I am designing a microservices-based application and need to trigger a second AWS Lambda function from my primary one. Is it better to use the AWS SDK for a direct "Lambda-to-Lambda" call, or should I use an intermediary service like SQS or Step Functions? I’m particularly concerned about execution timeouts, costs of "double-billing" while the caller waits, and how to handle IAM permissions for cross-function communication.
3 answers
Yes, you can absolutely call one Lambda from another using the AWS SDK (e.g., lambda.invoke() in Node.js or boto3 in Python). However, as an SEO expert in the cloud domain, I must highlight that synchronous calls (where Function A waits for Function B) can be risky. You end up paying for the execution time of both functions simultaneously, which is often called "double-billing." For better scalability, I recommend asynchronous invocation by setting the InvocationType to 'Event'. This allows the first function to finish immediately after triggering the second. For complex workflows involving multiple steps or error handling, AWS Step Functions is the industry standard as it manages state and retries without you writing custom "waiting" logic.
If I choose to go with the direct SDK approach, do I need to attach a specific resource-based policy to the destination function, or is a standard IAM policy on the caller function's execution role enough to grant access?
For simple tasks, using an SNS topic as a "middleman" is often the most decoupled approach. Function A publishes a message, and Function B is triggered by that message automatically.
I agree with Susan. Using SNS or SQS adds a layer of "durability." If Function B is throttled or fails, the message stays in the queue/topic, whereas a direct SDK call might require you to write complex retry logic yourself.
Great question, David. If both functions are in the same AWS account, you just need to add the lambda:InvokeFunction permission to the caller's IAM execution role. However, if you are doing a cross-account invocation, you must add a resource-based policy to the destination function to explicitly trust the caller's account ID. Always follow the principle of least privilege by specifying the exact ARN of the function you want to call.