SWI-Prolog's broadcast library provides a means that may be used to facilitate publish and subscribe communication regimes between anonymous members of a community of interest. The members of the community are however, necessarily limited to a single instance of Prolog. The UDP broadcast library removes that restriction. With this library loaded, any member on your local IP subnetwork that also has this library loaded may hear and respond to your broadcasts.
This module has only two public predicates. When the module is initialized, it starts a two listener threads that listen for broadcasts from others, received as UDP datagrams.
Unlike TIPC broadcast, UDP broadcast has only one scope,
broadcast/1 or broadcast_request/1 that is not directed to the listener
above, behaves as usual and is confined to the instance of Prolog that
originated it. But when so directed, the broadcast will be sent to all
participating systems, including itself, by way of UDP's multicast
addressing facility. A UDP broadcast or broadcast request takes the
broadcast(udp_subnet(+Term, +Timeout)). To prevent the
potential for feedback loops, the scope qualifier is stripped from the
message before transmission. The timeout is optional. It specifies the
amount to time to wait for replies to arrive in response to a
broadcast_request. The default period is 0.250 seconds. The timeout is
ignored for broadcasts.
An example of three separate processes cooperating on the same Node:
Process A: ?- listen(number(X), between(1, 5, X)). true. ?- Process B: ?- listen(number(X), between(7, 9, X)). true. ?- Process C: ?- findall(X, broadcast_request(udp_subnet(number(X))), Xs). Xs = [1, 2, 3, 4, 5, 7, 8, 9]. ?-
It is also possible to carry on a private dialog with a single responder. To do this, you supply a compound of the form, Term:PortId, to a UDP scoped broadcast/1 or broadcast_request/1, where PortId is the ip-address and port-id of the intended listener. If you supply an unbound variable, PortId, to broadcast_request, it will be unified with the address of the listener that responds to Term. You may send a directed broadcast to a specific member by simply providing this address in a similarly structured compound to a UDP scoped broadcast/1. The message is sent via unicast to that member only by way of the member's broadcast listener. It is received by the listener just as any other broadcast would be. The listener does not know the difference.
For example, in order to discover who responded with a particular value:
Host B Process 1: ?- listen(number(X), between(1, 5, X)). true. ?- Host A Process 1: ?- listen(number(X), between(7, 9, X)). true. ?- Host A Process 2: ?- listen(number(X), between(1, 5, X)). true. ?- bagof(X, broadcast_request(udp_subnet(number(X):From,1)), Xs). From = ip(192, 168, 1, 103):34855, Xs = [7, 8, 9] ; From = ip(192, 168, 1, 103):56331, Xs = [1, 2, 3, 4, 5] ; From = ip(192, 168, 1, 104):3217, Xs = [1, 2, 3, 4, 5].
While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:
- UDP broadcast requires an initialization step in order to launch the broadcast listener daemon. See udp_broadcast_initialize/2.
- Prolog's broadcast_request/1 is nondet. It sends the request, then evaluates the replies synchronously, backtracking as needed until a satisfactory reply is received. The remaining potential replies are not evaluated. This is not so when UDP is involved.
- A UDP broadcast/1 is completely asynchronous.
- A UDP broadcast_request/1 is partially synchronous. A broadcast_request/1 is sent, then the sender balks for a period of time (default: 250 ms) while the replies are collected. Any reply that is received after this period is silently discarded. A optional second argument is provided so that a sender may specify more (or less) time for replies.
- Replies are presented to the user as a choice point on arrival, until the broadcast request timer finally expires. This allows traffic to propagate through the system faster and provides the requestor with the opportunity to terminate a broadcast request early if desired, by simply cutting choice points.
- Please beware that broadcast request transactions remain active and resources consumed until broadcast_request finally fails on backtracking, an uncaught exception occurs, or until choice points are cut. Failure to properly manage this will likely result in chronic exhaustion of UDP sockets.
- If a listener is connected to a generator that always succeeds (e.g. a random number generator), then the broadcast request will never terminate and trouble is bound to ensue.
- broadcast_request/1 with
udp_subnetscope is not reentrant. If a listener performs a broadcast_request/1 with UDP scope recursively, then disaster looms certain. This caveat does not apply to a UDP scoped broadcast/1, which can safely be performed from a listener context.
- UDP broadcast's capacity is not infinite. While it can tolerate substantial bursts of activity, it is designed for short bursts of small messages. Unlike TIPC, UDP is unreliable and has no QOS protections. Congestion is likely to cause trouble in the form of non-Byzantine failure. That is, late, lost (e.g. infinitely late), or duplicate datagrams. Caveat emptor.
- A UDP broadcast_request/1 term that is grounded is considered to be a broadcast only. No replies are collected unless the there is at least one unbound variable to unify.
- A UDP broadcast/1 always succeeds, even if there are no listeners.
- A UDP broadcast_request/1 that receives no replies will fail.
- Replies may be coming from many different places in the network (or none at all). No ordering of replies is implied.
- Prolog terms are sent to others after first converting them to atoms using term_to_atom/2. Passing real numbers this way may result in a substantial truncation of precision.
- The broadcast model is based on anonymity and a presumption of trust--a perfect recipe for compromise. UDP is an Internet protocol. A UDP broadcast listener exposes a public port (20005), which is static and shared by all listeners, and a private port, which is semi-static and unique to the listener instance. Both can be seen from off-cluster nodes and networks. Usage of this module exposes the node and consequently, the cluster to significant security risks. So have a care when designing your application. You must talk only to those who share and contribute to your concerns using a carefully prescribed protocol.
- UDP broadcast categorically and silently ignores all message traffic originating from or terminating on nodes that are not members of the local subnet. This security measure only keeps honest people honest!
- udp_broadcast_service(?Domain, ?Address) is nondet
- provides the UDP broadcast address for a given Domain. At present,
only one domain is supported,
- udp_host_to_address(?Service, ?Address) is nondet
- locates a UDP service by name. Service is an atom or grounded term
representing the common name of the service. Address is a UDP
address structure. A server may advertise its services by name by
including the fact, udp:
host_to_address(+Service, +Address), somewhere in its source. This predicate can also be used to perform reverse searches. That is it will also resolve an Address to a Service name.
- udp_broadcast_initialize(+IPAddress, +SubnetMask) is semidet
- causes any required runtime initialization to occur. At present, proper operation of UDP broadcast depends on local information that is not easily obtained mechanically. In order to determine the appropriate UDP broadcast address, you must supply the IPAddress and SubnetMask for the node that is running this module. These data are supplied in the form of ip/4 terms. This is now required to be included in an applications intialization directive.