  end
end)

Usage:

# Make all model subclass datasets support concurrent eager loading
Sequel::Model.plugin :concurrent_eager_loading

# Make the Album class datasets support concurrent eager loading
Album.plugin :concurrent_eager_loading

# Make all model subclass datasets concurrently eager load by default
Sequel::Model.plugin :concurrent_eager_loading, always: true

Defined Under Namespace

Modules: ClassMethods, DatasetMethods

Class Method Summary collapse

Class Method Details

.configure(mod, opts = OPTS) ⇒ Object


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# File 'lib/sequel/plugins/concurrent_eager_loading.rb', line 105

def self.configure(mod, opts=OPTS)
  if opts.has_key?(:always)
    mod.instance_variable_set(:@always_eager_load_concurrently, opts[:always])
  end
end