# Multi-threading programming

Logtalk provides experimental support for multi-threading programming on selected Prolog compilers. Logtalk makes use of the low-level Prolog built-in predicates that implement message queues and interface with POSIX threads and mutexes (or a suitable emulation), providing a small set of high-level predicates and directives that allows programmers to easily take advantage of modern multi-processor and multi-core computers without worrying about the tricky details of creating, synchronizing, or communicating with threads, mutexes, and message queues. Logtalk multi-threading programming integrates with object-oriented programming providing a threaded engines API, enabling objects and categories to prove goals concurrently, and supporting synchronous and asynchronous messages.

## Enabling multi-threading support

Multi-threading support may be disabled by default. It can be enabled on the Prolog adapter files of supported compilers by setting the read-only threads compiler flag to `supported`.

## Enabling objects to make multi-threading calls

The threaded/0 object directive is used to enable an object to make multi-threading calls:

    :- threaded.

## Multi-threading built-in predicates

Logtalk provides a small set of built-in predicates for multi-threading programming. For simple tasks where you simply want to prove a set of goals, each one in its own thread, Logtalk provides a threaded/1 built-in predicate. The remaining predicates allow for fine-grained control, including postponing retrieving of thread goal results at a later time, supporting non-deterministic thread goals, and making one-way asynchronous calls. Together, these predicates provide high-level support for multi-threading programming, covering most common use cases.

### Proving goals concurrently using threads

A set of goals may be proved concurrently by calling the Logtalk built-in predicate threaded/1. Each goal in the set runs in its own thread.

When the `threaded/1` predicate argument is a conjunction of goals, the predicate call is akin to and-parallelism. For example, assume that we want to find all the prime numbers in a given interval, `[N, M]`. We can split the interval in two parts and then span two threads to compute the prime numbers in each sub-interval:

    prime_numbers(N, M, Primes) :-
        M > N,
        N1 is N + (M - N) // 2,
        N2 is N1 + 1,
        threaded((
            prime_numbers(N2, M, [], Acc),
            prime_numbers(N, N1, Acc, Primes)
        )).

    prime_numbers(N, M, Acc, Primes) :-
        ...

The `threaded/1` call terminates when the two implicit threads terminate. In a computer with two or more processors (or with a processor with two or more cores) the code above can be expected to provide better computation times when compared with single-threaded code for sufficiently large intervals.

When the `threaded/1` predicate argument is a disjunction of goals, the predicate call is akin to or-parallelism, here reinterpreted as a set of goals competing to find a solution. For example, consider the different methods that we can use to find the roots of real functions. Depending on the function, some methods will faster than others. Some methods will converge into the solution while others may diverge and never find it. We can try all the methods simultaneously by writing:

    find_root(Function, A, B, Error, Zero) :-
        threaded((
            bisection::find_root(Function, A, B, Error, Zero)
        ;   newton::find_root(Function, A, B, Error, Zero)
        ;   muller::find_root(Function, A, B, Error, Zero)
        )).

The above `threaded/1` goal succeeds when one of the implicit threads succeeds in finding the function root, leading to the termination of all the remaining competing threads.

The `threaded/1` built-in predicate is most useful for lengthy, independent deterministic computations where the computational costs of each goal outweigh the overhead of the implicit thread creation and management.

### Proving goals asynchronously using threads

A goal may be proved asynchronously using a new thread by calling the threaded_call/1-2 built-in predicate . Calls to this predicate are always true and return immediately (assuming a callable argument). The term representing the goal is copied, not shared with the thread. The thread computes the first solution to the goal, posts it to the implicit message queue of the object from where the `threaded_call/1` predicate was called, and suspends waiting for either a request for an alternative solution or for the program to commit to the current solution.

The results of proving a goal asynchronously in a new thread may be later retrieved by calling the threaded_exit/1-2 built-in predicate within the same object where the call to the `threaded_call/1` predicate was made. The `threaded_exit/1` calls suspend execution until the results of the `threaded_call/1` calls are sent back to the object message queue.

The `threaded_exit/1` predicate allow us to retrieve alternative solutions through backtracking (if you want to commit to the first solution, you may use the threaded_once/1-2 predicate instead of the `threaded_call/1` predicate). For example, assuming a `lists` object implementing the usual `member/2` predicate, we could write:

    | ?- threaded_call(lists::member(X, [1,2,3])).

    X = _G189
    yes

    | ?- threaded_exit(lists::member(X, [1,2,3])).

    X = 1 ;
    X = 2 ;
    X = 3 ;
    no

In this case, the `threaded_call/1` and the `threaded_exit/1` calls are made within the pseudo-object `user`. The implicit thread running the `lists::member/2` goal suspends itself after providing a solution, waiting for a request to an alternative solution; the thread is automatically terminated when the runtime engine detects that backtracking to the `threaded_exit/1` call is no longer possible.

Calls to the `threaded_exit/1` predicate block the caller until the object message queue receives the reply to the asynchronous call. The predicate threaded_peek/1-2 may be used to check if a reply is already available without removing it from the thread queue. The `threaded_peek/1` predicate call succeeds or fails immediately without blocking the caller. However, keep in mind that repeated use of this predicate is equivalent to polling a message queue, which may hurt performance.

Be careful when using the `threaded_exit/1` predicate inside failure-driven loops. When all the solutions have been found (and the thread generating them is therefore terminated), re-calling the predicate will generate an exception. Note that failing instead of throwing an exception is not an acceptable solution as it could be misinterpreted as a failure of the `threaded_call/1` argument.

The example on the previous section with prime numbers could be rewritten using the `threaded_call/1` and `threaded_exit/1` predicates:

    prime_numbers(N, M, Primes) :-
        M > N,
        N1 is N + (M - N) // 2,
        N2 is N1 + 1,
        threaded_call(prime_numbers(N2, M, [], Acc)),
        threaded_call(prime_numbers(N, N1, Acc, Primes)),
        threaded_exit(prime_numbers(N2, M, [], Acc)),
        threaded_exit(prime_numbers(N, N1, Acc, Primes)).

    prime_numbers(N, M, Acc, Primes) :-
        ...

When using asynchronous calls, the link between a `threaded_exit/1` call and the corresponding `threaded_call/1` call is established using unification. If there are multiple `threaded_call/1` calls for a matching `threaded_exit/1` call, the connection can potentially be established with any of them (this is akin to what happens with tabling). Nevertheless, you can easily use a call tag by using in alternative threaded_call/2, threaded_once/2, and threaded_exit/2 built-in predicates. For example:

    ?- threaded_call(member(X, [1,2,3]), Tag).

    Tag = 1
    yes

    ?- threaded_call(member(X, [1,2,3]), Tag).

    Tag = 2
    yes

    ?- threaded_exit(member(X, [1,2,3]), 2).

    X = 1 ;
    X = 2 ;
    X = 3
    yes

When using these predicates, the tags shall be considered as an opaque term; users shall not rely on its type. Tagged asynchronous calls can be canceled by using the threaded_cancel/1 predicate.

## One-way asynchronous calls

Sometimes we want to prove a goal in a new thread without caring about the results. This may be accomplished by using the built-in predicate threaded_ignore/1. For example, assume that we are developing a multi-agent application where an agent may send an “happy birthday” message to another agent. We could write:

    ..., threaded_ignore(agent::happy_birthday), ...

The call succeeds with no reply of the goal success, failure, or even exception ever being sent back to the object making the call. Note that this predicate implicitly performs a deterministic call of its argument.

## Asynchronous calls and synchronized predicates

Proving a goal asynchronously using a new thread may lead to problems when the goal results in side effects such as input/output operations or modifications to an object database. For example, if a new thread is started with the same goal before the first one finished its job, we may end up with mixed output, a corrupted database, or unexpected goal failures. In order to solve this problem, predicates (and grammar rule non-terminals) with side effects can be declared as synchronized by using the synchronized/1 predicate directive. Proving a query to a synchronized predicate (or synchronized non-terminal) is internally protected by a mutex, thus allowing for easy thread synchronization. For example:

    % ensure thread synchronization
    :- synchronized(db_update/1).

    db_update(Update) :-
        % predicate with side-effects
        ...

A second example: assume an object defining two predicates for writing, respectively, even and odd numbers in a given interval to the standard output. Given a large interval, a goal such as:

    | ?- threaded_call(obj::odd_numbers(1,100)),
         threaded_call(obj::even_numbers(1,100)).

    1 3 2 4 6 8 5 7 10 ...
    ...

will most likely result in a mixed up output. By declaring the `odd_numbers/2` and `even_numbers/2` predicates synchronized:

    :- synchronized([
        odd_numbers/2,
        even_numbers/2]).

one goal will only start after the other one finished:

    | ?- threaded_ignore(obj::odd_numbers(1,99)),
         threaded_ignore(obj::even_numbers(1,99)).

    1 3 5 7 9 11 ...
    ...
    2 4 6 8 10 12 ...
    ...

Note that, in a more realistic scenario, the two `threaded_ignore/1` calls would be made concurrently from different objects. Using the same synchronized directive for a set of predicates imply that they all use the same mutex, as required for this example.

As each Logtalk entity is independently compiled, this directive must be included in every object or category that contains a definition for the described predicate, even if the predicate declaration is inherited from another entity, in order to ensure proper compilation. Note that a synchronized predicate cannot be declared dynamic. To ensure atomic updates of a dynamic predicate, declare as synchronized the predicate performing the update.

Synchronized predicates may be used as wrappers to messages sent to objects that are not multi-threading aware. For example, assume a `log` object defining a `write_log_entry/2` predicate that writes log entries to a file, thus using side effects on its implementation. We can specify and define e.g. a `sync_write_log_entry/2` predicate as follows:

    :- synchronized(sync_write_log_entry/2).

    sync_write_log_entry(File, Entry) :-
        log::write_log_entry(File, Entry).

and then call the `sync_write_log_entry/2` predicate instead of the `write_log_entry/2` predicate from multi-threaded code.

The synchronization directive may be used when defining objects that may be reused in both single-threaded and multi-threaded Logtalk applications. The directive simply make calls to the synchronized predicates deterministic when the objects are used in a single-threaded application.

## Synchronizing threads through notifications

Declaring a set of predicates as synchronized can only ensure that they are not executed at the same time by different threads. Sometimes we need to suspend a thread not on a synchronization lock but on some condition that must hold true for a thread goal to proceed. I.e. we want a thread goal to be suspended until a condition becomes true instead of simply failing. The built-in predicate threaded_wait/1 allows us to suspend a predicate execution (running in its own thread) until a notification is received. Notifications are posted using the built-in predicate threaded_notify/1. A notification is a Prolog term that a programmer chooses to represent some condition becoming true. Any Prolog term can be used as a notification argument for these predicates. Related calls to the `threaded_wait/1` and `threaded_notify/1` must be made within the same object, this, as the object message queue is used internally for posting and retrieving notifications.

Each notification posted by a call to the `threaded_notify/1` predicate is consumed by a single `threaded_wait/1` predicate call (i.e. these predicates implement a peer-to-peer mechanism). Care should be taken to avoid deadlocks when two (or more) threads both wait and post notifications to each other.

## Threaded engines

Threaded engines provide an alternative to the multi-threading predicates described in the previous sections. An engine is a computing thread whose solutions can be lazily computed and retrieved. In addition, an engine also supports a term queue that allows passing arbitrary terms to the engine.

An engine is created by calling the threaded_engine_create/3 built-in predicate. For example:

    | ?- threaded_engine_create(X, member(X, [1,2,3]), worker).
    yes

The first argument is an answer template to be used for retrieving solution bindings. The user can name the engine, as in this example where the atom `worker` is used, or have the runtime generate a name, which should be treated as an opaque term.

Engines are scoped by the object within which the `threaded_engine_create/3` call takes place. Thus, different objects can create engines with the same names with no conflicts. Moreover, engines share the visible predicates of the object creating them.

The engine computes the first solution of its goal argument and suspends waiting for it to be retrieved. Solutions can be retrieved one at a time using the threaded_engine_next/2 built-in predicate:

    | ?- threaded_engine_next(worker, X).
    X = 1
    yes

The call blocks until a solution is available and fails if there are no solutions left. After returning a solution, this predicate signals the engine to start computing the next one. Note that this predicate is deterministic. In contrast with the `threaded_exit/1-2` built-in predicates, retrieving the next solution requires calling the predicate again instead of by backtracking into its call. For example:

    collect_all(Engine, [Answer| Answers]) :-
        threaded_engine_next(Engine, Answer),
        !,
        collect_all(Engine, Answers).
    collect_all(_, []).

There is also a reified alternative version of the predicate, threaded_engine_next_reified/2, which returns `the(Answer)`, `no`, and `exception(Error)` terms as answers. Using this predicate, collecting all solutions to an engine uses a different programming pattern:

    ... :-
        ...,
        threaded_engine_next_reified(Engine, Reified),
        collect_all_reified(Reified, Engine, Answers),
        ...

    collect_all_reified(no, _, []).
    collect_all_reified(the(Answer), Engine, [Answer| Answers]) :-
        threaded_engine_next_reified(Engine, Reified),
        collect_all_reified(Reified, Engine, Answers).

Engines must be explicitly terminated using the threaded_engine_destroy/1 built-in predicate:

    | ?- threaded_engine_destroy(worker).
    yes

A common usage pattern for engines is to define a recursive predicate that uses the engine term queue to retrieve a task to be performed. For example, assume we define the following predicate:

    loop :-
        threaded_engine_fetch(Task),
        handle(Task),
        loop.

The threaded_engine_fetch/1 built-in predicate fetches a task for the engine term queue. The engine clients would use the threaded_engine_post/2 built-in predicate to post tasks into the engine term queue. The engine would be created using the call:

    | ?- threaded_engine_create(none, loop, worker).

    yes

The `handle/1` predicate, after performing a task, can use the threaded_engine_yield/1 built-in predicate to make the task results available for consumption using the `threaded_engine_next/2` and `threaded_engine_next_reified/2` built-in predicates. Blocking semantics are used by these two predicates: the `threaded_engine_yield/1` predicate blocks until the returned solution is consumed while the `threaded_engine_next/2` predicate blocks until a solution becomes available.

## Multi-threading performance

The performance of multi-threading applications is highly dependent on the backend Prolog compiler, on the operating-system, and on the use of dynamic binding and dynamic predicates. All compatible backend Prolog compilers that support multi-threading features make use of POSIX threads or pthreads. The performance of the underlying pthreads implementation can exhibit significant differences between operating systems. An important point is synchronized access to dynamic predicates. As different threads may try to simultaneously access and update dynamic predicates, these operations may used a lock-free algorithm or be protected by a lock, usually implemented using a mutex. In the latter case, poor mutex lock operating-system performance, combined with a large number of collisions by several threads trying to acquire the same lock, can result in severe performance penalties. Thus, whenever possible, avoid using dynamic predicates and dynamic binding.