Provided by: libxs-dev_1.2.0-2build1_amd64 
NAME
xs_socket - create Crossroads socket
SYNOPSIS
void *xs_socket (void *context, int type);
DESCRIPTION
The xs_socket() function shall create a Crossroads socket within the specified context and
return an opaque handle to the newly created socket. The type argument specifies the
socket type, which determines the semantics of communication over the socket.
The newly created socket is initially unbound, and not associated with any endpoints. In
order to establish a message flow a socket must first be connected to at least one
endpoint with xs_connect(3), or at least one endpoint must be created for accepting
incoming connections with xs_bind(3).
Key differences to conventional sockets. Generally speaking, conventional sockets present
a synchronous interface to either connection-oriented reliable byte streams (SOCK_STREAM),
or connection-less unreliable datagrams (SOCK_DGRAM). In comparison, Crossroads sockets
present an abstraction of an asynchronous message queue, with the exact queueing semantics
depending on the socket type in use. Where conventional sockets transfer streams of bytes
or discrete datagrams, Crossroads sockets transfer discrete messages.
Crossroads sockets being asynchronous means that the timings of the physical connection
setup and tear down, reconnect and effective delivery are transparent to the user and
organized by Crossroads library itself. Further, messages may be queued in the event that
a peer is unavailable to receive them.
Conventional sockets allow only strict one-to-one (two peers), many-to-one (many clients,
one server), or in some cases one-to-many (multicast) relationships. With the exception of
XS_PAIR, Crossroads sockets may be connected to multiple endpoints using xs_connect(),
while simultaneously accepting incoming connections from multiple endpoints bound to the
socket using xs_bind(), thus allowing many-to-many relationships.
Thread safety. Crossroads sockets are not thread safe. Applications MUST NOT use a socket
from multiple threads except after migrating a socket from one thread to another with a
"full fence" memory barrier.
Socket types. Crossroads defines several messaging patterns which encapsulate exact
semantics of a particular topology. For example, publish-subscribe pattern defines data
distribution trees while request-reply defines networks of shared stateless services. Each
pattern defines several socket types (roles in the pattern).
The following sections present the socket types defined by Crossroads library:
Request-reply pattern
The request-reply pattern is used for sending requests from a client to one or more
instances of a stateless service, and receiving subsequent replies to each request sent.
XS_REQ
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
Compatible peer sockets
XS_REP
Send/receive pattern Send, Receive, Send, Receive,
...
Outgoing routing strategy Load-balanced
Incoming routing strategy Last peer
XS_HWM option action Block
XS_REP
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
Compatible peer sockets XS_REQ
Send/receive pattern Receive, Send, Receive, Send,
...
Incoming routing strategy Fair-queued
Outgoing routing strategy Last peer
XS_HWM option action Drop
XS_XREQ
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
Compatible peer sockets XS_XREP, XS_REP
Send/receive pattern Unrestricted
Outgoing routing strategy Load-balanced
Incoming routing strategy Fair-queued
XS_HWM option action Block
XS_XREP
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
Compatible peer sockets XS_XREQ, XS_REQ
Send/receive pattern Unrestricted
Outgoing routing strategy See text
Incoming routing strategy Fair-queued
XS_HWM option action Drop
Publish-subscribe pattern
The publish-subscribe pattern is used for one-to-many distribution of data from a single
publisher to multiple subscribers in a fan out fashion.
XS_PUB
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
Compatible peer sockets XS_SUB, XS_XSUB
Send/receive pattern Send only
Incoming routing strategy N/A
Outgoing routing strategy Fan out
XS_HWM option action Drop
XS_SUB
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
Compatible peer sockets XS_PUB, XS_XPUB
Send/receive pattern Receive only
Incoming routing strategy Fair-queued
Outgoing routing strategy N/A
XS_HWM option action Drop
XS_XPUB
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
Compatible peer sockets XS_SUB, XS_XSUB
Send/receive pattern Send messages, receive
subscriptions
Incoming routing strategy N/A
Outgoing routing strategy Fan out
XS_HWM option action Drop
XS_XSUB
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
Compatible peer sockets XS_PUB, XS_XPUB
Send/receive pattern Receive messages, send
subscriptions
Incoming routing strategy Fair-queued
Outgoing routing strategy N/A
XS_HWM option action Drop
Pipeline pattern
The pipeline pattern is used for distributing data to nodes arranged in a pipeline. Data
always flows down the pipeline, and each stage of the pipeline is connected to at least
one node. When a pipeline stage is connected to multiple nodes data is load-balanced among
all connected nodes.
XS_PUSH
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
Compatible peer sockets XS_PULL
Direction Unidirectional
Send/receive pattern Send only
Incoming routing strategy N/A
Outgoing routing strategy Load-balanced
XS_HWM option action Block
XS_PULL
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
Compatible peer sockets XS_PUSH
Direction Unidirectional
Send/receive pattern Receive only
Incoming routing strategy Fair-queued
Outgoing routing strategy N/A
XS_HWM option action N/A
Survey pattern
Survey pattern can be used to post a survey to a set of notes and collect responses from
them. The survey is distributed from surveyor to all connected respondents. Responses are
routed back to the original surveyor.
XS_SURVEYOR
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
Compatible peer sockets XS_RESPONDENT, XS_XRESPONDENT
Direction Bidirectional
Send/receive pattern Send one message, receive many
messages.
Incoming routing strategy Fair-queued
Outgoing routing strategy Fan out
XS_HWM option action Drop
XS_RESPONDENT
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
Compatible peer sockets XS_SURVEYOR, XS_XSURVEYOR
Direction Bidirectional
Send/receive pattern Receive a survey, send one
response.
Incoming routing strategy Fair-queued
Outgoing routing strategy Last peer
XS_HWM option action Drop
XS_XSURVEYOR
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
Compatible peer sockets XS_RESPONDENT, XS_XRESPONDENT
Direction Bidirectional
Send/receive pattern Send surveys, receive responses.
Incoming routing strategy Fair-queued
Outgoing routing strategy Fan out
XS_HWM option action Drop
XS_XRESPONDENT
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
Compatible peer sockets XS_SURVEYOR, XS_XSURVEYOR
Direction Bidirectional
Send/receive pattern Receive surveys, send responses.
Incoming routing strategy Fair-queued
Outgoing routing strategy See text
XS_HWM option action Drop
Exclusive pair pattern
The exclusive pair is an advanced pattern used for communicating exclusively between two
peers.
XS_PAIR
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.
Note
XS_PAIR sockets are experimental, and are currently missing several features such
as auto-reconnection.
Table 15. Summary of XS_PAIR characteristics
Compatible peer sockets XS_PAIR
Direction Bidirectional
Send/receive pattern Unrestricted
Incoming routing strategy N/A
Outgoing routing strategy N/A
XS_HWM option action Block
RETURN VALUE
The xs_socket() function shall return an opaque handle to the newly created socket if
successful. Otherwise, it shall return NULL and set errno to one of the values defined
below.
ERRORS
EINVAL
The requested socket type is invalid.
EFAULT
The provided context is invalid.
EMFILE
The limit on the total number of open Crossroads sockets has been reached.
ETERM
The context specified was terminated.
SEE ALSO
xs_init(3) xs_setsockopt(3) xs_bind(3) xs_connect(3) xs_send(3) xs_recv(3) xs(7)
AUTHORS
The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and
Martin Lucina <martin@lucina.net[2]>.
NOTES
1. sustrik@250bpm.com
mailto:sustrik@250bpm.com
2. martin@lucina.net
mailto:martin@lucina.net