- Reference manual
- Jeffrey Rosenwald (JeffRose@acm.org)
- See also
- http://tipc.sf.net, http://www.erlang.org
- Linux only
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:
- [det]tipc_socket(-SocketId, +SocketType)
- Creates a TIPC-domain socket of the type specified by
SocketType, and unifies it to an identifier, SocketId.
SocketType is one of the following atoms:
- rdm - unnumbered, reliable datagram service,
- dgram - unnumbered, unreliable datagram service,
- seqpacket - numbered, reliable datagram service, and
- stream - reliable, connection-oriented byte-stream service
socket_error('Address family not supported by protocol')is thrown if a TIPC server is not available on the current host.
- Closes the indicated socket, making SocketId invalid. In
stream applications, sockets are closed by closing both stream handles
returned by tipc_open_socket/3.
There are two cases where
used because there are no stream-handles:
- After tipc_accept/3, the server does a fork/1 to handle the client in a sub-process. In this case the accepted socket is not longer needed from the main server and must be discarded using tipc_close_socket/1.
- If, after discovering the connecting client with tipc_accept/3, the server does not want to accept the connection, it should discard the accepted socket immediately using tipc_close_socket/1.
SocketId the socket identifier returned by tipc_socket/2 or tipc_accept/3.
- socket_error('Invalid argument) is thrown if an attempt is made to close a socket identifier that has already been closed.
- [det]tipc_open_socket(+SocketId, -InStream, -OutStream)
- Opens two SWI-Prolog I/O-streams, one to deal with input from the socket and one with output to the socket. If tipc_bind/3 has been called on the socket, OutStream is useless and will not be created. After closing both InStream and OutStream, the socket itself is discarded.
- [det]tipc_bind(+Socket, +Address, +ScopingOption)
- Associates/disassociates a socket with the name/3
or name_seq/3 address specified in Address.
It also registers/unregisters it in the topology server name table. This
makes the address visible/invisible to the rest of the network according
to the scope specified in ScopingOption. ScopingOption
is a grounded term that is one of:
- where Scope is one of:
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.
- where Scope is as defined above. An application may target a specific
address for removal from its collection of addresses by specifying the
address and its scope. The scoping option,
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.
- [det]tipc_listen(+Socket, +Backlog)
- Listens for incoming requests for connections. Backlog indicates how many pending connection requests are allowed. Pending requests are requests that are not yet acknowledged using tipc_accept/3. If the indicated number is exceeded, the requesting client will be signalled that the service is currently not available. A suggested default value is 5.
- [det]tipc_accept(+Socket, -Slave, -Peer)
- Blocks on a server socket and waits for connection requests from clients. On success, it creates a new socket for the client and binds the identifier to Slave. Peer is bound to the TIPC address, port_id/2, of the client.
- [det]tipc_connect(+Socket, +TIPC_address)
- Provides a connection-oriented, client-interface to connect a socket to
a given TIPC_address. After successful completion,
tipc_open_socket/3 may be
used to create I/O-Streams to the remote socket.
socket_error('Connection refused'), if there are no servers bound to the specified address.
socket_error('Connection timed out'), if no server that is bound to the specified address accepts the connect request within the specified time limit. See also tipc_setopt/2.
- [det]tipc_get_name(+Socket, -TIPC_address)
- Unifies TIPC_address with the port-id assigned to the socket.
- [det]tipc_get_peer_name(+Socket, -TIPC_address)
- Unifies TIPC_address with the port-id assigned to the socket
that this socket is connected to.
socket_error('Transport endpoint is not connected'), if an attempt is made to obtain a peer's name of an unconnected socket.
- [det]tipc_setopt(+Socket, +Option)
- Sets options on the socket. Defined options are:
- Allow sockets to assign a priority to their traffic. Priority is one of
- Allow TIPC to silently discard packets in congested situations, rather than queuing them for later transmission.
- Allow TIPC to silently discard packets in congested situations, rather than returning them to the sender as undeliverable.
- Specifies the time interval that tipc_connect/2 will use before abandoning a connection attempt. Default: 8.000 sec.
- [det]tipc_receive(+Socket, -Data, -From, +OptionList)
- Waits for, and returns the next datagram. Like its UDP counterpart, the
data are returned as a Prolog string object (see string_codes/2). From
is an address structure of the form port_id/2,
indicating the sender of the message.
Defined options are:
- Defines the returned term-type. Type is one of atom, codes or string (default).
- Poll the socket and return immediately. If a message is present, it is
returned. If not, then an exception,
error(socket_error('Resource temporarily unavailable'), _), 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).
- [det]tipc_send(+Socket, +Data, +To, +Options)
- sends a TIPC datagram to one or more destinations. Like its UDP
counterpart, Data is a string, atom or code-list providing
the data to be sent. To is a name/3, name_seq/3,
or port_id/2 address structure. See
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
port_id(0,0), is returned if the destination address was invalid. Senders and receivers should beware of this possibility.
- [det]tipc_canonical_address(-CanonicalAddress, +PortId)
- Translates a port_id/2 address into
canonical TIPC form:
- tipc_address(Zone, Cluster, Node, Reference)
- It is provided for debugging an printing purposes only. The canonical address is not used for any other purpose.
- [semidet]tipc_service_exists(+Address, +Timeout)
- Interrogates the TIPC topology server to see if a service is available
at an advertised Address.
Address is one of:
name(Type, Instance, Domain)or
name_seq(Type, Lower, Upper). A name/3, address is translated to a name_seq/3, following, where Lower and Upper are assigned the value of Instance. Domain is unused and must be zero. A
name_seq(Type, Lower, Upper)is a multi-cast address. This predicate succeeds if there is at least one service that would answer according to multi-cast addressing rules.
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.
- [nondet]tipc_service_probe(?Address, ?PortId)
- Allows a user to discover the instance ranges and/or port-ids for a
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.
- [det]tipc_service_port_monitor(+Addresses, :Goal)
- [det]tipc_service_port_monitor(+Addresses, :Goal, ?Timeout)
- Monitors a collection of worker threads that are bound to a list of Addresses.
A single port monitor may be used to provide surveillance over workers
that are providing a number of different services. For a given address
type, discontiguous port ranges may be specified, but overlapping port
ranges may not. Goal for example, may simply choose to
broadcast the notification, thus delegating the notification event
handling to others.
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 structure:
- when the worker binds its socket to the address.
- when the worker unbinds its socket from the address.
Timeout is optional. It is one of:
- a non-negative real number that specifies the number of seconds that surveillance is to be continued.
- causes the monitor to run forever in the current thread (e.g. never returns).
- causes the monitor to run forever as a separate thread. ThreadId is unified with the thread identifier of the monitor thread. This is useful when the monitor is required to provide continuous surveillance, while operating in the background.
- causes the TIPC service and the TIPC stack to be initialized and made
ready for service. An application must call this predicate as part of
its initialization prior to any use of TIPC predicates. Please note
the change of the API.
socket_error('Address family not supported by protocol')if a TIPC server is not available on the current host.
- 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
- Jeffrey Rosenwald (JeffRose@acm.org)
- See also
- Linux only, tipc_broadcast
This module provides a replicated data store that is coordinated using a variation on Lamport's Paxos concensus protocol. The original method is described in his paper entitled, "The Part-time Parliament", which was published in 1998. The algorithm is tolerant of non-Byzantine failure. That is late or lost delivery or reply, but not senseless delivery or reply. The present algorithm takes advantage of the convenience offered by multicast to the quorum's membership, who can remain anonymous and who can come and go as they please without effecting Liveness or Safety properties.
Paxos' quorum is a set of one or more attentive members, whose processes respond to queries within some known time limit (< 20ms), which includes roundtrip delivery delay. This property is easy to satisfy given that every coordinator is necessarily a member of the quorum as well, and a quorum of one is permitted. An inattentive member (e.g. one whose actions are late or lost) is deemed to be "not-present" for the purposes of the present transaction and consistency cannot be assured for that member. As long as there is at least one attentive member of the quorum, then persistence of the database is assured.
Each member maintains a ledger of terms along with information about when they were originally recorded. The member's ledger is deterministic. That is to say that there can only be one entry per functor/arity combination. No member will accept a new term proposal that has a line number that is equal-to or lower-than the one that is already recorded in the ledger.
Paxos is a three-phase protocol:
1: A coordinator first prepares the quorum for a new proposal by broadcasting a proposed term. The quorum responds by returning the last known line number for that functor/arity combination that is recorded in their respective ledgers.
2: The coordinator selects the highest line number it receives, increments it by one, and then asks the quorum to finally accept the new term with the new line number. The quorum checks their respective ledgers once again and if there is still no other ledger entry for that functor/arity combination that is equal-to or higher than the specified line, then each member records the term in the ledger at the specified line. The member indicates consent by returning the specified line number back to the coordinator. If consent is withheld by a member, then the member returns a
nackinstead. The coordinator requires unanimous consent. If it isn't achieved then the proposal fails and the coordinator must start over from the beginning.
3: Finally, the coordinator concludes the successful negotiation by broadcasting the agreement to the quorum in the form of a
paxos_changed(Term)event. This is the only event that should be of interest to user programs.
For practical reasons, we rely on the partially synchronous behavior (e.g. limited upper time bound for replies) of broadcast_request/1 over TIPC to ensure Progress. Perhaps more importantly, we rely on the fact that the TIPC broadcast listener state machine guarantees the atomicity of broadcast_request/1 at the process level, thus obviating the need for external mutual exclusion mechanisms.
Note that this algorithm does not guarantee the rightness of the value proposed. It only guarantees that if successful, the value proposed is identical for all attentive members of the quorum.
Note also that tipc_paxos now requires an initialization step. See tipc_initialize/0.
- [semidet]tipc_paxos_set(?Term, +Retries)
- negotiates to have Term recorded in the ledger for each of
the quorum's members. This predicate succeeds if the quorum unanimously
accepts the proposed term. If no such entry exists in the Paxon's
ledger, then one is silently created. tipc_paxos_set/1
will retry the transaction several times (default: 20) before failing.
Failure is rare and is usually the result of a collision of two or more
writers writing to the same term at precisely the same time. On failure,
it may be useful to wait some random period of time, and then retry the
transaction. By specifying a retry count of zero, tipc_paxos_set/2
will succeed iff the first ballot succeeds.
On success, tipc_paxos_set/1 will also broadcast the term
paxos_changed(Term), to the quorum.
Term is a compound that may have unbound variables. Retries (optional) is a non-negative integer specifying the number of retries that will be performed before a set is abandoned.
- unifies Term with the entry retrieved from the Paxon's
ledger. If no such entry exists in the member's local cache, then the
quorum is asked to provide a value, which is verified for consistency.
An implied tipc_paxos_set/1
follows. This predicate succeeds if a term with the same functor and
arity exists in the Paxon's ledger, and fails otherwise.
Term is a compound. Any unbound variables are unified with those provided in the ledger entry.
- declares that Term is to be automatically replicated to the
quorum each time it becomes grounded. It uses the behavior afforded by
Term is an ungrounded Term
- [det]tipc_paxos_on_change(?Term, :Goal)
- executes the specified Goal when Term changes. tipc_paxos_on_change/2
listens for paxos_changed/1 notifications
for Term, which are emitted as the result of successful tipc_paxos_set/1
transactions. When one is received for Term, then Goal
is executed in a separate thread of execution.
Term is a compound, identical to that used for tipc_paxos_get/1. Goal is one of:
- a callable atom or term, or
- the atom
ignore, which causes monitoring for Term to be discontinued.
- See tipc:tipc_initialize/0.
- Jeffrey A. Rosenwald
- See also
- Nicholas Carriero and David Gelernter. How to Write Parallel Programs: A First Course. The MIT Press, Cambridge, MA, 1990.
- - SWI-Prolog for Linux only
- tipc_broadcast library
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.
- Starts a Linda-server in this process. The network address is written to
current output stream as a TIPC
port_id/2 reference (e.g.
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.
- Establishes a connection to a Linda-server providing a named tuple
space. Domain is an atom specifying a particular tuple-space,
selected from a universe of tuple-spaces. At present however, only one
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.
- Closes the connection to the Linda-server. Causes the server to release resources associated with this client.
- [semidet]linda_timeout(?OldTime, ?NewTime)
- Controls Linda's message-passing timeout. It specifies the time window
where clients will accept server replies in response to
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.
- Temporarily sets Linda's timeout. Internally, the original timeout is saved and then the timeout is set to NewTime. NewTime is as described in linda_timeout/2. The original timeout is restored automatically on cut of choice points, failure on backtracking, or uncaught exception.
- Places a Tuple in Linda's tuple-space.
- Atomically removes the tuple Tuple from Linda's tuple-space if it is there. The tuple will be returned to exactly one requestor. If no tuple is available, the predicate blocks until it is available (that is, someone performs an out/1).
- Atomically removes the tuple Tuple from Linda's tuple-space if it is there. If not, the predicate fails. This predicate can fail due to a timeout.
- [det]in(+TupleList, -Tuple)
- As in/1 but succeeds when any one of the tuples in TupleList is available. Tuple is unified with the fetched tuple.
- Succeeds nondeterministically if Tuple is available in the tuple-space, suspends otherwise until it is available. Compare this with in/1: the tuple is not removed.
- Succeeds nondeterministically if Tuple is available in the tuple-space, fails otherwise. This predicate can fail due to a timeout.
- [nondet]rd(?TupleList, -Tuple)
- As in/2 but provides a choice point that does not remove any tuples.
- [nondet]bagof_in_noblock(?Template, ?Tuple, -Bag)
- [nondet]bagof_rd_noblock(?Template, ?Tuple, -Bag)
- Bag is the list of all instances of Template such
that Tuple exists in the tuple-space. The behavior of
variables in Tuple and Template is as in bagof/3.
The variables could be existentially quantified with ^/2
as in bagof/3. The operation is performed
as an atomic operation. This predicate can fail due to a timeout.
Example: Assume that only one client is connected to the server and that
the tuple-space initially is empty.
?- 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.
- [det]linda_eval(?Head, :Goal)
- [det]linda_eval_detached(?Head, :Goal)
- Causes Goal to be evaluated in parallel with a parent
predicate. The child thread is a full-fledged client, possessing the
same capabilities as the parent. Upon successful completion of Goal,
unbound variables are unified and the result is sent to the Linda server
via out/1, where it is made available
to others. linda_eval/2
evaluates Goal, then unifies the result with Head,
providing a means of customizing the resulting output structure. In linda_eval/1, Head,
Goal are identical, except that the module name for Head
is stripped before output. If the child fails or receives an uncaught
exception, no such output occurs.
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).
- [det]tuple(?Head, :Goal)
- registers Head as a virtual tuple in TIPC Linda's tuple
space. On success, any client on the cluster may reference the tuple, Head,
using rd/1 or rd_noblock/1.
On reference, Goal is executed by a separate thread of
execution in the host client's Prolog process. The result is unified
with Head, which is then returned to the guest client. As in
linda_eval/(1-2) above, Goal is evaluated using forall/2.
The virtual tuple is unregistered on backtracking into a tuple/(1-2),
receipt of uncaught exception, or cut of choice-points. In tuple/1,
Head and Goal are identical, except that the
module name is stripped from Head.
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 cluster. See tipc_linda_server/0, below.
- Acts as a stand-alone Linda server. This predicate initializes the TIPC
stack and then starts a Linda server in the current thread. If a client
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 22.214.171.124 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
- See tipc:tipc_initialize/0.