rippled/doc/3_5_composed.qbk

[/
    Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
]

[section Writing Composed Operations]
[block'''<?dbhtml stop-chunking?>''']

Asynchronous operations are started by calling a free function or member
function known as an asynchronous ['__async_initfn__]. This function accepts
parameters specific to the operation as well as a "completion token." The
token is either a completion handler, or a type defining how the caller is
informed of the asynchronous operation result. __Asio__ comes with the
special tokens __use_future__ and __yield_context__ for using futures
and coroutines respectively. This system of customizing the return value
and method of completion notification is known as the
['Extensible Asynchronous Model] described in __N3747__, and a built in
to __N4588__. Here is an example of an initiating function which reads a
line from the stream and echoes it back. This function is developed
further in the next section:

[example_core_echo_op_1]

Authors using Beast can reuse the library's primitives to create their
own initiating functions for performing a series of other, intermediate
asynchronous operations before invoking a final completion handler.
The set of intermediate actions produced by an initiating function is
known as a
[@http://blog.think-async.com/2009/08/composed-operations-coroutines-and-code.html ['composed operation]].
To ensure full interoperability and well-defined behavior, __Asio__ imposes
requirements on the implementation of composed operations. These classes
and functions make it easier to develop initiating functions and their
composed operations:

[table Asynchronous Helpers
[[Name][Description]]
[[
    [link beast.ref.beast__async_completion `async_completion`]
][
    This class aggregates the completion handler customization point and
    the asynchronous initiation function return value customization point
    into a single object which exposes the appropriate output types for the
    given input types, and also contains boilerplate that is necessary to
    implement an initiation function using the Extensible Model.
]]
[[
    [link beast.ref.beast__async_return_type `async_return_type`]
][
    This template alias determines the return value of an asynchronous
    initiation function given the completion token and signature. It is used
    by asynchronous initiation functions to meet the requirements of the
    Extensible Asynchronous Model.
]]
[[
    [link beast.ref.beast__bind_handler `bind_handler`]
][
    This function returns a new, nullary completion handler which when
    invoked with no arguments invokes the original completion handler with a
    list of bound arguments. The invocation is made from the same implicit
    or explicit strand as that which would be used to invoke the original
    handler. This is accomplished by using the correct overload of
    `asio_handler_invoke` associated with the original completion handler.

]]
[[
    [link beast.ref.beast__handler_alloc `handler_alloc`]
][
    This class meets the requirements of [*Allocator], and uses any custom
    memory allocation and deallocation hooks associated with a given handler.
    It is useful for when a composed operation requires temporary dynamic
    allocations to achieve its result. Memory allocated using this allocator
    must be freed before the final completion handler is invoked.
]]
[[
    [link beast.ref.beast__handler_ptr `handler_ptr`]
][
    This is a smart pointer container used to manage the internal state of a
    composed operation. It is useful when the state is non trivial. For example
    when the state has non-copyable or expensive to copy types. The container
    takes ownership of the final completion handler, and provides boilerplate
    to invoke the final handler in a way that also deletes the internal state.
    The internal state is allocated using the final completion handler's
    associated allocator, benefiting from all handler memory management
    optimizations transparently.
]]
[[
    [link beast.ref.beast__handler_type `handler_type`]
][
    This template alias converts a completion token and signature to the
    correct completion handler type. It is used in the implementation of
    asynchronous initiation functions to meet the requirements of the
    Extensible Asynchronous Model.
]]
]



[section Echo]

This example develops an initiating function called [*echo].
The operation will read up to the first newline on a stream, and
then write the same line including the newline back on the stream.
The implementation performs both reading and writing, and has a
non-trivially-copyable state.
First we define the input parameters and results, then declare our
initiation function. For our echo operation the only inputs are the
stream and the completion token. The output is the error code which
is usually included in all completion handler signatures.

[example_core_echo_op_2]

Now that we have a declaration, we will define the body of the function.
We want to achieve the following goals: perform static type checking on
the input parameters, set up the return value as per __N3747__, and launch
the composed operation by constructing the object and invoking it.

[example_core_echo_op_3]

The initiating function contains a few relatively simple parts. There is
the customization of the return value type, static type checking, building
the return value type using the helper, and creating and launching the
composed operation object. The [*`echo_op`] object does most of the work
here, and has a somewhat non-trivial structure. This structure is necessary
to meet the stringent requirements of composed operations (described in more
detail in the __Asio__ documentation). We will touch on these requirements
without explaining them in depth.

Here is the boilerplate present in all composed operations written
in this style:

[example_core_echo_op_4]

Next is to implement the function call operator. Our strategy is to make our
composed object meet the requirements of a completion handler by being copyable
(also movable), and by providing the function call operator with the correct
signature. Rather than using `std::bind` or `boost::bind`, which destroys
the type information and therefore breaks the allocation and invocation
hooks, we will simply pass `std::move(*this)` as the completion handler
parameter for any operations that we initiate. For the move to work correctly,
care must be taken to ensure that no access to data members are made after the
move takes place. Here is the implementation of the function call operator for
this echo operation:

[example_core_echo_op_5]

This is the most important element of writing a composed operation, and
the part which is often neglected or implemented incorrectly. It is the
declaration and definition of the "handler hooks". There are four hooks:

[table Handler Hooks
[[Name][Description]]
[[
    [@http://www.boost.org/doc/html/boost_asio/reference/asio_handler_invoke.html `asio_handler_invoke`]
][
    Default invoke function for handlers. This hooking function ensures
    that the invoked method used for the final handler is accessible at
    each intermediate step.
]]
[[
    [@http://www.boost.org/doc/html/boost_asio/reference/asio_handler_allocate.html `asio_handler_allocate`]
][
    Default allocation function for handlers. Implement `asio_handler_allocate`
    and `asio_handler_deallocate` for your own handlers to provide custom
    allocation for temporary objects.
]]
[[
    [@http://www.boost.org/doc/html/boost_asio/reference/asio_handler_deallocate.html `asio_handler_deallocate`]
][
    Default deallocation function for handlers. Implement `asio_handler_allocate`
    and `asio_handler_deallocate` for your own handlers to provide custom
    allocation for temporary objects.
]]
[[
    [@http://www.boost.org/doc/html/boost_asio/reference/asio_handler_is_continuation.html `asio_handler_is_continuation`]
][
    Default continuation function for handlers. Implement
    `asio_handler_is_continuation` for your own handlers to indicate when
    a handler represents a continuation.
]]
]

Our composed operation stores the final handler and performs its own
intermediate asynchronous operations. To ensure that I/O objects, in this
case the stream, are accessed safely it is important to use the same method
to invoke intermediate handlers as that used to invoke the final handler.
Similarly, for the memory allocation hooks our composed operation should use
the same hooks as those used by the final handler. And finally for the
`asio_is_continuation` hook, we want to return `true` for any intermediate
asynchronous operations we perform after the first one, since those represent
continuations. For the first asynchronous operation we perform, the hook should
return `true` only if the final handler also represents a continuation. Our
implementation of the hooks will forward the call to the corresponding
overloads of the final handler:

[example_core_echo_op_6]

There are some common mistakes that should be avoided when writing
composed operations:

* Type erasing the final handler. This will cause undefined behavior.

* Not using `std::addressof` to get the address of the handler.

* Forgetting to include a return statement after calling an
  initiating function.

* Calling a synchronous function by accident. In general composed
  operations should not block for long periods of time, since this
  ties up a thread running on the __io_service__.

* Forgetting to overload `asio_handler_invoke` for the composed
  operation. This will cause undefined behavior if someone calls
  the initiating function with a strand-wrapped function object,
  and there is more than thread running on the `io_service`.

* For operations which complete immediately (i.e. without calling an
  intermediate initiating function), forgetting to use `io_service::post`
  to invoke the final handler. This breaks the following initiating
  function guarantee: ['Regardless of whether the asynchronous operation
  completes immediately or not, the handler will not be invoked from
  within this function. Invocation of the handler will be performed
  in a manner equivalent to using `boost::asio::io_service::post`].
  The function
  [link beast.ref.beast__bind_handler `bind_handler`]
  is provided for this purpose.

A complete, runnable version of this example may be found in the examples
directory.

[endsect]



[endsect]