WIP: Async and deferred loaders

examples/http_loader.rb

@@ -43,27 +43,43 @@ def initialize(host:, size: 4, timeout: 4)
       @host = host
       @size = size
       @timeout = timeout
+      @futures = {}
     end
 
-    def perform(operations)
+    def perform_on_wait(operations)
       # This fans out and starts off all the concurrent work, which starts and
       # immediately returns Concurrent::Promises::Future` objects for each operation.
+      operations.each do |operation|
+        future(operation)
+      end
+    end
+
+    def perform(operations)
+      # Defer to let other non-async loaders run to completion first.
+      defer
+
+      # Collect the futures (and possibly trigger any newly added ones)
       futures = operations.map do |operation|
-        Concurrent::Promises.future do
-          pool.with { |connection| operation.call(connection) }
-        end
+        future(operation)
       end
+
       # At this point, all of the concurrent work has been started.
 
       # This converges back in, waiting on each concurrent future to finish, and fulfilling each
       # (non-concurrent) Promise.rb promise.
       operations.each_with_index.each do |operation, index|
-        fulfill(operation, futures[index].value) # .value is a blocking call
+        fulfill(operation, futures[index].value!) # .value is a blocking call
       end
     end
 
   private
 
+    def future(operation)
+      @futures[operation] ||= Concurrent::Promises.future do
+        pool.with { |connection| operation.call(connection) }
+      end
+    end
+
     def pool
       @pool ||= ConnectionPool.new(size: @size, timeout: @timeout) do
         HTTP.persistent(@host)

lib/graphql/batch/executor.rb

@@ -53,6 +53,19 @@ def resolve(loader)
       @loading = was_loading
     end
 
+    def defer(loader)
+      while (non_deferred_loader = @loaders.find { |_, loader| !loader.deferred })
+        resolve(non_deferred_loader)
+      end
+    end
+
+    def on_wait
+      # FIXME: Better name?
+      @loaders.each do |_, loader|
+        loader.on_any_wait
+      end
+    end
+
     def tick
       resolve(@loaders.shift.last)
     end

lib/graphql/batch/loader.rb

@@ -42,13 +42,14 @@ def current_executor
       end
     end
 
-    attr_accessor :loader_key, :executor
+    attr_accessor :loader_key, :executor, :deferred
 
     def initialize
       @loader_key = nil
       @executor = nil
       @queue = nil
       @cache = nil
+      @deferred = false
     end
 
     def load(key)
@@ -66,6 +67,14 @@ def prime(key, value)
       cache[cache_key(key)] ||= ::Promise.resolve(value).tap { |p| p.source = self }
     end
 
+    def on_any_wait
+      return if resolved?
+      load_keys = queue # "Peek" the queue, but don't consume it.
+      # TODO: Should we have a "peek queue" / "async queue", that we can consume here, to prevent
+      # duplicate calls to perform_on_wait? (perform_on_wait should be idempotent anyway, but...)
+      perform_on_wait(load_keys)
+    end
+
     def resolve # :nodoc:
       return if resolved?
       load_keys = queue
@@ -88,6 +97,7 @@ def around_perform
     # For Promise#sync
     def wait # :nodoc:
       if executor
+        executor.on_wait
         executor.resolve(self)
       else
         resolve
@@ -126,6 +136,36 @@ def fulfilled?(key)
       promise.pending? && promise.source != self
     end
 
+    def perform_on_wait(keys)
+      # FIXME: Better name?
+      # Interface to add custom code to e.g. trigger async operations when any loader starts waiting.
+      # Example:
+      #
+      #   def initialize
+      #     super()
+      #     @futures = {}
+      #   end
+      #
+      #   def perform_on_wait(keys)
+      #     keys.each do |key|
+      #       future(key)
+      #     end
+      #   end
+      #
+      #   def perform(keys)
+      #     defer # let other non-async loaders run to completion first.
+      #     keys.each do |key|
+      #       future(key).value!
+      #     end
+      #   end
+      #
+      #   def future(key)
+      #     @futures[key] ||= Concurrent::Promises.future do
+      #       # Perform the async operation
+      #     end
+      #   end
+    end
+
     # Must override to load the keys and call #fulfill for each key
     def perform(keys)
       raise NotImplementedError
@@ -146,6 +186,13 @@ def finish_resolve(key)
       end
     end
 
+    def defer
+      @deferred = true
+      executor.defer(self)
+    ensure
+      @deferred = false
+    end
+
     def cache
       @cache ||= {}
     end