Interface describing Observer wrappers that are thread-safe (can receive concurrent events) and that return an immediate Continue when receiving onNext events. Meant to be used by data sources that cannot uphold the no-concurrent events and the back-pressure related requirements (i.e. data-sources that cannot wait on Future[Ack] for sending the next event).

Implementations of this interface have the following contract:

• onNext / onError / onComplete of this interface MAY be called concurrently
• onNext SHOULD return an immediate Continue, as long as the buffer is not full and the underlying observer hasn't signaled Stop (N.B. due to the asynchronous nature, Stop signaled by the underlying observer may be noticed later, so implementations of this interface make no guarantee about queued events - which could be generated, queued and dropped on the floor later)
• onNext MUST return an immediate Stop result, after it notices that the underlying observer signaled Stop (due to the asynchronous nature of observers, this may happen later and queued events might get dropped on the floor)
• in general the contract for the underlying Observer is fully respected (grammar, non-concurrent notifications, etc...)
• when the underlying observer canceled (by returning Stop), or when a concurrent upstream data source triggered an error, this SHOULD eventually be noticed and acted upon
• as long as the buffer isn't full and the underlying observer isn't Stop, then implementations of this interface SHOULD not lose events in the process
• the buffer MAY BE either unbounded or bounded, in case of bounded buffers, then an appropriate overflowStrategy needs to be set for when the buffer overflows - either an onError triggered in the underlying observer coupled with a Stop signaled to the upstream data sources, or dropping events from the head or the tail of the queue, or attempting to apply back-pressure, etc...

See OverflowStrategy for the buffer policies available.