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If you’re trying to understand how Lambda and SNS permissions work together, here’s a simple explanation with a solution to make sure everything runs smoothly.
First, your Lambda function can publish messages to an SNS topic because the Lambda’s execution role has the right permissions. Specifically, it has permission to use “sns:publish” on all SNS resources. When Lambda runs, it takes on the permissions of its execution role. This role is like a set of ID badges that tell AWS what actions Lambda is allowed to perform. Since the role includes the “sns:publish” permission, Lambda is authorized to send messages to any SNS topic.
Now, if you remove the “sns:publish” permission from that role but try to allow access through the SNS topic’s access policy, the Lambda function still won’t be able to publish. That’s because Lambda uses its execution role to determine what it can do, not the access policy on the SNS topic. The access policy controls who can access SNS, but it doesn’t give permission to the Lambda function unless the function’s role has the right permissions.
For your Lambda function to successfully send messages to SNS, you need to make sure that the permission is given in the Lambda’s execution role. The SNS topic’s access policy can specify who is allowed to access it, but the Lambda’s execution role is what grants the permissions to perform actions like publishing.
In summary, the key to resolving permission issues is to ensure your Lambda’s execution role includes the proper “sns:publish” permission. Without that, even if the SNS topic allows access, Lambda won’t be able to publish messages.
Sources:
- Defining Lambda function permissions with an execution role
- Using identity-based policies with Amazon SNS
- How Lambda works
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