A producer of items (and related control messages) received by Subscribers. Each current Flow.Subscriber receives the same items (via method onNext) in the same order, unless drops or errors are encountered. If a Publisher encounters an error that does not allow items to be issued to a Subscriber, that Subscriber receives onError, and then receives no further messages. Otherwise, when it is known that no further messages will be issued to it, a subscriber receives onComplete. Publishers ensure that Subscriber method invocations for each subscription are strictly ordered in happens-before order.

Publishers may vary in policy about whether drops (failures to issue an item because of resource limitations) are treated as unrecoverable errors. Publishers may also vary about whether Subscribers receive items that were produced or available before they subscribed.