ECS: Fargate service replacement doesn't preserve desired task count

[zkello]

Describe the bug

My team recently ran into this bug which caused our Fargate service to be replaced instead of updated during a deployment. Not ideal, but during the replacement we observed some behaviors which surprised us. Basically the service was replaced as soon as a single task in the new service was up and running. This caused the service to suddenly be under-scaled for the traffic it was receiving. Replacements of Fargate services are rare, but it seems like CDK/ECS could be smarter about how it's done.

Expected Behavior

1. The old Fargate service shouldn't be replaced until the new service reaches its desired task count or the very least its minimum task count.
2. The new service should have the desired task count set to that of the old service at time of deployment. This will prevent the service from being brought online under-scaled.

Current Behavior

1. Traffic from our NLB to the old Fargate service was cut off as soon as a single task was up in running in the new Fargate service, despite the minimum task count in the service being set to 3
2. The desired task count was set the minimum task count despite the desired task count of the old service being set much higher.

Reproduction Steps

• Have a Fargate service deployed with the following:
  • Minimum task count set above 1
  • Desired task count set above minium task count based on autoscaling policy (cpu in our case)
• Make a CDK change that requires a replacement of the service.
• Observe service is replaced as soon as new service has one task healthy

Possible Solution

No response

Additional Information/Context

No response

CDK CLI Version

2.59.0

Framework Version

No response

Node.js Version

18

OS

Linux

Language

Typescript

Language Version

No response

Other information

No response

[peterwoodworth]

Could you provide reproduction code please?

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