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
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.
After initialization and, in the case of a unicast network managing
the set of peers, communication happens through broadcast/1,
broadcast_request/1 and listen/1,2,3.
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
?- listen(number(X), between(1, 5, X)).
?- listen(number(X), between(7, 9, X)).
?- 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)).
Host A Process 1:
?- listen(number(X), between(7, 9, X)).
Host A Process 2:
?- listen(number(X), between(1, 5, X)).
?- 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
- 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_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
- 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
- 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
- - Jeffrey Rosenwald (JeffRose@acm.org), Jan Wielemaker
- See also
- - BSD-2
- udp_broadcast_service(?Scope, ?Address) is nondet[private]
- provides the UDP broadcast address for a given Scope. At present,
only one scope is supported,
- udp_scope(?ScopeName, ?ScopeDef)[private]
- make_private_socket is det[private]
- Create our private socket. This socket is used for messages that are
directed to me. Note that we only need this for broadcast networks.
If we use a unicast network we use our public port to contact this
- make_public_socket(+ScopeData, +Scope)[private]
- Create the public port Scope.
- Dispatch inbound traffic. This loop uses wait_for_input/3 to wait
for one or more UDP sockets and dispatches the requests using the
internal broadcast service. For an incomming broadcast request we
send the reply only to the requester and therefore we must use a
socket that is not in broadcast mode.
- ld_dispatch(+PrivateSocket, +Term, +From, +Scope)[private]
- Locally dispatch Term received from From. If it concerns a broadcast
request, send the replies to PrivateSocket to From. The multifile
hook black_list/1 can be used to ignore certain messages.
- Update the UDP relaying proxy service. The proxy consists of three
- Listen on our scope. If any messages are received, hand them
to udp_broadcast/3 to be broadcasted to scope or sent to a
- Listen on the scope public port. Incomming messages are
relayed to the internal broadcast mechanism and replies are sent
to from our private socket.
- Listen on our private port and reply using the same port.
- Close a UDP broadcast scope.
- udp_broadcast(+What, +Scope, +TimeOut)[private]
- Send a broadcast request to my UDP peers in Scope. What is either of
Term:Address to send Term to a specific address or query
the address from which term is answered or it is a plain Term.
If Term is nonground, it is considered is a request (see
broadcast_request/1) and the predicate succeeds for each answer
received within TimeOut seconds. If Term is ground it is considered
an asynchronous broadcast and udp_broadcast/3 is deterministic.
- udp_basic_broadcast(+Term, +Dest) is multi[private]
- Create a UDP private socket and use it to send Term to Address. If
Address is our broadcast address, set the socket in broadcast mode.
This predicate succeeds with a choice point. Committing the choice
point closes S.
|Dest||- is one of |
- 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
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
- 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.
|Address||- has canonical form |
- 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
|Scope||- is the scope for which the message is broadcasted. This
can be used to use different serialization for different scopes.|
|Term||- encapsulates the term broadcasted by the application as
- Is sent by
- request(Id, ApplTerm)
- Is sent by broadcast_request/1, where Id is a unique large
(64 bit) integer.
- reply(Id, ApplTerm)
- Is sent to reply on a broadcast_request/1 request that has
been received. Arguments are the same as above.
- - The hook may throw
udp(invalid_message) to stop processing
- udp_term_string(+Scope, +Term, -String) is det[private]
- udp_term_string(+Scope, -Term, +String) is semidet[private]
- Serialize an arbitrary Prolog term as a string. The string is
prefixed by a magic key to ensure we only accept messages that are
meant for us.
In mode (+,-), Term is written with the options
This predicate first calls udp_term_string_hook/3.
- unicast_out_of_scope_request(+Scope, +From, +Data) is semidet[private]
- 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
:- multifile udp_broadcast:udp_unicast_join_hook/3.
udp_broadcast:udp_unicast_join_hook(Scope, From, join(welcome)) :-
join_request(Scope, Address, Reply) :-
?- join_request(myscope, "18.104.22.168":10001, Reply).
Reply = welcome.
The following predicates are exported, but not or incorrectly documented.
- udp_peer(Arg1, Arg2)
- udp_peer_del(Arg1, Arg2)