[undertow-dev] How to handle exceptions thrown from worker threads

[Wolinsky, Jared]

For me, adding the exception onto the exchange would be fine, along with a mention in the error handling section of the docs.  I think it's ok if the DefaultResponseListener isn't called if the exchange is ended because that is some handler explicitly taking care of the response and overriding the default case.  That said, I don't think an explicit error handling interface would be a bad way to go either.

I was actually able to work around this using Bill O'Neil's advice and dispatching the request every time in my ErrorHandler, which is the outer handler in my chain.  Then I try-catch the next.handleRequest call, and do my exception handling there.  

Thanks,
Jared
________________________________________
From: Stuart Douglas [sdouglas at redhat.com]
Sent: Wednesday, June 15, 2016 10:37 PM
To: Wolinsky, Jared
Cc: undertow-dev at lists.jboss.org
Subject: Re: [undertow-dev] How to handle exceptions thrown from worker threads

I think you are correct that we don't handle this case very well, the
basic error handing strategy has always been to just catch the error
in a handler, but when you don't have control over the dispatch site
this gets tricky.

We should definitely make the error available in
DefaultResponseListener, probably just as an attachment on the
exchange. Would this be enough to meet your requirements, or would
some kind of actual error handling interface be better?
DefaultResponseListener may not always be called if the exchange has
already been finished when the error occurs (i.e. the page has already
been sent to the user).

Stuart

On Wed, Jun 15, 2016 at 2:01 AM, Wolinsky, Jared <jared.wolinsky at mlb.com> wrote:
> Good morning,
>
> We're using Undertow in a new web service at MLB, and I haven't been able to
> find a solution for handling errors that occur in requests that have been
> dispatched to worker threads.  Here is a link to my SO question:
> http://stackoverflow.com/questions/37753726/how-to-handle-exceptions-thrown-from-undertow-worker-threads.
> Ignoring worker threads, my model for error handling is to have an outer
> handler that maintains a map of exception type -> {log level, http status
> code}.  Then I allow exceptions to propagate out from the actual API
> handlers, hitting the error handler which logs the error and responds to the
> client with the correct status code based on the exception type.  When
> exceptions are thrown from worker threads, however, they break out of the
> normal handler chain and never hit the error handler, resulting in a 500
> response every time.  Here are options I've considered for handling this
> problem:
>
> Inside my error handler, set a DefaultResponseListener
> (https://http2.undertow.io/documentation/core/error-handling.html) on the
> exchange that contains my error handling logic.  I hit a wall here after
> realizing the listener only has access to the Exchange, not to the exception
> that will be thrown.
> Set an UncaughtExceptionHandler on each handler that's on a worker thread.
> This code would be added to my base handler class, do a
> !exchange.isInIoThread() check, set the error handler, then call the
> handleRequest method.  The caveat here is that I can only do this for
> handlers that I have control over, not built-in handlers that dispatch to
> worker threads such as the AuthenticationCallHandler.
> Use an AspectJ-based approach where I create an @Around advice that executes
> for all of my handlers that will wrap the execution in a try-catch, and take
> care of the error handling.
>
>
> Are there best practices for handling this?
>
>
> Thanks,
>
> Jared Wolinsky
>
> Senior Software Engineer, MLB Advanced Media
>
>
>
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