Exceptions are always marked as success invocations

[tlfbrito]

when throwing any exception on php, for example:

throw new Exception('Ups!');

It's always identified as a success invocation on AWS monitor :

I'm using Symfony Framework

[mnapoli]

Hi, thanks for the report but we would need more information:

• Bref version
• which runtime/layer is used
• where is the exception thrown

[tlfbrito]

Bref version: 0.5.24
runtime/layer: (bref:layer.php-74-fpm) (it also happens with bref:layer.php-73-fpm)
Exception is thrown in a regular Symfony Controller:

    /**
     * @Route("/ping", name="ping")
     */
    public function index()
    {
        throw new \Exception('Because I can');
    }

[mnapoli]

OK, that is the expected behavior: Symfony catches exceptions to return a response (display their custom 500 error page).

I'm not sure there is anything we can do here :/

[deleugpn]

I remember investigating this about 6 months ago and I shared this with my team. Laravel has the same behavior: every API call will always result in a response. Maybe the HTTP Code is 500 or 400, but Lambda doesn't interpret that, it's part of Load Balancer / API Gateway. So long the Lambda returns a valid response, the services treat it as a successful execution. Since we use Load Balancer, I remember saying that we should watch the ALB 5xx Cloud Watch metric instead of failed invocations.

[tlfbrito]

OK, that is the expected behavior: Symfony catches exceptions to return a response (display their custom 500 error page).

I'm not sure there is anything we can do here :/

Yes, Symfony catches the exception... but it will respect the http return codes.
I wonder what would be the behaviour on JS applications?

[mnapoli]

As @deleugpn said, the HTTP status code is not taken into account by Lambda.

In JS or other languages, if you catch exceptions to return a specific HTTP response, no error is counted.

It all boils down to: does your code (including your framework) returns an exception or not. Here, Symfony will catch it so there is no exception reaching Lambda.

I remember saying that we should watch the ALB 5xx Cloud Watch metric instead of failed invocations.

Yes this is a good indicator. You can watch logs as well.

In short, you are in the same situation as on any other hosting provider: watch HTTP response codes as well as logs.

[tlfbrito]

I notice this because I was exploring canary deployments with serverless-plugin-canary-deployments but it only checks for lambda invocation errors, which doesn't seem to happen.

[mnapoli]

Yes, I think you can add custom rules for canary deployments so you could check the logs or the HTTP response codes.