Add thread pool and concurrency_max_threads configuration option

lib/graphiti/configuration.rb

@@ -8,6 +8,20 @@ class Configuration
     #   Defaults to false OR if classes are cached (Rails-only)
     attr_accessor :concurrency
 
+    # This number must be considered in accordance with the database
+    # connection pool size configured in `database.yml`. The connection
+    # pool should be large enough to accommodate both the foreground
+    # threads (ie. web server or job worker threads) and background
+    # threads. For each process, Graphiti will create one global
+    # executor that uses this many threads to sideload resources
+    # asynchronously. Thus, the pool size should be at least
+    # `thread_count + concurrency_max_threads + 1`. For example, if your
+    # web server has a maximum of 3 threads, and
+    # `concurrency_max_threads` is set to 4, then your pool size should
+    # be at least 8.
+    # @return [Integer] Maximum number of threads to use when fetching sideloads concurrently
+    attr_accessor :concurrency_max_threads
+
     attr_accessor :respond_to
     attr_accessor :context_for_endpoint
     attr_accessor :links_on_demand
@@ -26,6 +40,7 @@ class Configuration
     def initialize
       @raise_on_missing_sideload = true
       @concurrency = false
+      @concurrency_max_threads = 4
       @respond_to = [:json, :jsonapi, :xml]
       @links_on_demand = false
       @pagination_links_on_demand = false

lib/graphiti/scope.rb

@@ -2,6 +2,23 @@ module Graphiti
   class Scope
     attr_accessor :object, :unpaginated_object
     attr_reader :pagination
+
+    @thread_pool_executor_mutex = Mutex.new
+
+    def self.thread_pool_executor
+      return @thread_pool_executor if @thread_pool_executor
+
+      concurrency = Graphiti.config.concurrency_max_threads || 4
+      @thread_pool_executor_mutex.synchronize do
+        @thread_pool_executor ||= Concurrent::ThreadPoolExecutor.new(
+          min_threads: 0,
+          max_threads: concurrency,
+          max_queue: concurrency * 4,
+          fallback_policy: :caller_runs
+        )
+      end
+    end
+
     def initialize(object, resource, query, opts = {})
       @object = object
       @resource = resource
@@ -49,7 +66,7 @@ def resolve_sideloads(results)
           @resource.adapter.close if concurrent
         }
         if concurrent
-          promises << Concurrent::Promise.execute(&resolve_sideload)
+          promises << Concurrent::Promise.execute(executor: self.class.thread_pool_executor, &resolve_sideload)
         else
           resolve_sideload.call
         end

spec/configuration_spec.rb

@@ -150,6 +150,21 @@
     end
   end
 
+  describe "#concurrency_max_threads" do
+    include_context "with config", :concurrency_max_threads
+
+    it "defaults" do
+      expect(Graphiti.config.concurrency_max_threads).to eq(4)
+    end
+
+    it "is overridable" do
+      Graphiti.configure do |c|
+        c.concurrency_max_threads = 1
+      end
+      expect(Graphiti.config.concurrency_max_threads).to eq(1)
+    end
+  end
+
   describe "#raise_on_missing_sideload" do
     include_context "with config", :raise_on_missing_sideload