Task definitions in AWS - Step-by-Step Execution

Process Flow - Task definitions

Write Task Definition JSON

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Register Task Definition in ECS

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ECS stores Task Definition

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Use Task Definition to Run Tasks or Services

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ECS launches containers as defined

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Tasks run with specified settings

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Monitor & Update

You create a task definition JSON, register it with ECS, then ECS uses it to run containers with the settings you defined.

Execution Sample

AWS

{
  "family": "my-task",
  "containerDefinitions": [
    {
      "name": "app",
      "image": "nginx",
      "memory": 128
    }
  ]
}

Defines a task with one container named 'app' using the nginx image and 128 MB memory.

Process Table

Step	Action	Input/State	Result/Output
1	Write task definition JSON	Define family 'my-task', container 'app', image 'nginx', memory 128	JSON ready to register
2	Register task definition	Send JSON to ECS	ECS stores task definition with revision 1
3	Run task using definition	Request ECS to run 'my-task:1'	ECS launches container 'app' with nginx image
4	Container starts	Container runs with 128 MB memory	App container running
5	Update task definition	Change memory to 256	New revision 2 registered
6	Run new task	Request ECS to run 'my-task:2'	ECS launches container with updated memory
7	Stop tasks	Request ECS to stop running tasks	Containers stopped
8	Exit	No more tasks to run	Execution ends

💡 No more tasks requested, ECS stops containers and ends execution

Status Tracker

Variable	Start	After Step 2	After Step 5	Final
Task Definition Revision	None	1	2	2
Container Memory (MB)	128 (initial JSON)	128	256 (updated)	256
Task Running State	No	Yes (step 3)	Yes (step 6)	No (step 7)

Key Moments - 3 Insights

Why do we need to register a task definition before running tasks?

Because ECS needs a stored definition to know what containers to launch and how. See execution_table step 2 where registration stores the definition.

What happens if you update the task definition JSON but don't register it?

ECS will still use the old registered version. Only registered revisions are used to run tasks, as shown in steps 5 and 6.

Can you run tasks with different settings without changing the original task definition?

No, you must register a new revision with updated settings. ECS runs tasks only from registered revisions (step 5 and 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the task definition revision after step 2?

A Revision 0

B Revision 1

C Revision 2

D No revision yet

💡 Hint

Check the 'Result/Output' column at step 2 in execution_table

At which step does ECS launch the container with updated memory?

A Step 6

B Step 5

C Step 3

D Step 7

💡 Hint

Look for when the task definition revision changes and a new task runs in execution_table

If you stop tasks at step 7, what is the running state after that?

A Yes, tasks still running

B Unknown

C No, tasks stopped

D Tasks paused

💡 Hint

Check 'Task Running State' in variable_tracker after step 7

Concept Snapshot

Task definitions are JSON files describing containers to run.
You register them with ECS to create revisions.
ECS uses these revisions to launch containers.
Update by registering new revisions.
Run tasks by specifying revision.
Stop tasks to end containers.

Full Transcript

Task definitions in AWS ECS are JSON documents that describe how to run one or more containers. You write the JSON with container details like image and memory. Then you register this JSON with ECS, which stores it as a revision. When you want to run containers, you tell ECS which task definition revision to use. ECS launches containers based on that definition. If you want to change settings, you update the JSON and register a new revision. Running tasks always use a registered revision. You can stop running tasks to end containers. This flow ensures ECS knows exactly what to run and how.