Publisher rules

Publisher has the following interface definition:

  public static interface Publisher<T> { 
       public void subscribe(Subscriber<? super T> subscriber); 
  }

Rule number 1.1 says, the total number of onNext signaled by a Publisher to Subscriber must be less than, or equal to, the total number of elements requested by that Subscriber Subscription at all times. There are multiple facets to this definition here. Let’s try to analyze them one by one:

• First and foremost, there has to be a request for a message from Subscriber (total number → 1 - N) to Publisher. Therefore, Publisher cannot start sending messages to unsuspecting subscribers on its own as these subscribers might still be deciding when to start receiving messages. Furthermore, some might still be performing some initial tasks in order to start receiving the message.
• Secondly, only after the request is received by Publisher can it begin transmitting the messages to Subscriber. In response to the request for messages from Publisher, Subscriber receives Subscription. Now Subscriber can use Subscription to interact with Publisher and vice versa. How many messages Publisher should send is mentioned in Subscription so the requested messages by Subscribers should be less than or equal to that number [message count <= total number]. 
• Thirdly, Publisher cannot send more messages to Subscriber than requested by Subscriber.

These three points together form a part of the backpressure we mentioned when we began with Reactive Streams.

And yes, the count requested by  Subscriber from Publisher is not binding on Publisher as per the other rule, not binding with respect to the count of messages. Publisher is allowed to send less than the requested count of messages from Subscriber. This can be described with the following.