Building a Card Playing Twitter Bot: when AWS Lambdas timeout

node.js AWS Lambdas have a default timeout of 3 seconds when you create a new Lambda via the AWS Console. This can be increased with the slider in the Console, but if you keep the default and you get a timeout, you’ll see this error in CloudWatch:

2018-06-14T01:16:56.953Z 9f86... Task timed out after 3.00 seconds

From looking at the logs, my Blackjack card playing Twitter bot Lambda typically executes in around 2 secs, so the default of 3  secs probably doesn’t have enough wiggle room for unexpected slowdowns, and while testing I’ve seen this error a couple of times. When it does time out, it seems the execution just stops in the middle of whatever it was currently doing at that point, which leads to some unexpected results. In the case of this Twitter bot, the execution is being triggered via a 5 minute Cloudwatch event, so the only way I know it’s failed is to look at the logs (it’s not called via a webpage, so there’s no error being returned to the caller). Increasing the timeout to 10 secs seems to work fine for this particular Lambda.

Remember you are billed per GB-s of execution time, so keep an eye on how long your Lambdas execute for. If you’re aiming for low cost, then quicker execution times are obviously better.