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# SUB protocol
The {{i:*SUB* protocol}}{{hi:*SUB*}} is one half of a publisher/{{i: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_][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`][nng_sub_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.
- {{i:`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.
[nng_sub_open]: TODO.md
[nng_socket_set]: TODO.md
[pub]: ./pub.md
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