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Here is my use case:

I have a scheduler lamdba and a executor lambda. In the scheduler lambda, I receive a list of (time, message) tuples indicating that, at time I would like to invoke the executor lambda with event message.

Here is what I have tried

  1. In the scheduler lambda, first clear all triggers from the executor lambda. Then create a EventBridge scheduled event for each (time, message) tuple. This has a few drawbacks...
    • It's quite difficult to remove all triggers from a lambda, as the Lambda API doesn't let you do that (I believe I have to do it through the EventBridge API with proper tagging)
    • Adding and removing ~100 triggers every day seems uneconomical and is not the intended use case of event bridge
  2. Running a dedicated EC2 instance to call the lambda function
    • I'm cheap and I don't want to pay for an instance that will lay idle for ~99.9% of the time.
    • Not serverless

Is there a serverless way of trigger a lambda in a non-periodic fashion?

EDToaster
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  • You can try saving `(time, message)` tuples into DynamoDB and run a lambda function to check the db periodically to determine whether to invoke `executor ` lambda. – jellycsc Sep 27 '20 at 22:02
  • One of the issues for this is that I would like to be able to set the times in 1 minute intervals. This means that I would have to schedule the lambda to run every 1 minute :/ – EDToaster Sep 27 '20 at 22:05
  • The cost of running a lambda every 1 minute is less than running an EC2 24/7. If you don't care about the precision of scheduled lambda execution, you can try the dynamodb+ttl+stream approach. – jellycsc Sep 27 '20 at 22:16
  • You can read more here. https://theburningmonk.com/2019/03/dynamodb-ttl-as-an-ad-hoc-scheduling-mechanism/ – jellycsc Sep 27 '20 at 22:16

1 Answers1

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A bit of a departure, but could you use dynamodb with a ttl? The scheduler could simply write to the table with the message, and format the ttl column to expire at the time you're adding to the tuple.

You could subscribe the executor lambda to the DynamoDb events, and only respond to events that are removed, and if you use New and old images you can retrieve the message from the old image (otherwise I believe it's empty when the item is deleted).

Joseph King
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  • If i'm reading https://theburningmonk.com/2019/03/dynamodb-ttl-as-an-ad-hoc-scheduling-mechanism/ correctly, it shows that the delay can be upwards of about 11 minutes? – EDToaster Sep 28 '20 at 02:33
  • Definitely depends on your risk threshold, but it looks like AWS says up to 48 hours! Have you looked at Step Functions? You could possibly just use a WaitState that pulls the wait time from the input, see docs here! https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-wait-state.html – Joseph King Sep 28 '20 at 12:35