XS_SOCKET(3) Crossroads I/O Manual XS_SOCKET(3)NAMExs_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 ALSOxs_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
Crossroads I/O 1.2.0 02/17/2016 XS_SOCKET(3)
