SUB protocol

The SUB protocol is one half of a publisher/subscriber pattern. In this pattern, a publisher sends data, which is broadcast to all subscribers. The subscribing applications only see the data to which they have subscribed.

The SUB protocol is the subscriber side, and the PUB protocol is the publisher side.

note

The publisher delivers all messages to all subscribers. The subscribers maintain their own subscriptions, and filter them locally. Thus, this pattern should not be used in an attempt to reduce bandwidth consumption.

The topics that subscribers subscribe to is compared to the leading bytes of the message body. Applications should construct their messages accordingly.

Socket Operations

The nng_sub0_open functions create a SUB socket. This socket may be used to receive messages, but is unable to send them. Attempts to send messages will result in NNG_ENOTSUP.

Protocol Versions

Only version 0 of this protocol is supported. (At the time of writing, no other versions of this protocol have been defined.)

Protocol Options

The following protocol-specific option is available.

  • NNG_OPT_SUB_PREFNEW:
    (bool)

    This read/write option specifies the behavior of the subscriber when the queue is full. When true (the default), the subscriber will make room in the queue by removing the oldest message. When false, the subscriber will reject messages if the message queue does not have room.

Protocol Headers

The SUB protocol has no protocol-specific headers.