MONGOID-5445 Add load_async to criteria (#5454)

Co-authored-by: Neil Shweky <[email protected]>
Co-authored-by: Oleg Pudeyev <[email protected]>

docs/reference/configuration.txt

@@ -265,6 +265,16 @@ for details on driver options.
       # if the database name is not explicitly defined. (default: nil)
       app_name: MyApplicationName
 
+      # Type of executor for queries scheduled using ``load_async`` method.
+      #
+      # There are two possible values for this option:
+      #
+      #   - :immediate - Queries will be immediately executed on a current thread.
+      #       This is the default option.
+      #   - :global_thread_pool - Queries will be executed asynchronously in
+      #       background using a thread pool.
+      #async_query_executor: :immediate
+
       # Mark belongs_to associations as required by default, so that saving a
       # model with a missing belongs_to association will trigger a validation
       # error. (default: true)
@@ -358,6 +368,11 @@ for details on driver options.
       # Raise an exception when a field is redefined. (default: false)
       duplicate_fields_exception: false
 
+      # Defines how many asynchronous queries can be executed concurrently.
+      # This option should be set only if `async_query_executor` option is set
+      # to `:global_thread_pool`.
+      #global_executor_concurrency: nil
+
       # Include the root model name in json serialization. (default: false)
       include_root_in_json: false
 

docs/reference/queries.txt

@@ -2410,5 +2410,72 @@ will query the database and separately cache their results.
 To use the cached results, call ``all.to_a.first`` on the model class.
 
 
-.. _legacy-query-cache-limitations:
+.. _load-async:
 
+Asynchronous Queries
+====================
+
+Mongoid allows running database queries asynchronously in the background.
+This can be beneficial when there is a need to get documents from different
+collections.
+
+In order to schedule an asynchronous query call the ``load_async`` method on a
+``Criteria``:
+
+.. code-block:: ruby
+
+  class PagesController < ApplicationController
+    def index
+      @active_bands = Band.where(active: true).load_async
+      @best_events = Event.best.load_async
+      @public_articles = Article.where(public: true).load_async
+    end
+  end
+
+In the above example three queries will be scheduled for asynchronous execution.
+Results of the queries can be later accessed as usual:
+
+.. code-block:: html
+
+  <ul>
+    <%- @active_bands.each do -%>
+      <li><%= band.name %></li>
+    <%- end -%>
+  </ul>
+
+Even if a query is scheduled for asynchronous execution, it might be executed
+synchronously on the caller's thread. There are three possible scenarios depending
+on when the query results are being accessed:
+
+#. If the scheduled asynchronous task has been already executed, the results are returned.
+#. If the task has been started, but not finished yet, the caller's thread blocks until the task is finished.
+#. If the task has not been started yet, it is removed from the execution queue, and the query is executed synchronously on the caller's thread.
+
+.. note::
+
+  Even though ``load_async`` method returns a ``Criteria`` object, you should not
+  do any operations on this object except accessing query results. The query is
+  scheduled for execution immediately after calling ``load_async``, therefore
+  later changes to the `Criteria`` object may not be applied.
+
+
+Configuring asynchronous query execution
+----------------------------------------
+
+Asynchronous queries are disabled by default. When asynchronous queries are
+disabled, ``load_async`` will execute the query immediately on the current thread,
+blocking as necessary. Therefore, calling ``load_async`` on criteria in this case
+is roughly the equivalent of calling ``to_a`` to force query execution.
+
+In order to enable asynchronous query execution, the following config options
+must be set:
+
+.. code-block:: yaml
+
+  development:
+    ...
+    options:
+      # Execute asynchronous queries using a global thread pool.
+      async_query_executor: :global_thread_pool
+      # Number of threads in the pool. The default is 4.
+      # global_executor_concurrency: 4

docs/release-notes/mongoid-8.1.txt

@@ -17,6 +17,11 @@ The complete list of releases is available `on GitHub
 please consult GitHub releases for detailed release notes and JIRA for
 the complete list of issues fixed in each release, including bug fixes.
 
+Added ``load_async`` method on ``Criteria`` to asynchronously load documents
+----------------------------------------------------------------------------
+
+The new ``load_async`` method on ``Criteria`` allows :ref:`running database queries asynchronously <load-async>`.
+
 
 Added ``attribute_before_last_save``, ``saved_change_to_attribute``, ``saved_change_to_attribute?``, and ``will_save_change_to_attribute?``  methods
 ----------------------------------------------------------------------------------------------------------------------------------------------------
@@ -245,8 +250,8 @@ for more details.
 Added ``readonly!`` method and ``legacy_readonly`` feature flag
 ---------------------------------------------------------------
 
-Mongoid 8.1 changes the meaning of read-only documents. In Mongoid 8.1 with 
-this feature flag turned off, a document becomes read-only when calling the 
+Mongoid 8.1 changes the meaning of read-only documents. In Mongoid 8.1 with
+this feature flag turned off, a document becomes read-only when calling the
 ``readonly!`` method:
 
 .. code:: ruby
@@ -261,8 +266,8 @@ this feature flag turned off, a document becomes read-only when calling the
 With this feature flag turned off, a ``ReadonlyDocument`` error will be
 raised when destroying or deleting, as well as when saving or updating.
 
-Prior to Mongoid 8.1 and in 8.1 with the ``legacy_readonly`` feature flag 
-turned on, documents become read-only when they are projected (i.e. using 
+Prior to Mongoid 8.1 and in 8.1 with the ``legacy_readonly`` feature flag
+turned on, documents become read-only when they are projected (i.e. using
 ``#only`` or ``#without``).
 
 .. code:: ruby
@@ -278,7 +283,7 @@ turned on, documents become read-only when they are projected (i.e. using
   band.destroy # => raises ReadonlyDocument error
 
 Note that with this feature flag on, a ``ReadonlyDocument`` error will only be
-raised when destroying or deleting, and not on saving or updating. See the 
+raised when destroying or deleting, and not on saving or updating. See the
 section on :ref:`Read-only Documents <readonly-documents>` for more details.
 
 

lib/config/locales/en.yml

@@ -122,6 +122,14 @@ en:
             A collation option is only supported if the query is executed on a MongoDB server
             with version >= 3.4."
           resolution: "Remove the collation option from the query."
+        invalid_async_query_executor:
+          message: "Invalid async_query_executor option: %{executor}."
+          summary: "A invalid async query executor was specified.
+            The valid options are: %{options}."
+          resolution: "Pick an allowed option or fix the typo. If you were
+            expecting the option to be there, please consult the following page
+            with respect to Mongoid's configuration:\n\n
+            \_\_https://www.mongodb.com/docs/mongoid/current/reference/configuration/#mongoid-configuration-options"
         invalid_config_file:
           message: "Invalid configuration file: %{path}."
           summary: "Your mongoid.yml configuration file does not contain the
@@ -222,6 +230,13 @@ en:
           resolution: "Please provide a valid type value for the field.
             Refer to:
             https://docs.mongodb.com/mongoid/current/reference/fields/#using-symbols-or-strings-instead-of-classes"
+        invalid_global_executor_concurrency:
+          message: "Invalid global_executor_concurrency option."
+          summary: "You set global_executor_concurrency while async_query_executor
+            option is not set to :global_thread_pool. The global_executor_concurrency is
+            allowed only for the global thread pool executor."
+          resolution: "Set global_executor_concurrency option to :global_thread_pool
+            or remove global_executor_concurrency option."
         invalid_includes:
           message: "Invalid includes directive: %{klass}.includes(%{args})"
           summary: "Eager loading in Mongoid only supports providing arguments

lib/mongoid.rb

@@ -12,6 +12,8 @@
 require "active_support/time_with_zone"
 require "active_model"
 
+require 'concurrent-ruby'
+
 require "mongo"
 require 'mongo/active_support'
 

lib/mongoid/config.rb

@@ -127,7 +127,20 @@ module Config
     # always return a Hash.
     option :legacy_attributes, default: false
 
-    # When this flag is false, a document will become read-only only once the 
+    # Sets the async_query_executor for the application. By default the thread pool executor
+    #   is set to `:immediate. Options are:
+    #
+    #   - :immediate - Initializes a single +Concurrent::ImmediateExecutor+
+    #   - :global_thread_pool - Initializes a single +Concurrent::ThreadPoolExecutor+
+    #      that uses the +async_query_concurrency+ for the +max_threads+ value.
+    option :async_query_executor, default: :immediate
+
+    # Defines how many asynchronous queries can be executed concurrently.
+    # This option should be set only if `async_query_executor` is set
+    # to `:global_thread_pool`.
+    option :global_executor_concurrency, default: nil
+
+    # When this flag is false, a document will become read-only only once the
     # #readonly! method is called, and an error will be raised on attempting
     # to save or update such documents, instead of just on delete. When this
     # flag is true, a document is only read-only if it has been projected
@@ -308,6 +321,7 @@ def truncate!
     # @param [ Hash ] options The configuration options.
     def options=(options)
       if options
+        Validators::AsyncQueryExecutor.validate(options)
         options.each_pair do |option, value|
           Validators::Option.validate(option)
           send("#{option}=", value)

lib/mongoid/config/validators.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
 
+require "mongoid/config/validators/async_query_executor"
 require "mongoid/config/validators/option"
 require "mongoid/config/validators/client"

lib/mongoid/config/validators/async_query_executor.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Mongoid
+  module Config
+    module Validators
+
+      # Validator for async query executor configuration.
+      #
+      # @api private
+      module AsyncQueryExecutor
+        extend self
+
+
+        def validate(options)
+          if options.key?(:async_query_executor)
+            if options[:async_query_executor].to_sym == :immediate && !options[:global_executor_concurrency].nil?
+              raise Errors::InvalidGlobalExecutorConcurrency
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/mongoid/contextual.rb

@@ -35,6 +35,18 @@ def context
       @context ||= create_context
     end
 
+    # Instructs the context to schedule an asynchronous loading of documents
+    # specified by the criteria.
+    #
+    # Note that depending on the context and on the Mongoid configuration,
+    # documents can be loaded synchronously on the caller's thread.
+    #
+    # @return [ Criteria ] Returns self.
+    def load_async
+      context.load_async if context.respond_to?(:load_async)
+      self
+    end
+
     private
 
     # Create the context for the queries to execute. Will be memory for

lib/mongoid/contextual/mongo.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "mongoid/contextual/mongo/documents_loader"
 require "mongoid/contextual/atomic"
 require "mongoid/contextual/aggregable/mongo"
 require "mongoid/contextual/command"
@@ -37,6 +38,8 @@ class Mongo
       # @attribute [r] view The Mongo collection view.
       attr_reader :view
 
+      attr_reader :documents_loader
+
       # Get the number of documents matching the query.
       #
       # @example Get the number of matching documents.
@@ -773,6 +776,17 @@ def third_to_last!
         third_to_last || raise_document_not_found_error
       end
 
+      # Schedule a task to load documents for the context.
+      #
+      # Depending on the Mongoid configuration, the scheduled task can be executed
+      # immediately on the caller's thread, or can be scheduled for an
+      # asynchronous execution.
+      #
+      # @api private
+      def load_async
+        @documents_loader ||= DocumentsLoader.new(view, klass, criteria)
+      end
+
       private
 
       # Update the documents for the provided method.
@@ -840,24 +854,29 @@ def inverse_sorting
         Hash[sort.map{|k, v| [k, -1*v]}]
       end
 
-      # Get the documents the context should iterate. This follows 3 rules:
-      #
-      # 1. If the query is cached, and we already have documents loaded, use
-      #   them.
-      # 2. If we are eager loading, then eager load the documents and use
-      #   those.
-      # 3. Use the query.
-      #
-      # @api private
+      # Get the documents the context should iterate.
       #
-      # @example Get the documents for iteration.
-      #   context.documents_for_iteration
+      # If the documents have been already preloaded by `Document::Loader`
+      # instance, they will be used.
       #
       # @return [ Array<Document> | Mongo::Collection::View ] The docs to iterate.
+      #
+      # @api private
       def documents_for_iteration
-        return view unless eager_loadable?
-        docs = view.map{ |doc| Factory.from_db(klass, doc, criteria) }
-        eager_load(docs)
+        if @documents_loader
+          if @documents_loader.started?
+            @documents_loader.value!
+          else
+            @documents_loader.unschedule
+            @documents_loader.execute
+          end
+        else
+          return view unless eager_loadable?
+          docs = view.map do |doc|
+            Factory.from_db(klass, doc, criteria)
+          end
+          eager_load(docs)
+        end
       end
 
       # Yield to the document.