This is a post(set of questions really) regarding the implications of 
at-most-once(maybe-once) delivery reliability offered by akka actors on 
APIs and software based on akka. From my understanding, even messages sent 
by actors to self can be lost.

Till now, I have been in scenarios where delivery guarantee is not 
necessary so life was good. But now, I need reliability from my service at 
least to the extent that a subscriber should know that a message has been 
lost.
1. If messages can be lost in actor system, and akka-streams are based on 
actors, does that mean akka-streams are leaky? Thats pretty scary to think 
about. Specifically does a Flow(or GraphStage) ensure that stream entities 
are not lost?
    For example, is the below code(based on Source.single doc 
<https://doc.akka.io/docs/akka/2.5/stream/operators/Source/single.html>) 
guaranteed to print `[hello]`?
    import akka.stream.*;

    CompletionStage<List<String>> future = Source.single("hello").runWith(
Sink.seq(), materializer);
    CompletableFuture<List<String>> completableFuture = future.
toCompletableFuture();
    completableFuture.thenAccept(result -> System.out.println(result));

2. If messages can be lost in actor system, and akka-http is based on 
actors, does that mean akka-http APIs, lets say websocket API randomly 
drops messages.
    For example, in:
    http.singleWebSocketRequest(WebSocketRequest.create(
"wss://api.example.com"), flow, mat)

    is it at least guaranteed that my flow will reliably receive everything 
on the websocket in order?
3. In actor world, I would have a sequence number on each message and let 
the receiver actor take remedial measures on a lost(probably never because 
of at-most-once reliability issues so far) sequence number. In case we live 
in a dystopian world where the answers to 1. and 2. are yes, this wouldn't 
work because the external web service I consume does not implement sequence 
numbers. Because any sequence number I generate within the application will 
only add sequence numbers to messages that are not already lost. Is this a 
correct understanding? 

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