A socket of type XS_REQ is used by a client to send requests to and receive replies from a service. This socket type allows only an alternating sequence of xs_send(request) and subsequent xs_recv(reply) calls. Each request sent is load-balanced among all services, and each reply received is matched with the last issued request.

When a XS_REQ socket enters an exceptional state due to having reached the high water mark for all services, or if there are no services at all, then any xs_send(3) operations on the socket shall block until the exceptional state ends or at least one service becomes available for sending; messages are not discarded.

Table 1. Summary of XS_REQ characteristics

A socket of type XS_REP is used by a service to receive requests from and send replies to a client. This socket type allows only an alternating sequence of xs_recv(request) and subsequent xs_send(reply) calls. Each request received is fair-queued from among all clients, and each reply sent is routed to the client that issued the last request. If the original requester doesn’t exist any more the reply is silently discarded.

When a XS_REP socket enters an exceptional state due to having reached the high water mark for a client, then any replies sent to the client in question shall be dropped until the exceptional state ends.

Table 2. Summary of XS_REP characteristics

A socket of type XS_XREQ is a socket type underlying XS_REQ. It doesn’t impose the strict order of sends and recvs as XS_REQ does and it is intended for use in intermediate devices in request-reply topologies.

Each message sent is load-balanced among all connected peers, and each message received is fair-queued from all connected peers.

When a XS_XREQ socket enters an exceptional state due to having reached the high water mark for all peers, or if there are no peers at all, then any xs_send(3) operations on the socket shall block until the exceptional state ends or at least one peer becomes available for sending; messages are not discarded.

Table 3. Summary of XS_XREQ characteristics

A socket of type XS_XREP is a socket type underlying XS_REP. It doesn’t impose the strict order of sends and recvs as XS_REQ does and it is intended for use in intermediate devices in request-reply topologies.

Messages received are fair-queued from among all connected peers. The outbound messages are routed to a specific peer, as explained below.

When a XS_XREP socket enters an exceptional state due to having reached the high water mark for all peers, or if there are no peers at all, then any messages sent to the socket shall be dropped until the exceptional state ends. Likewise, any messages to be routed to a non-existent peer or a peer for which the individual high water mark has been reached shall also be dropped.

Table 4. Summary of XS_XREP characteristics

A socket of type XS_PUB is used by a publisher to distribute data. Messages sent are distributed in a fan out fashion to all connected peers. The xs_recv(3) function is not implemented for this socket type.

When a XS_PUB socket enters an exceptional state due to having reached the high water mark for a subscriber, then any messages that would be sent to the subscriber in question shall instead be dropped until the exceptional state ends. The xs_send() function shall never block for this socket type.

Table 5. Summary of XS_PUB characteristics

A socket of type XS_SUB is used by a subscriber to subscribe to data distributed by a publisher. Initially a XS_SUB socket is not subscribed to any messages, use the XS_SUBSCRIBE option of xs_setsockopt(3) to specify which messages to subscribe to. The xs_send() function is not implemented for this socket type.

Table 6. Summary of XS_SUB characteristics

Same as XS_PUB except that you can receive subscriptions from the peers in form of incoming messages. Subscription message is a byte 1 (for subscriptions) or byte 0 (for unsubscriptions) followed by the subscription body.

Table 7. Summary of XS_XPUB characteristics

Same as XS_SUB except that you subscribe by sending subscription messages to the socket. Subscription message is a byte 1 (for subscriptions) or byte 0 (for unsubscriptions) followed by the subscription body.

Table 8. Summary of XS_XSUB characteristics

A socket of type XS_PUSH is used by a pipeline node to send messages to downstream pipeline nodes. Messages are load-balanced to all connected downstream nodes. The xs_recv() function is not implemented for this socket type.

When a XS_PUSH socket enters an exceptional state due to having reached the high water mark for all downstream nodes, or if there are no downstream nodes at all, then any xs_send(3) operations on the socket shall block until the exceptional state ends or at least one downstream node becomes available for sending; messages are not discarded.

Table 9. Summary of XS_PUSH characteristics

A socket of type XS_PULL is used by a pipeline node to receive messages from upstream pipeline nodes. Messages are fair-queued from among all connected upstream nodes. The xs_send() function is not implemented for this socket type.

Table 10. Summary of XS_PULL characteristics

XS_SURVEYOR socket type can be used to send surveys to all respondents in the topology and receive the replies from all of them. Each survey sent is distributed to all connected peers, and incoming replies are fair-queue. As you don’t know the number of respondents in the topology you don’t know the number of responses you are going to get, therefore you should use XS_SURVEY_TIMEOUT socket option to set the deadline for the survey.

Table 11. Summary of XS_SURVEYOR characteristics

This socket type receives surveys from surveyors and sends responses. Incoming surveys are fair-queued. Outgoing responses are routed back to the original surveyor.

Table 12. Summary of XS_RESPONDENT characteristics

A socket of type XS_XSURVEYOR is a socket type underlying XS_SURVEYOR. It doesn’t impose the strict order of sends and recvs as XS_SURVEYOR does and it is intended for use in intermediate devices in survey topologies.

Table 13. Summary of XS_XSURVEYOR characteristics

A socket of type XS_XRESPONDENT is a socket type underlying XS_RESPONDENT. It doesn’t impose the strict order of sends and recvs as XS_RESPONDENT does and it is intended for use in intermediate devices in survey topologies.

Incoming surveys are fair-queued. Each survey is prefixed by a message part identifying the surveyor it was received from. Outgoing responses are routed to the original surveyor based on the first message part.

Table 14. Summary of XS_XRESPONDENT characteristics

A socket of type XS_PAIR can only be connected to a single peer at any one time. No message routing or filtering is performed on messages sent over a XS_PAIR socket.

When a XS_PAIR socket enters an exceptional state due to having reached the high water mark for the connected peer, or if no peer is connected, then any xs_send(3) operations on the socket shall block until the peer becomes available for sending; messages are not discarded.

Table 15. Summary of XS_PAIR characteristics