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 library support three styles of networking as described below. Each of these networks have their own advantages and disadvantages. Please study the literature to understand the consequences.
- Broadcast messages are sent to the LAN subnet. The broadcast implementation uses two UDP ports: a public to address the whole group and a private one to address a specific node. Broadcasting is generally a good choice if the subnet is small and traffic is low.
- Unicast sends copies of packages to known peers. Unicast networks can easily be routed. The unicast version uses a single UDP port per node. Unicast is generally a good choice for a small party, in particular if the peers are in different networks.
- Multicast is like broadcast, but it can be configured to work accross networks and may work more efficiently on VLAN networks. Like the broadcast setup, two UDP ports are used. Multicasting can in general deliver the most efficient LAN and WAN networks, but requires properly configured routing between the peers.
A broadcast/1 or broadcast_request/1 of the shape
udp(Scope, Term) or
udp(Scope, Term, TimeOut) is forwarded over the UDP network to all peers
that joined the same Scope. To prevent the potential for feedback
loops, only the plain Term is broadcasted locally. The timeout is
optional. It specifies the amount to time to wait for replies to arrive
in response to a broadcast_request/1. The default period is 0.250
seconds. The timeout is ignored for broadcasts.
An example of three separate processes cooperating in the same scope
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(peers, 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(peers,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].
All incomming trafic is handled by a single thread with the alias
udp_inbound_proxy. This thread also performs the internal dispatching
using broadcast/1 and broadcast_request/1. Future versions may provide
for handling these requests in seperate threads.
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 proxy. 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. With UDP, all peers will send all answers to the query. The receiver may however stop listening.
- 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_string/3. Serialization does not deal with cycles, attributes or sharing. The hook udp_term_string_hook/3 may be defined to change the message serialization and support different message formats and/or encryption.
- 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, 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!
- Close a UDP broadcast scope.
- udp_broadcast_initialize(+IPAddress, +Options) is semidet
- Initialized UDP broadcast bridge. IPAddress is the IP address on the
network we want to broadcast on. IP addresses are terms
ip(A,B,C,D)or an atom or string of the format
A.B.C.D. Options processed:
- Name of the scope. Default is
- Subnet to broadcast on. This uses the same syntax as IPAddress. Default classifies the network as class A, B or C depending on the the first octet and applies the default mask.
- Public port to use. Default is 20005.
- Method to send a message to multiple peers. One of
- Use UDP broadcast messages to the LAN. This is the default
- Use UDP multicast messages. This can be used on WAN networks, provided the intermediate routers understand multicast.
- Send the messages individually to all registered peers.
For compatibility reasons Options may be the subnet mask.
- udp_peer_add(+Scope, +Address) is det
- udp_peer_del(+Scope, ?Address) is det
- udp_peer(?Scope, ?Address) is nondet
- Manage and query the set of known peers for a unicast network. Address is either a term IP:Port or a plain IP address. In the latter case the default port registered with the scope is used.
- udp_term_string_hook(+Scope, +Term, -String) is det[multifile]
- udp_term_string_hook(+Scope, -Term, +String) is semidet[multifile]
- Hook for serializing the message Term. The default writes
%prolog\n, followed by the Prolog term in quoted notation while ignoring operators. This hook may use alternative serialization such as fast_term_serialized/2, use library(ssl) to realise encrypted messages, etc.
- udp_unicast_join_hook(+Scope, +From, +Data) is semidet[multifile]
- This multifile hook is called if an UDP package is received on the
port of the unicast network identified by Scope. From is the origin
IP and port and Data is the message data that is deserialized as
defined for the scope (see udp_term_string/3).
This hook is intended to initiate a new node joining the network of peers. We could in theory also omit the in-scope test and use a normal broadcast to join. Using a different channal however provides a basic level of security. A possibe implementation is below. The first fragment is a hook added to the server, the second is a predicate added to a client and the last initiates the request in the client. The excanged term (
join(X)) can be used to exchange a welcome handshake.
:- multifile udp_broadcast:udp_unicast_join_hook/3. udp_broadcast:udp_unicast_join_hook(Scope, From, join(welcome)) :- udp_peer_add(Scope, From),
join_request(Scope, Address, Reply) :- udp_peer_add(Scope, Address), broadcast_request(udp(Scope, join(X))).
?- join_request(myscope, "188.8.131.52":10001, Reply). Reply = welcome.
The following predicates are exported, but not or incorrectly documented.