AWS Lambda Function Handler Errors

The point of entry for AWS Lambda functions is always the configured “handler”. Jets configures the Jets Shim looks as the handler and it looks something like this:

handlers/controller.rb

# Outside handler. Gets called once only during cold-start / boot.
Jets.shim.boot

def lambda_handler(event:, context:)
  # Inside the handler. Gets call on every invocation.
  Jets.shim.handler(event, context)
end

Errors can happen outside or inside the lambda_handler.

  • Errors outside the handler can be tougher to track because AWS Lambda will not show the stackTrace as part of the invoke response. I’m guessing that AWS Lambda removes stack trace for security reasons.
  • They do show up in the AWS Lambda Log though.

Here are some debugging tips if you’re AWS Lambda does not seem to be working correctly.

jets logs

Logs are you’re friend.

jets log -f

jets exec

The jets exec REPL can be also used to see the errors.

jets exec

With jets exec you can run commands as if you are in a bash shell within the AWS Lambda server to check whether your application is set up correctly.

Rails Tips

Here are some additional Rails tips.

  • Consider turning on config.consider_all_requests_local = true turning this on temporarliy for production. This can be a security concern as it’ll reveal your stack trace in the browser. If you are ok with it though, it can help. Again, you can see this same error with jets log.
  • If the error is deep in some gem, you can try to remove the Rails backtrace cleaner. Put this Rails.backtrace_cleaner.remove_silencers! in config/initializers/backtrace.rb.

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