Call an authenticate method before retrying

[reisenberger]

@robertbaker asked under a separate issue (moving here as worth calling out as own issue!)

I have a scenario for web requests.

Say I make a httpClient call that throws a 403. On a 403, I want to
call my authenticate method first then retry the method.

I don't think this is possible currently with polly. If it is please give me an example!

[reisenberger]

Hey @robertbaker. This came up on StackOverflow recently, but I think the q has been deleted. It's very possible. Configure a policy with an onRetry delegate. Polly calls the onRetry delegate before the next try. In the onRetry delegate, call your authentication method.

The code at the top of #107 gives an example. [#107 uncovered an obscure compiler gotcha that led to an async sequencing issue, now fixed, but the code example matches your need I think!]

Let us know if you need any further assistance!

EDIT: The example in #107 is async, but you can also do this with sync policies.

[reisenberger]

@robertbaker If an additional issue is that HttpClient returns 403 as a status code rather than throws an exception, HttpResponseMessage.EnsureSuccessStatusCode() can be used to make that an exception, as described here. This provides a way of getting Polly to handle outcomes from HttpClient calls that might be either exceptions (eg timeouts) or status codes. There's a sketch of an async example using continuations here. EDIT: Or of course you can throw a specific exception only on 403 yourself.

We are looking at allowing Policies to handle return values as well as exceptions natively - see #14, and wider discussion in the Roadmap. Any feedback you have towards this feature would be welcome! Thanks.

UPDATE: The feature #14 is now delivered. The main Polly readme describes much more elegant ways of handling exceptions and HttpStatusCodes combied.

[ghost]

Thanks, I'll try to implement this soon.

[reisenberger]

@robertbaker No problem. @PhilipAnthonyMurray was essentially working on sthg similar I think. We are doing the same thing in-house and achieved this pattern (refreshing authentication on retry) against both straight https authentication and SAML authentication: but the pattern isn't any more complicated than outlined above.

[reisenberger]

@adamhathcock You raised OAuth and Polly on #90, see this thread #122. One pattern is to use the onRetry delegate (which gets executed between tries), so, in its simplest schematic form, something like:

var authorisationEnsuringPolicy = Policy
  .Handle<AuthorizationException>()
  .Retry(1, onRetry: (e, i) => RefreshAuthorization());

authorisationEnsuringPolicy.Execute(() => DoSomethingThatRequiresAuthorization());

[adamhathcock]

Thanks @reisenberger I was sure there was some way to do it without having to bring in some kind of new concept.

[reisenberger]

Closing historic issue. Fully documented on wiki