Services and tasks in AWS - Step-by-Step Execution

Process Flow - Services and tasks

Create Task Definition

↓

Register Task Definition

↓

Create Service

↓

Service Launches Tasks

↓

Tasks Run Containers

↓

Monitor & Scale Tasks

↓

Update Service

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Replace Tasks

This flow shows how you define a task, register it, create a service to run tasks, and how the service manages running and updating tasks.

Execution Sample

AWS

1. Define task with container info
2. Register task definition
3. Create service with task definition
4. Service launches tasks
5. Tasks run containers

This sequence shows how AWS ECS runs containers by defining tasks and services.

Process Table

Step	Action	Input	Result	Notes
1	Define Task	Container image: nginx	Task Definition created	Defines container and resources
2	Register Task	Task Definition JSON	Task Definition ARN	Task is saved and versioned
3	Create Service	Task Definition ARN, desired count=2	Service created	Service manages tasks
4	Service Launches Tasks	Desired count=2	2 Tasks started	Tasks run containers
5	Tasks Running	Container image nginx	Containers running	Containers serve traffic
6	Monitor & Scale	CPU usage high	Service scales to 3 tasks	More tasks launched
7	Update Service	New Task Definition version	Service updates tasks	Old tasks replaced
8	Tasks Replaced	Old tasks stopped	New tasks running	Service stable with new version
9	Exit	Desired count met	Stable service	No further action needed

💡 Service reaches desired count and tasks run containers successfully

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	After Step 8	Final
Task Definition	None	Registered v1	Registered v1	Registered v1	Registered v2	Registered v2
Service Desired Count	0	2	2	3	3	3
Running Tasks	0	0	2	3	3	3
Task Version	None	v1	v1	v1	v2	v2

Key Moments - 3 Insights

Why does the service launch multiple tasks instead of just one?

Because the service's desired count is set to more than one (2 in step 3), so it launches that many tasks to run containers (see execution_table step 4).

What happens when the service updates to a new task definition version?

The service replaces old tasks with new tasks running the updated version (see execution_table steps 7 and 8). This ensures smooth updates without downtime.

How does the service decide to scale up the number of tasks?

It monitors metrics like CPU usage and scales tasks up or down to meet demand (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, how many tasks does the service launch initially?

A 1

B 2

C 3

D 0

💡 Hint

Check step 4 in the execution_table where the service launches tasks.

At which step does the service update to a new task definition version?

A Step 5

B Step 6

C Step 7

D Step 8

💡 Hint

Look for the step labeled 'Update Service' in the execution_table.

If the desired count changes from 2 to 3, what happens in the execution table?

A Service scales to 3 tasks

B Service deletes task definitions

C Service stops tasks

D No change

💡 Hint

See step 6 where the service scales tasks based on CPU usage.

Concept Snapshot

AWS ECS runs containers using tasks and services.
Define a task with container info.
Register task definition to save it.
Create a service to run tasks.
Service launches and manages tasks.
Update service to replace tasks with new versions.

Full Transcript

This visual execution shows how AWS ECS uses tasks and services to run containers. First, you define a task with container details like the image. Then you register this task definition, which saves it with a version. Next, you create a service that uses this task definition and sets how many tasks to run. The service launches the tasks, which run containers. The service monitors the tasks and can scale them up or down based on demand. When you update the task definition, the service replaces old tasks with new ones running the updated version. This process ensures your containers run reliably and can be updated smoothly.

Practice

1. What is the main role of a service in AWS when managing your app tasks?

easy

A. To define the resources and containers for the app

B. To keep the app tasks running and replace them if they fail

C. To store data for the app

D. To monitor network traffic

Solution

1. Step 1: Understand what a service does

   A service manages running copies of your app tasks and ensures they stay running.

2. Step 2: Compare with other options

   Defining resources and containers is the task's role, not the service's. Storing data and monitoring traffic are unrelated.

3. Final Answer:

   To keep the app tasks running and replace them if they fail -> Option B

4. Quick Check:

   Service manages tasks = B [OK]

Hint: Services keep tasks running and replace failed ones [OK]

Common Mistakes:

• Confusing service with task definition
• Thinking service stores data
• Assuming service monitors network

2. Which of the following is the correct way to define a task in AWS ECS?

easy

A. A database schema for app data

B. A YAML file listing EC2 instances

C. A JSON file specifying containers, CPU, and memory

D. A script to launch EC2 instances

Solution

1. Step 1: Identify task definition format

   A task in AWS ECS is defined by a JSON file that specifies containers, CPU, and memory requirements.

2. Step 2: Eliminate incorrect options

   YAML listing EC2 instances is not a task definition. Scripts to launch EC2 or database schemas are unrelated.

3. Final Answer:

   A JSON file specifying containers, CPU, and memory -> Option C

4. Quick Check:

   Task definition = JSON file with containers [OK]

Hint: Task definitions use JSON to specify containers and resources [OK]

Common Mistakes:

• Confusing task definition with instance listing
• Using scripts instead of JSON for tasks
• Mixing database schemas with task definitions

3. If you create a service with a desired count of 3 tasks, but one task fails, what will AWS ECS do?

medium

A. Replace the failed task to maintain 3 running tasks

B. Stop all tasks and alert the user

C. Ignore the failure and keep 2 tasks running

D. Scale down the service to 2 tasks

Solution

1. Step 1: Understand service desired count behavior

   The service tries to keep the number of running tasks equal to the desired count.

2. Step 2: What happens on task failure?

   If a task fails, the service replaces it to maintain the desired count of 3 tasks.

3. Final Answer:

   Replace the failed task to maintain 3 running tasks -> Option A

4. Quick Check:

   Service maintains desired count = A [OK]

Hint: Service replaces failed tasks to keep desired count [OK]

Common Mistakes:

• Thinking service stops all tasks on failure
• Assuming service ignores failures
• Believing service scales down automatically

4. You defined a task with 512 MB memory but your service fails to start tasks due to insufficient resources. What is the likely problem?

medium

A. The container image is missing

B. The task definition JSON is invalid

C. The service desired count is set to zero

D. The cluster has no available memory to run the task

Solution

1. Step 1: Analyze the error cause

   Insufficient resources usually mean the cluster lacks enough memory to run the task.

2. Step 2: Check other options

   Invalid JSON or zero desired count would cause different errors. Missing image causes container start failure, not resource error.

3. Final Answer:

   The cluster has no available memory to run the task -> Option D

4. Quick Check:

   Insufficient resources = cluster memory shortage [OK]

Hint: Check cluster memory before blaming task definition [OK]

Common Mistakes:

• Assuming JSON syntax error causes resource failure
• Ignoring cluster resource limits
• Confusing desired count zero with resource shortage

5. You want to run a web app that scales automatically based on CPU usage. Which combination should you use in AWS ECS?

hard

A. Create a service with a task definition and enable auto scaling on the service

B. Create only tasks without a service and manually start more tasks

C. Use EC2 instances without ECS to run containers

D. Create a service but disable task definitions

Solution

1. Step 1: Identify components for scaling

   A service manages tasks and can be configured with auto scaling based on CPU.

2. Step 2: Evaluate other options

   Running tasks without a service lacks auto scaling. Using EC2 alone misses ECS benefits. Disabling task definitions is invalid.

3. Final Answer:

   Create a service with a task definition and enable auto scaling on the service -> Option A

4. Quick Check:

   Service + task + auto scaling = C [OK]

Hint: Use service with auto scaling for CPU-based scaling [OK]

Common Mistakes:

• Skipping service and expecting auto scaling
• Confusing EC2 usage without ECS
• Disabling task definitions breaks deployment