Question

I have two methods that both return an IObservable

IObservable<Something[]> QueryLocal();

and

IObservable<Something[]> QueryWeb();

QueryLocal is always successful. QueryWeb is susceptible to both a timeout and possible web errors.

I wish to implement a QueryLocalAndWeb() that calls both and combines their results.
So far I have:

IObservable<Something[]> QueryLocalAndWeb()
{
  var a = QueryLocal();
  var b = QueryWeb();
  var plan = a.And(b).Then((x, y) => x.Concat(y).ToArray());
  return Observable.When(plan).Timeout(TimeSpan.FromSeconds(10), a);
}

However, I’m not sure that it handles the case where QueryWeb yields an error.
In the future I might have a QueryWeb2() that also needs to be taken into account.

So, how do I combine the results from a number of IObservables ignoring the ones that throw errors (or time out)?

Answer 1

I guess OnErrorResumeNext should be able to handle this scenario:

From MSDN:

Continues an observable sequence that is terminated normally or by an
exception with the next observable sequence.

IObservable<Something[]> QueryLocalAndWeb()
{
  var a = QueryLocal();
  var b = QueryWeb().Timeout(TimeSpan.FromSeconds(10));
  return Observable.OnErrorResumeNext(b, a);
}

You can do concat of array by using Aggregation on the returned observable.
I am assuming that both local and web are cold observable i.e they start producing values only when someone subscribes to them.