- Jeffrey Rosenwald (JeffRose@acm.org)
- See also
- Linux only
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 TIPC broadcast library removes that restriction. With this library loaded, any member of a TIPC network that also has this library loaded may hear and respond to your broadcasts. Using TIPC Broadcast, it becomes a nearly trivial matter to build an instance of supercomputer that researchers within the High Performance Computer community refer to as "Beowulf Class Cluster Computers."
This module has no public predicates. When this module is initialized, it does three things:
- It starts a listener daemon thread that listens for broadcasts from others, received as TIPC datagrams, and
- It registers three listeners: tipc_node/1, tipc_cluster/1, and tipc_zone/1, and
- It registers three listeners: tipc_node/2, tipc_cluster/2, and tipc_zone/2.
A broadcast/1 or broadcast_request/1
that is not directed to one of the six listeners 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 TIPC's multicast addressing facility. A TIPC
broadcast or broadcast request takes the typical form:
The principal functors
tipc_zone, specify the scope of the broadcast. The functor
tipc_node, specifies that the broadcast is to be confined
to members of a present TIPC node. Likewise,
tipc_zone, specify that the traffic should be confined
to members of a present TIPC cluster and zone, respectively. 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(tipc_node(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 TIPC scoped broadcast/1 or broadcast_request/1, where PortId is the 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 TIPC 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.
Although this capability is needed under some circumstances, it has a tendency to compromise the resilience of the broadcast model. You should not rely on it too heavily, or fault tolerance will suffer.
For example, in order to discover who responded with a particular value:
Process A: ?- listen(number(X), between(1, 3, X)). true. ?- Process B: ?- listen(number(X), between(7, 9, X)). true. ?- Process C: ?- broadcast_request(tipc_node(number(X):From)). X = 7, From = port_id('<1.1.1:3971170279>') ; X = 8, From = port_id('<1.1.1:3971170279>') ; X = 9, From = port_id('<1.1.1:3971170279>') ; X = 1, From = port_id('<1.1.1:3971170280>') ; X = 2, From = port_id('<1.1.1:3971170280>') ; X = 3, From = port_id('<1.1.1:3971170280>') ; false. ?-
While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:
- TIPC broadcast now requires an initialization step in order to launch the broadcast listener daemon. See tipc_initialize/0.
- 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 TIPC is involved.
- A TIPC broadcast/1 is completely asynchronous.
- A TIPC 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. An optional second argument is provided so that a sender may specify more (or less) time for replies.
- Replies are no longer collected using findall/3. Replies are presented to the user as a choice point on arrival, until the broadcast request timer finally expires. This change 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 will now 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 TIPC 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 TIPC scope is not reentrant (at least, not now anyway). If a listener performs a broadcast_request/1 with TIPC scope recursively, then disaster looms certain. This caveat does not apply to a TIPC scoped broadcast/1, which can safely be performed from a listener context.
- TIPC's capacity is not infinite. While TIPC can tolerate substantial bursts of activity, it is designed for short bursts of small messages. It can tolerate several thousand replies in response to a broadcast_request/1 without trouble, but it will begin to encounter congestion beyond that. And in congested conditions, things will start to become unreliable as TIPC begins prioritizing and/or discarding traffic.
- A TIPC 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 TIPC broadcast/1 always succeeds, even if there are no listeners.
- A TIPC 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. See prolog flag option, 'float_format', of current_prolog_flag/2.
- [nondet]tipc_host_to_address(?Service, ?Address)
- locates a TIPC service by name. Service is an atom or
grounded term representing the common name of the service. Address
is a TIPC address structure. A server may advertise its services by name
by including the fact, tipc:
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. The search is zone-wide. Locating a service however, does not imply that the service is actually reachable from any particular node within the zone.
- See tipc:tipc_initialize/0