AWS IoT & Lambda: Triggering function using rule A works but not using rule B

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Four components are related to this question:

  1. The "IoT client": A program that runs in IoT devices, connects to AWS and publishes/subscribes to topics,
  2. a Lambda function with two different versions: 1 and 2,
  3. an AWS IoT rule (#1) that triggers version 1 of the Lambda function, and
  4. an AWS IoT rule (#2) that triggers version 2 of the Lambda function.

Version 2 of the Lambda function introduced breaking changes requiring a new version of the IoT client.

IoT rule #1 uses $aws/rules/DoSomething/${device_id} IoT rule #2 uses $aws/rules/do_something/${device_id}

Version 1 of the IoT client uses rule #1 and version 2 of the IoT client uses rule #2. At this point in the IoT client's code, nothing else has changed except changing DoSomething to do_something.

Using the AWS IoT console, I duplicated rule #2, renamed it to do_something instead of DoSomething and changed the target to version 2 of the Lambda function instead of version 1.

I have checked the configuration->permissions settings of both versions, and they are identical (allowing writing to the Lambda function's CloudWatch Logs, not version-specific). Both versions are triggered using the same IAM role.

Lambda function v1 has IoT rule #1 as a trigger. Lambda function v1 is also an action of IoT rule #1. Lambda function v2 has IoT rule #2 as a trigger. Lambda function v2 is also an action of IoT rule #2.

Yet, when the IoT client uses IoT rule #2, the function is never triggered. If I change the rule to IoT rule #1, it triggers the function.

The only thing I need to change to make it trigger some function is do_something -> DoSometing, but I don't want DoSomething, I want do_something.

Comparing the two rules in the console, nothing is changed (as far as I can see) except DoSomething -> do_something and the Lambda function version under Actions. The SQL statements are the same.

The two function versions are also identical (as far as I can see) except their code, the version and which rule is hooked up as a trigger.

What did I miss?

aws-iot-core aws-iot iot aws-lambda amazon-web-services
1 Answers

Check the policy attached to the certificate, which is in turn attached to the thing(device) which is used to publish.

There is a chance that the topic is limited to a certain format like starting with or containing some string.AWS IoT Core action resources

For example, the policy may dictate to have all topics start like $aws/rules/DoSomething/

You can make use of the MQTT Test client in order to check whether the topic used for publishing is proper, It's available under IOT core itself.AWS IoT MQTT client

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