This content originally appeared on DEV Community and was authored by bikash119
Understanding the Concepts
Understanding IAM Through a Real-World Analogy
Imagine a FedEx delivery to a house inside a gated community. This scenario perfectly illustrates how AWS IAM works:
1. Trust Policy – The Community Protocol
The gated community management creates a protocol stating: “Only verified FedEx employees can be assigned the role of package delivery within our community.” This is your Trust Policy – it defines which entities (FedEx employees = AWS services like EC2, ECS) are trusted to assume a specific role.
2. Role – The Delivery Role
Using this protocol, the security team at the gate creates a “Package Delivery Role.” This role exists as a defined position with specific responsibilities, but no one holds it permanently. This is your IAM Role – a set of permissions that can be temporarily assumed by trusted entities.
3. Permissions – What the Role Can Do
The role comes with specific permissions: “Can access all residential areas, cannot access the car charging stations, must use designated parking spots.” These rules define what anyone with this role can or cannot do within the community. This represents your Role Policies – the specific AWS permissions attached to the role.
4. Instance Profile – The Badge Vending Machine
When a FedEx driver arrives, security verifies their identity and issues a temporary access badge with all the delivery permissions encoded into it. Once the delivery is complete, the badge expires automatically. The driver loses all community access privileges upon exit. This is your Instance Profile – it provides temporary credentials to AWS resources, ensuring access is time-limited and automatically revoked.
This system ensures that even legitimate delivery personnel only have access for exactly as long as needed, with precisely the permissions required for their task.
Why Instance Profiles? EC2 vs ECS Role Assignment
EC2 Instances Have Their Own Job in ECS
When an EC2 instance joins an ECS cluster, it becomes a worker node with specific responsibilities:
- Register itself with the ECS service
- Report available resources (CPU, memory, storage)
- Download and start containers as instructed by ECS
- Monitor container health and report status
- Manage container lifecycle events
To perform these cluster management tasks, the EC2 instance needs the AmazonEC2ContainerServiceforEC2Role policy. Since EC2 is a general-purpose compute service, it uses instance profiles to securely deliver these credentials.
ECS Tasks Assume Roles Directly
ECS tasks, however, don’t need instance profiles because:
- They’re purpose-built containerized workloads with specific, known requirements
- ECS can directly inject appropriate credentials into containers
- The ECS service handles credential management natively
Two Separate Layers of Permissions
Think of it as two distinct jobs:
- Infrastructure Layer (EC2 + Instance Profile): “I’m a worker node that needs to communicate with ECS control plane”
- Application Layer (ECS Tasks + Task Roles): “I’m a specific application that needs to access AWS services”
This separation allows the same EC2 instance to host multiple different ECS tasks, each with their own specific permissions, while the instance itself maintains its cluster management permissions.
This content originally appeared on DEV Community and was authored by bikash119