Duplicate event fired

Question

Hello everyone,

I am facing an odd situation here. 
I have some events since a few days that are fired 2 times in the same bus (default). They are exactly the same : content and id. 
And so they triggered some lambdas two times messing with our event process. 
I thought it should be impossible. 
I assume that if there are two logs in events/debug, there are two event fired.

Look at the photo. You can see the same id in the JSON at the same hour.

![Duplicate log](/media/postImages/original/IMZY4XnsBAQoSpbfzey5DcZw)

If you have any idea about what can cause that.

Thanks for your help.

EDIT 1 : The events are generated by a lambda using aws sdk for nodeJs  and method putEvents.

Answer

Thanks for your response.

I was aware that a rule could be triggered multiple times. 
But here it is more that the event is fired two times. (maybe it means the same for AWS, but I think there is slight difference)

In any case make the lambdas idempotent should be the solution. 
As a best practice, for the unique id, is it better to use the id generated by event bridge or an id in the payload coming from our internal process ?

Thanks

Answer

This can happen in a large distributed system (which Amazon EventBridge is). See the documentation here: https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-troubleshooting.html#eb-rule-triggered-more-than-once

A best practice is to make events idempotent, for more than this rare occurrence. You may need to replay events under some recovery scenarios. See the AWS Well-Architected Framework Best Practice, [Make all responses idempotent](https://docs.aws.amazon.com/wellarchitected/latest/framework/rel_prevent_interaction_failure_idempotent.html).

Duplicate event fired

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