[ecs-patterns]: allow use of existing load balancer in ApplicationMultipleTargetGroupsEc2Service

This is a 🚀 Feature Request

Description

Would like to be able to use an existing load balancer with ApplicationMultipleTargetGroupsEc2Service.

Load balancer pricing includes a fee per hour of load balancer running, which currently means about $18 per month. Running many services, each with its own balancer, can quickly add up.

Additionally, creating a separate load balancer for each service means we can’t run multiple services under the same DNS name. So it’s not possible to call different microservices depending on the path, for example (probably a common pattern?). Or group multiple subdomains on the same load balancer.

Use Case

const service1 = new ApplicationMultipleTargetGroupsEc2Service(this, "Service1",{
    ...
    targetGroups: [
             {
                containerPort: 80,
                listener: "listener",
                **pathPattern: "service1",**
                priority: 1,
                protocol: Protocol.TCP,
              },
});
const service2 = new ApplicationMultipleTargetGroupsEc2Service(this, "Service2",{ 
    targetGroups: [
              {
                containerPort: 80,
                listener: "listener",
                **pathPattern: "service2",**
                priority: 2,
                protocol: Protocol.TCP,
              },
});

I would like that that two gets deployed to (Use the same LoadBalancer, instead of creating one for Each Service):

• myalb.amazonaws.com/service1
• myalb.amazonaws.com/service2

Proposed Solution

Not have a workaround. Would need to skip ECS Patterns for this.

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

[pmaoui]

@ggallotti If you use 1 port to 1 service, ApplicationMultipleTargetGroupsEc2Service doesn’t look like the way to go as you would need only one target group per service. From what I learned, ECS Patterns are suitable for fairly generic needs and doesn’t support usage of existing resources like load balancers.
To use an existing ALB, I would recommend to import its listener and use the Ec2Service class . You can create many services with many target groups and linked them to one listener.

Here an example to attach one service to one target group to an existing listener:

const yourALBListener = ApplicationListener.fromLookup(this, 'appListener', {
    listenerArn: 'the:arn:of-your-elb-listener-here',
})

const service1 = new Ec2Service(this, 'Service1', {
    ...
})

const service1Target = service1.loadBalancerTarget({
    containerName: 'your-container-name-from-service1',
    containerPort: 80,
    protocol: Protocol.TCP,
})

const group1 = new ApplicationTargetGroup(this, 'DashboardTargetGroup', {
    vpc: YourVpc,
    port: 80,
    targetType: TargetType.IP,
})

const group1Rule = new ApplicationListenerRule(this, 'group1rule', {
    pathPattern: 'service1',
    listener: yourALBListener,
    priority: 10,
})

service1Target.attachToApplicationTargetGroup(group1)
group1Rule.addTargetGroup(group1)

Of course, I would be interested if someone is aware of a more concise approach.

[ggallotti]

Thanks for the code @poupougnac. Seems very simple and clean. Yes, I think that for now I would need to Skip ECS Patterns for this.