Error handling

[lpil]

Hello!

Is there a recommended way of handling errors so that an application using Elli serves 500 with a suitable body whenever the handler crashes?

Thanks,
Louis

[yurrriq]

Is this behaviour insufficient?

• https://github.com/elli-lib/elli/blob/2.0.2/src/elli_example_callback.erl#L118-L121
• https://github.com/elli-lib/elli/blob/2.0.2/test/elli_tests.erl#L238-L243

[lpil]

At my company we have a specific format for error bodies.

Currently I'm catching at exceptions and thrown values in the callback and returning said body. Is this the recommended way to do this? :)

[yurrriq]

I would suggest to use postprocess/3 something like this:

postprocess(Req, {500, Body}, Config) ->
  custom_error(Req, [], Config);
postprocess(Req, {500, Headers, Body}, Config) ->
  custom_error(Req, Headers, Config);
postprocess(_, Res, _) ->
  Res.


custom_error(Req, Headers, Config) ->
  {500, Headers, CustomBody}.

[lpil]

Wonderful, thank you.

I see that postprocess isn't part of the elli_handler behaviour. I presume it goes in the same module?

[yurrriq]

Yeah! I'd love to codify an elli_middleware behaviour.

[lpil]

That would be great. It's quite unclear how to use the middlewear features at the moment I think.

[yurrriq]

This seems pretty related to elli-lib/elli_fileserve#7 and #3. Also note that I've spec'd out init/2, preprocess/2 and postprocess/3 as optional callbacks in elli_handler in 71a7c9a.

[lpil]

That's much clearer for me, thanks :)

[yurrriq]

Great! I’ll publish a new release in the next few days.

[yurrriq]

https://hex.pm/packages/elli/2.1.2

Feel free to re-open if need be, but I'm closing this for now.