fix(aws-ecs): don't emit DesiredCount in daemon mode

packages/@aws-cdk/aws-ecs/lib/base/base-service.ts

@@ -104,7 +104,7 @@ export abstract class BaseService extends cdk.Construct
     this.taskDefinition = taskDefinition;
 
     this.resource = new cloudformation.ServiceResource(this, "Service", {
-      desiredCount: props.desiredCount || 1,
+      desiredCount: props.desiredCount,
       serviceName: props.serviceName,
       loadBalancers: new cdk.Token(() => this.loadBalancers),
       deploymentConfiguration: {

packages/@aws-cdk/aws-ecs/lib/ec2/ec2-service.ts

@@ -81,7 +81,12 @@ export class Ec2Service extends BaseService implements elb.ILoadBalancerTarget {
       throw new Error('Supplied TaskDefinition is not configured for compatibility with EC2');
     }
 
-    super(parent, name, props, {
+    super(parent, name, {
+      ...props,
+      // If daemon, desiredCount must be undefined and that's what we want. Otherwise, default to 1.
+      desiredCount: props.daemon || props.desiredCount !== undefined ? props.desiredCount : 1,
+    },
+    {
       cluster: props.cluster.clusterName,
       taskDefinition: props.taskDefinition.taskDefinitionArn,
       launchType: 'EC2',

packages/@aws-cdk/aws-ecs/lib/fargate/fargate-service.ts

@@ -60,7 +60,10 @@ export class FargateService extends BaseService {
       throw new Error('Supplied TaskDefinition is not configured for compatibility with Fargate');
     }
 
-    super(parent, name, props, {
+    super(parent, name, {
+      ...props,
+      desiredCount: props.desiredCount !== undefined ? props.desiredCount : 1,
+    }, {
       cluster: props.cluster.clusterName,
       taskDefinition: props.taskDefinition.taskDefinitionArn,
       launchType: 'FARGATE',

packages/@aws-cdk/aws-ecs/test/ec2/test.ec2-service.ts

@@ -56,6 +56,10 @@ export = {
       const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
       cluster.addDefaultAutoScalingGroupCapacity({ instanceType: new ec2.InstanceType('t2.micro') });
       const taskDefinition = new ecs.Ec2TaskDefinition(stack, 'Ec2TaskDef');
+      taskDefinition.addContainer('BaseContainer', {
+        image: ecs.ContainerImage.fromDockerHub('test'),
+        memoryReservationMiB: 10,
+      });
 
       // THEN
       test.throws(() => {
@@ -65,8 +69,35 @@ export = {
           daemon: true,
           desiredCount: 2
         });
+      }, /Don't supply desiredCount/);
+
+      test.done();
+    },
+
+    'Output does not contain DesiredCount if daemon mode is set'(test: Test) {
+      // GIVEN
+      const stack = new cdk.Stack();
+      const vpc = new ec2.VpcNetwork(stack, 'MyVpc', {});
+      const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
+      cluster.addDefaultAutoScalingGroupCapacity({ instanceType: new ec2.InstanceType('t2.micro') });
+      const taskDefinition = new ecs.Ec2TaskDefinition(stack, 'Ec2TaskDef');
+      taskDefinition.addContainer('BaseContainer', {
+        image: ecs.ContainerImage.fromDockerHub('test'),
+        memoryReservationMiB: 10,
       });
 
+      // WHEN
+      new ecs.Ec2Service(stack, "Ec2Service", {
+        cluster,
+        taskDefinition,
+        daemon: true,
+      });
+
+      // THEN
+      expect(stack).to(haveResource('AWS::ECS::Service', (service: any) => {
+        return service.LaunchType === 'EC2' && service.DesiredCount === undefined;
+      }));
+
       test.done();
     },