Linda is a framework for building systems that are composed of programs that cooperate among themselves in order to realize a larger goal. A Linda application is composed of two or more processes acting in concert. One process acts as a server and the others act as clients. Fine-grained communications between client and server is provided by way of message passing over sockets and support networks, TIPC sockets in this case. Clients interact indirectly by way of the server. The server is in principle an eraseable blackboard that clients can use to write (out/1), read (rd/1) and remove (in/1) messages called tuples. Some predicates will fail if a requested tuple is not present on the blackboard. Others will block until a tuple instance becomes available. Tuple instances are made available to clients by writing them on the blackboard using out/1.
In TIPC Linda, there is a subtle difference between the in
and the
rd predicates that is worth noting. The in
predicates succeed exactly once for each tuple placed in the tuple
space. The tuple is provided to exactly one requesting client. Clients
can contend for tuples in this way, thus enabling multi-server
operations. The rd predicates succeed nondeterministically,
providing all matching tuples in the tuple space at a given time to the
requesting client as a choice point without disturbing them.
TIPC Linda is inspired by and adapted from the SICStus Prolog API. But unlike SICStus TCP Linda, TIPC Linda is connectionless. There is no specific session between client and server. The server receives and responds to datagrams originated by clients in an epiperiodic manner.
Example: A simple producer-consumer.
In client 1:
init_producer :-
linda_client(global),
producer.
producer :-
produce(X),
out(p(X)),
producer.
produce(X) :- .....
In client 2:
init_consumer :-
linda_client(global),
consumer.
consumer :-
in(p(A)),
consume(A),
consumer.
consume(A) :- .....
Example: Synchronization
...,
in(ready), %Waits here until someone does out(ready)
...,
Example: A critical region
...,
in(region_free), % wait for region to be free
critical_part,
out(region_free), % let next one in
...,
Example: Reading global data
...,
rd(data(Data)),
...,
or, without blocking:
...,
(rd_noblock(data(Data)) ->
do_something(Data)
; write('Data not available!'),nl
),
...,
Example: Waiting for any one of several events
...,
in([e(1),e(2),...,e(n)], E),
% Here is E instantiated to the first tuple that became available
...,
Example: Producers and Consumers in the same process using linda_eval
threads and/or tuple predicates
consumer1 :-
repeat,
in([p(_), quit], Y),
( Y = p(Z) -> writeln(consuming(Z)); !),
fail.
producer1 :-
forall(between(1,40, X), out(p(X))).
producer_consumer1 :-
linda_eval(consumer1),
call_cleanup(producer1, out(quit)), !.
%
%
consumer2 :-
between(1,4,_),
in_noblock(p(X)), !,
writeln(consuming(X)),
consumer2.
producer2 :-
linda_eval(p(X), between(1,40, X)).
producer_consumer2 :-
producer2,
linda_eval(consumer2), !.
%
%
consumer3 :-
forall(rd_noblock(p(X)), writeln(consuming(X))).
producer3 :-
tuple(p(X), between(1,40, X)).
producer_consumer3 :-
producer3,
linda_eval(done, consumer3),
in(done), !.
The server is the process running the "blackboard process". It is part of TIPC Linda. It is a collection of predicates that are registered as tipc_broadcast listeners. The server process can be run on a separate machine if necessary.
To load the package, enter the query:
?- use_module(library(tipc/tipc_linda)).
?- linda.
TIPC Linda server now listening at: port_id('<1.1.1:3200515722>')
true.
The clients are one or more Prolog processes that have connection(s)
to the server.
To load the package, enter the query:
?- use_module(library(tipc/tipc_linda)).
?- linda_client(global).
TIPC Linda server listening at: port_id('<1.1.1:3200515722>')
true.
port_id('<1.1.1:3200515722>')
). This predicates looks to see if a server is already listening on the
cluster. If so, it reports the address of the existing server.
Otherwise, it registers a new server and reports its address.
?- linda.
TIPC Linda server now listening at: port_id('<1.1.1:3200515722>')
true.
?- linda.
TIPC Linda server still listening at: port_id('<1.1.1:3200515722>')
true.
The following will call my_init/0 in the current module after the server is successfully started or is found already listening. my_init/0 could start client-processes, initialize the tuple space, etc.
?- linda(my_init).
global, is supported. A client may interact
with any server reachable on the TIPC cluster. This predicate will fail
if no server is reachable for that tuple space.in
and rd requests. Replies arriving outside of this window
are silently ignored. OldTime is unified with the old timeout
and then timeout is set to NewTime. NewTime is of
the form Seconds:Milliseconds. A non-negative real number, seconds, is
also recognized. The default is 0.250 seconds. This timeout is thread
local and is not inherited from its parent. New threads are
initialized to the default.
Note: The synchronous behavior afforded by in/1 and rd/1 is implemented by periodically polling the server. The poll rate is set according to this timeout. Setting the timeout too small may result in substantial network traffic that is of little value.
error(feature_not_supported). SICStus Linda can disable the
timeout by specifying off as NewTime. This
feature does not exist for safety reasons.?- out(x(a,3)), out(x(a,4)), out(x(b,3)), out(x(c,3)). true. ?- bagof_rd_noblock(C-N, x(C,N), L). L = [a-3,a-4,b-3,c-3] . true. ?- bagof_rd_noblock(C, N^x(C,N), L). L = [a,a,b,c] . true.
Joining Threads: Threads created using linda_eval/(1-2) are not allowed to linger. They are joined (blocking the parent, if necessary) under three conditions: backtracking on failure into an linda_eval/(1-2), receipt of an uncaught exception, and cut of choice-points. Goals are evaluated using forall/2. They are expected to provide nondeterministic behavior. That is they may succeed zero or more times on backtracking. They must however, eventually fail or succeed deterministically. Otherwise, the thread will hang, which will eventually hang the parent thread. Cutting choice points in the parent's body has the effect of joining all children created by the parent. This provides a barrier that guarantees that all child instances of Goal have run to completion before the parent proceeds. Detached threads behave as above, except that they operate independently and cannot be joined. They will continue to run while the host process continues to run.
Here is an example of a parallel quicksort:
qksort([], []).
qksort([X | List], Sorted) :-
partition(@>(X), List, Less, More),
linda_eval(qksort(More, SortedMore)),
qksort(Less, SortedLess), !,
in_noblock(qksort(More, SortedMore)),
append(SortedLess, [X | SortedMore], Sorted).
Note: A virtual tuple is an extension of the server. Even
though it is operating in the client's Prolog environment, it is
restricted in the server operations that it may perform. It is generally
safe for tuple predicates to perform out/1
operations, but it is unsafe for them to perform any variant of in
or rd, either directly or indirectly. This restriction is
however, relaxed if the server and client are operating in separate
heavyweight processes (not threads) on the node or cluster. This is most
easily achieved by starting a stand-alone Linda server somewhere on the
cluster. See
tipc_linda_server/0,
below.
out(server_quit), the server's Prolog process
will exit via halt/1. It is intended for
use in scripting as follows:
swipl -q -g 'use_module(library(tipc/tipc_linda)),
tipc_linda_server' -t 'halt(1)'
See also manual section 2.10.2.1 Using PrologScript.
Note: Prolog will return a non-zero exit status if this predicate is executed on a cluster that already has an active server. An exit status of zero is returned on graceful shutdown.
permission_error(halt,thread,2),context(halt/1,Only
from thread’main')), if this predicate is executed in a thread
other than main.