1. The Request Lifecycle (Client to Task)
When a user makes a request to your application, the flow is strictly hierarchical. The Load Balancer does not "know" about the Service logic; it only knows IP addresses.
- Client: Sends a request (e.g.,
GET /api) to the Load Balancer's DNS name.
- Load Balancer (ALB): Receives the request on a specific Listener (e.g., Port 80).
- Target Group (The Registry): The ALB checks the Target Group associated with that Listener. The Target Group contains a list of IP addresses and ports of currently healthy Tasks.
- Task (The Worker): The ALB picks one IP from the Target Group and forwards the request directly to that Task. The Task processes it and responds.
2. Service vs. Task: Who does what?
- ECS Service (The Manager):
- It is the "Brain" or "Scheduler".
- It ensures that X number of copies (Desired Count) are always running.
- It registers new Tasks into the Target Group automatically.
- It is part of the Control Plane.
- ECS Task (The Worker):
- It is the actual running container (Application code).
- It handles the actual traffic.
- It is part of the Data Plane.
3. Key Takeaway: "The Service Crash Scenario"
Scenario: Service Management Logic Fails
If your Service crashes (the management logic fails or CloudFormation stack is stuck), your application will stay online.
Why? Because the Load Balancer talks to the Tasks (via the Target Group registry), not to the Service. As long as the Tasks (containers) are alive on the EC2/Fargate infrastructure, traffic flows.
The Catch (No Auto-Healing)
Because the "Manager" (Service) is dead, if a Task fails now (e.g., runs out of memory), a new Task will NOT start automatically. The brain is gone, so the body cannot heal itself.
4. Additional Q&A
Q: If I delete my Service, do Tasks delete?
A: Yes. When you explicitly delete a Service via Console/CLI, the Service sends a "Stop" signal to all its Tasks before it dies. This is different from the Service "crashing" or being "unreachable."
Q: Can 3 tasks in 1 Service have different Target Groups?
A: No. Tasks in a Service are clones. The Service registers all its tasks to the Target Group(s) you defined. You cannot route Task A to TG-1 and Task B to TG-2. They all go to the same place(s).
