19.1 Caveats

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_subnet scope 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!

[nondet]udp_broadcast_service(?Domain, ?Address)

provides the UDP broadcast address for a given Domain. At present, only one domain is supported, udp_subnet.

[nondet]udp_host_to_address(?Service, ?Address)

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.

[semidet]udp_broadcast_initialize(+IPAddress, +SubnetMask)

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.