Replay subject subscription behaviour - system.reactive

Question

The following code works as expected but I am puzzled by the way it behaves when I uncomment the line 'o.OnCompleted();'
The code joins all subscribers to the result of a single long operation and caches the result for further subscribers for 2 seconds. Any subscription after this time starts the process again.
Subscriptions will come from other threads (simulated with the thread pool).
var obs = Observable.Create((IObserver<Guid> o) =>
{
Console.WriteLine("Start");
Thread.Sleep(1000); // process
Console.WriteLine("End");
o.OnNext(Guid.NewGuid());
//o.OnCompleted(); // <-- uncomment this
return Disposable.Empty;
})
.Replay(TimeSpan.FromSeconds(2))
.RefCount()
.Take(1);
ThreadPool.QueueUserWorkItem(delegate
{
// simulate request from threadpool
obs.Subscribe(x => Console.WriteLine($"1: {x}"), () => Console.WriteLine($"1: complete"));
});
ThreadPool.QueueUserWorkItem(delegate
{
obs.Subscribe(x => Console.WriteLine($"2: {x}"), () => Console.WriteLine($"2: complete"));
});
Thread.Sleep(4000);
ThreadPool.QueueUserWorkItem(delegate
{
obs.Subscribe(x => Console.WriteLine($"3: {x}"), () => Console.WriteLine($"3: complete"));
});
Here is the result:
Start
End
1: 255BEFDC-2F14-40AD-AE77-2B005C5A3AA9
2: 255BEFDC-2F14-40AD-AE77-2B005C5A3AA9
1: complete
2: complete
Start
End
3: 1214DC63-F688-475A-9CB7-C3784054A4AC
3: complete
The odd behaviour is if I uncomment the line 'o.OnCompleted()' the result changes to this:
Start
End
1: 255BEFDC-2F14-40AD-AE77-2B005C5A3AA9
2: 255BEFDC-2F14-40AD-AE77-2B005C5A3AA9
1: complete
2: complete
Start
End
3: complete
The 3rd subscriber causes another subscription to the root observable but the result is missing. It appears the ReplaySubject caches the result of the previous observable having completed but still causes a new subscription. This seems unintuitive. I would like to understand why it doesn't work.
Note: I originially tried this using Defer instead of Create which had the same result as the second run above (for obvious reasons).

Answer 1

When you use the Replay/RefCount pair you create an observable that shares a common subscription to the source observable.
From the source:
Returns a connectable observable sequence that shares a single subscription to the underlying sequence replaying all notifications.
Now, it's important to remember that an observable produces a series of zero or more values, followed by either a complete or error signal. It cannot produce values after a complete or error is produced.
Since you are sharing a common subscription to the source and if your source producing a complete then it cannot produce more values. So when you call o.OnCompleted() then you're doing exactly that.
Also, as a side-note, you should avoid ever writing return Disposable.Empty; inside a Create. It means you're creating an observable than can complete before the subscription has returned and that can lead to race conditions.
The way to write your code without it is:
var obs =
Observable
.Defer(() => Observable.Return(Guid.NewGuid()).Concat(Observable.Never<Guid>()))
.Replay(TimeSpan.FromSeconds(2.0))
.RefCount()
.Take(1);
But this is the same as not calling o.OnCompleted().