refill Docker container pool in small batches

config/docker.yml.erb

@@ -2,7 +2,8 @@ default: &default
   connection_timeout: 3
   pool:
     active: false
-    interval: 60
+    interval: 15
+    maximum_refill_count: 32
   ports: !ruby/range 4500..4600
 
 development:

lib/docker_container_pool.rb

@@ -36,7 +36,7 @@ class DockerContainerPool
 
   def self.refill
     ExecutionEnvironment.all.each do |execution_environment|
-      refill_count = execution_environment.pool_size - @containers[execution_environment.id].length
+      refill_count = [execution_environment.pool_size - @containers[execution_environment.id].length, config[:maximum_refill_count]].min
       if refill_count > 0
         Concurrent::Future.execute do
           @containers[execution_environment.id] += refill_count.times.map { create_container(execution_environment) }

spec/lib/docker_container_pool_spec.rb

@@ -91,7 +91,16 @@ describe DockerContainerPool do
     end
 
     context 'with something to refill' do
-      before(:each) { @execution_environment.update(pool_size: 1) }
+      let(:maximum_refill_count) { 5 }
+      before(:each) { @execution_environment.update(pool_size: 10) }
+
+      it 'complies with the maximum batch size' do
+        expect(DockerContainerPool::config).to receive(:[]).with(:maximum_refill_count).and_return(maximum_refill_count)
+        expect_any_instance_of(Concurrent::Future).to receive(:execute) do |future|
+          expect(DockerContainerPool).to receive(:create_container).with(@execution_environment).exactly(maximum_refill_count).times
+          future.instance_variable_get(:@task).call
+        end
+      end
 
       it 'works asynchronously' do
         expect(Concurrent::Future).to receive(:execute)