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Parseq: Better living through asynchronicity!

Douglas Crockford
2018-04-25
Public Domain

Parseq provides a straight-forward, functional approach to the management of
asynchronicity. Parseq embraces the paradigm of distributed message passing.

You should structure your application as a set of requestor functions that each
perform or manage a unit of work. This is a good design pattern in general. The
temporal relationships between the requestors is specified by assembling the
requestors into sets that are passed to the parseq factories.

parseq.js is a module that exports a parseq object that contains five factory
functions:

    parseq.fallback
    parseq.parallel
    parseq.parallel_object
    parseq.race
    parseq.sequence

Each of these factory functions returns a requestor function. A factory function
may throw an exception if it finds problems in its parameters.

A requestor function is any function that takes a callback and a value.

    a_requestor(callback, value)

The requestor will do some work or send a message to another process or system.
When the work is done, the requestor signals the result by passing a value or
an error report to its callback. The callback could be called in a future turn,
so the requestor does not need to block, nor should it ever block.

A requestor will pass its 'value' parameter to any requestors that it starts. A
sequence will use the 'value' parameter to its first requestor. It will then
pass the result of a requestor to the next requestor.

The requestor should not throw an exception. It should communicate all failures
through its callback. A requestor function may return a cancel function.

A callback function takes two arguments: 'success' and 'error'.

    a_callback(success, error)

If 'success' is undefined, then failure is being signalled. If 'success' is not
undefined, then the value of 'success' is the result. 'error' may contain
information explaining the failure.

A cancel function takes a 'reason' argument that might be propogated as the
'error' of some callback.

    cancel(reason)

The cancel function attempts to stop the operation of the requestor. If a
program decides that it no longer needs the result of a requestor, it can call
the cancel function that the requestor returned. This is not an undo operation.
It is just a way of stopping unneeded work.

All of these factories (except for parallel_object) take an array of requestor
functions. The parallel factory can take two arrays of requestor functions.

All of the factories can take a time limit expressed in milliseconds. The
requestor that the factory returns will fail if it can not finish its work in
the allotted time. If 'milliseconds' is 0 or undefined, then there will be no
time limit.

Three of the factories (parallel, parallel_object, and race) can take a
'throttle' argument. Normally, these factories want to start all of their
requestors at once. Unfortunately, that can cause some incompetent systems to
fail due to resource exhaustion or other limitations. The 'throttle' puts an
upper limit on the number of requestors that can be running at once. Please be
aware that some of your requestors might not start running until others have
finished. You need to factor that delay into your time limits.


The factories:

parseq.fallback(requestor_array, milliseconds)

    parseq.fallback returns a requestor function. When the requestor is called,
    it will call the first element in 'requestor_array'. If that is ultimately
    successful, its value will be passed to the callback. But if the requestor
    fails, the next element will be called, and so on. If none of the elements
    is successful, then the fallback fails. If any succeeds, then the fallback
    succeeds.

    If 'milliseconds' is 0 or undefined, then there is no time limit. If
    'milliseconds' is greater than zero, then that imposes a time limit. The
    fallback requestor will fail if it can not finish in time.

    The fallback requestor will return a 'cancel' function that can be
    called when the result is no longer needed.


parseq.parallel(required_array, optional_array, milliseconds, throttle, option)

    parseq.parallel returns a requestor that processes many requestors in
    parallel, producing an array of all of the successful results. It does not
    add parallelism to JavaScript. It allows you to exploit the natural
    parallelism of the universe.

    parseq.parallel can take two arrays of requestors: Those that are required
    to produce results, and those that may optionally produce results. The
    parallel operation only succeeds if all of the required requestors succeed.
    Failure of optional requestors does not cause the parallel operation to
    fail.

    The result maps the success values of the required requestors and optional
    requestors into a single array. The value produced by the first element of
    the requestors array provides the first element of the result.

    If the 'milliseconds' argument is supplied, then a time limit is imposed.
    The result must be complete before the time expires.

    If there is no time limit, and if there are required requestors, then the
    parallel operation is finished when all of the required requestors are done.
    All unfinished optional requestors will be cancelled.

    If 'throttle' is not undefined, then there will be a limit on the number of
    requestors that will be active at a time.

    The 'option' parameter works when there are both requireds and optionals.
    It can have one of three values:

         undefined       The requireds must finish before the time limit.
                         The optionals must finish before the requireds
                         finish.

         true            The requireds and the optionals must all finish before
                         the time limit.

         false           The requireds have no time limit. The optionals
                         must finish before the requireds finish and the
                         time limit, whichever is later.


parseq.parallel_object(
    required_object, optional_object, milliseconds, throttle, option
)

    parseq.parallel_object is like parseq.parallel except that it operates on
    objects of requestors instead of on arrays of requestors. It successful,
    it will deliver an object of results. A key from a requestor object will
    be used as the key for the requestor's result.


parseq.race(requestor_array, milliseconds, throttle)

    parseq.race returns a requestor that starts all of the functions in the
    requestors array in parallel. Its result is the result of the first of those
    requestors to successfully finish. All of the other requestors will be
    cancelled. If all of those requestors fail, then the race fails.

    If the 'milliseconds' argument is supplied, then if no requestor has been
    successful in the allotted time, then the race fails, and all pending
    requestors are cancelled.

    If 'throttle' is not undefined, then there will be a limit on the
    number of requestors that will be active at a time.


parseq.sequence(requestor_array, milliseconds)

    parseq.sequence returns a requestor that processes each element of the
    requestors array one at a time. Each requestor will be passed the result of
    the previous requestor as its 'value' argument. If all succeed, then the
    sequence succeeds, giving the result of the last of the requestors. If any
    fail, then the sequence fails.

    If the optional 'milliseconds' argument is supplied, then if all of the
    requestors have not all completed in the allotted time, then the
    sequence fails and the pending requestor is cancelled.