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|The TIPC libraries: library(tipc/...)|
Transparent Inter-Process Communication (TIPC) provides a flexible,
reliable, fault-tolerant, high-speed, and low-overhead framework for
inter-process communication between federations of trusted peers,
operating as a unit. It was developed by Ericsson AB, as a means to
provide for communications between Common Control Systems processes and
Network Element peers in telephone switching systems, sometimes
operating at arm's length on different line cards or mainframes.
Delegation of responsibility in this way is one of the fundamental
precepts of the Erlang programming system, also developed at Ericsson.
TIPC represents a more generalized version of the same behavioral design
pattern. For an overview, please see:
The TIPC module uses the error handling functions from
and therefore all the functions below may throw
where Code is the lowercase version of the C-macro error code
and Message is an atom describing the error in a human
friendly format, depending on the current locale. See the socket library
|SocketType||is one of the following atoms:
|SocketId||the socket identifier returned by tipc_socket/2 or tipc_accept/3.|
node. Servers may bind to more than one address by making successive calls to tipc_bind/3, one for each address that it wishes to advertise. The server will receive traffic for all of them. A server may, for example, register one address with node scope, another with cluster scope, and a third with zone scope. A client may then limit the scope of its transmission by specifying the appropriate address.
no_scope(all), may be used to unbind the socket from all of its registered addresses. This feature allows an application to gracefully exit from service. Because the socket remains open, the application may continue to service current transactions to completion. TIPC however, will not schedule any new work for the server instance. If no other servers are available, the work will be rejected or dropped according to the socket options specified by the client.
Note that clients do not need to bind to any address. Its port-id is sufficient for this role. And server sockets (e.g. those that are bound to name/3 or name_seq/3, addresses) may not act as clients. That is, they may not originate connections from the socket using tipc_connect/2. Servers however, may originate datagrams from bound sockets using tipc_send/4. Please see the TIPC programmers's guide for other restrictions.
Defined options are:
error(socket_error(eagain, Message), _), will be thrown. Users are cautioned not to "spin" unnecessarily on non-blocking receives as they may prevent the system from servicing other background activities such as XPCE event dispatching.
The typical sequence to receive a connectionless TIPC datagram is:
receive :- tipc_socket(S, dgram), tipc_bind(S, name(18888, 10, 0), scope(zone)), repeat, tipc_receive(Socket, Data, From, [as(atom)]), format('Got ~q from ~q~n', [Data, From]), Data == quit, !, tipc_close_socket(S).
tipc_overview.txt, for more information on TIPC Address Structures. Options is currently unused.
A simple example to send a connectionless TIPC datagram is:
send(Message) :- tipc_socket(S, dgram), tipc_send(S, Message, name(18888, 10,0), ), tipc_close_socket(S).
Messages are delivered silently unless some form of congestion was
encountered and the
dest_droppable(false) option was issued
on the sender's socket. In this case, the send succeeds but a
notification in the form of an empty message is returned to the sender
from the receiver, indicating some kind of delivery failure. The port-id
of the receiver is returned in congestion conditions. A
is returned if the destination address was invalid. Senders and
receivers should beware of this possibility.
|Address||is one of: |
|Timeout||is optional. It is a non-negative real number that specifies the amount of time in seconds to block and wait for a service to become available. Fractions of a second are also permissible.|
|Address||is a name_seq/3 address. The address type must be grounded.|
|PortId||is unified with the port-id for a specific name_sequence address.|
|Addresses||is a list of name/3 or name_seq/3 addresses for the services to be monitored.|
|Goal||is a predicate that will be called when
a worker's publication status changes. The Goal is called
exactly once per event with its the last argument unified with the
|Timeout||is optional. It is one of:
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:
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:
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.
This module provides compatibility for using paxos over TIPC. As of
SWI-Prolog 7.7.15 the core of this module has been moved to the core
library(paxos) and can be used with other
distributed implementations of
library(broadcast) such as
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
rd predicates that is worth noting. The
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
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) :- .....
..., 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
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.
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.
rdrequests. 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
offas 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
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
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 220.127.116.11 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