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