IAM Role Vs Resource Policy

You're right that both IAM roles and resource policies are used to control access between AWS resources, but their use cases and behavior differ. Let me clarify the scenarios where you would use one over the other and how many resources can assume an IAM role.

1. IAM Role vs Resource Policy

• IAM Role:

  • Use Case: An IAM role is primarily used when one AWS resource or entity (such as an EC2 instance, Lambda function, or AWS service) needs to assume a set of permissions to interact with other AWS resources. The role itself doesn’t have credentials; instead, entities assume the role and inherit its permissions.
  • Scenario: For example, if you want an EC2 instance to have permission to read from an S3 bucket, you would create an IAM role with the necessary permissions and attach it to the EC2 instance. Similarly, Lambda functions can assume roles to access other AWS services.
  • Assumable by: IAM roles can be assumed by AWS services (like EC2, Lambda, etc.), federated users, or even by other IAM roles. Each entity that assumes the role can inherit its permissions temporarily.
  • Example:
    • An EC2 instance assumes a role that allows it to access S3.
    • A Lambda function assumes a role to read from a DynamoDB table.

• Resource Policy:

  • Use Case: Resource policies are DIRECTLY attached to resources like S3 buckets, SNS topics, or SQS queues to specify which principals (AWS users, accounts, or services) can access them. These are directly associated with the resource, not a user or entity.
  • Scenario: For example, if you want to allow an external AWS account to read from your S3 bucket, you would attach a resource policy (bucket policy) to the S3 bucket to specify that access. The policy defines the permissions and who can access the resource.
  • Use Case for Cross-Account Access: You can use a resource policy to grant access to a resource (like a bucket or a queue) from other AWS accounts or services without using IAM roles.
  • Example:
    • A bucket policy allows an external AWS account to access an S3 bucket.
    • A resource policy on an SNS topic allows only a specific IAM role or AWS service to publish messages to it.

2. When to Use One vs the Other

• Use IAM Role: When you need to allow a service or resource to assume permissions dynamically (e.g., for EC2 instances, Lambda functions, or other services).
  • You can grant permissions to AWS services (like EC2) to access other AWS resources (like S3, DynamoDB, etc.).
  • IAM roles are ideal when the principal needs to take actions across multiple services or resources and might need temporary permissions.
• Use Resource Policy: When you need to control access to a resource directly and specify which principals (IAM users, roles, services, or accounts) can access it.
  • This is useful when controlling direct access to a specific resource (e.g., an S3 bucket, an SNS topic, or an SQS queue).
  • Use resource policies for cross-account access or if you want to explicitly grant access to specific entities without needing to involve an IAM role.

3. How Many Resources Can Assume an IAM Role?

• There’s no strict limit to how many resources can assume a single IAM role, as long as the role's trust policy allows it. For example:
  • An EC2 instance can assume an IAM role to access resources like S3, DynamoDB, or other AWS services.
  • A Lambda function can assume an IAM role to perform actions like accessing databases or writing to logs.
  • An Assumed role can also be shared across multiple entities if needed, as long as the trust relationships and permissions are correctly defined.
• The main constraint is how the role’s trust policy is configured — it determines which entities can assume the role, and you can allow multiple entities (such as different EC2 instances, Lambda functions, etc.) to assume the same role.