ecs-patterns: allow use of existing load balancer in Load Balanced Services

[bgdnlp]

🚀 Feature Request

General Information

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

Description

Would like to be able to use an existing load balancer with LoadBalancedFargateService and LoadBalancedEc2Service.

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.

Proposed Solution

Allow passing an existing load balancer as a parameter, the same way it is now possible to specify an existing vpc or cluster.

If an existing load balancer is used, passing any parameter that changes the behavior of the load balancer ( #3691 ) should probably return an error or be ignored, as it might interfere with the behavior of other services. Adding a new listener, or adding a target group to an existing listener may be ok, but changing subnets for an existing load balancer probably shouldn't be done here.

Environment

• **CDK CLI Version:1.4.0
• **Module Version:1.4.0
• **OS:all
• **Language:all

Other information

[piradeepk]

@bgdnlp Thanks for the feature request! We're looking into the request and will get back to you once we have more information to provide.

[piradeepk]

Resolved as part of #4213

[ggallotti]

Could we reuse ALB with ApplicationMultipleTargetGroupsEc2Service?

So we can have multiples ApplicationMultipleTargetGroupsEc2Service using the same Load Balancer?