In mutiny, I want to retry a Uni<> when it fails, but the only condition to generate a failure is to throw an exception?

[jiayaoO3O]

I was wondering, do I have to use an exception to determine when onFailure() occurs? Do I have to throw an exception to trigger a retry when the http request returns a non-200 status code, which is the effect I want?

[cescoffier]

What you can do is the following:

// This is the Uni representing your call to the remote service (HTTP in your case)
Uni<Response> call = callYourService();

// Verify and retry
Uni<Response> uni = call
  .onItem().transform(resp -> {
      if (resp.statusCode() != 200) { throw new RuntimeException("not ok: " + resp.statusCode()); }
     return resp; 
  })
  .onFailure().retry().atMost(5); // Your retry

The idea is to forward a failure if the response is not a 200 and recover (retry) in that case. If would retry for every failure. You can limit the retry by failing with a specific exception and use onFailure(MySpecificException.class).retry()...

[jiayaoO3O]

So, if I want to use the fail-retryfeature, I have to throw an exception? Take the following example :
call.onFailure(e->e.getLocalizedMessage().contains("abc")).retry().atMost(3);
The incoming parameters of this function is Predicate<? super Throwable> predicate, but the effect I want is
call.onFailure(resp ->resp.statusCode() != 200).retry().atMost(3);
Why force the entry to be Predicate<? super Throwable> predicate?

[cescoffier]

Because a retry is a failure recovery action. It happens when it receives a failure, that's why you need to trigger a failure to use it.

Having a response status != 200 is not a failure, in the sense that for HTTP it's expected. However, in your code, this should be considered as a failure. So you need to add the code that emits a failure (there are several ways for doing this) in this case.

Throwing an exception is one way. You can also do something like:

Uni<Response> uni = call
  .onItem().transformToUni(resp -> {
      if (resp.statusCode() != 200) { return Uni.createFrom().failure(new Exception("..."); }
     return Uni.createFrom().item(resp); 
  })
  .onFailure().retry().atMost(5); // Your retry

We discussed a few months (maybe even a year) ago about adding:

uni.onItem().fail(Predicate<T>, Function<T, Throwable>);
// and
uni.onItem().fail(Predicate<T>);

We didn't add them yet. The implementation is trivial.

[jiayaoO3O]

Thank you so much : )