.. Copyright (C) 2012 Adrian Serio Copyright (C) 2012 Vinay C Amatya Copyright (C) 2015 Hartmut Kaiser SPDX-License-Identifier: BSL-1.0 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) .. _examples_accumulator: ====================== Components and actions ====================== The accumulator examples demonstrate the use of components. Components are C++ classes that expose methods as a type of |hpx| action. These actions are called component actions. There are three examples: - accumulator - template accumulator - template function accumulator Components are globally named, meaning that a component action can be called remotely (e.g., from another machine). There are two accumulator examples in |hpx|. In the :ref:`examples_fibonacci` and the :ref:`examples_hello_world`, we introduced plain actions, which wrapped global functions. The target of a plain action is an identifier which refers to a particular machine involved in the computation. For plain actions, the target is the machine where the action will be executed. Component actions, however, do not target machines. Instead, they target component instances. The instance may live on the machine that we've invoked the component action from, or it may live on another machine. The components in these examples expose three different functions: * ``reset()`` - Resets the accumulator value to 0. * ``add(arg)`` - Adds ``arg`` to the accumulators value. * ``query()`` - Queries the value of the accumulator. These examples create an instance of the (template or template function) accumulator, and then allow the user to enter commands at a prompt, which subsequently invoke actions on the accumulator instance. Accumulator =========== Setup ----- The source code for this example can be found here: :download:`accumulator_client.cpp <../../examples/accumulators/accumulator_client.cpp>`. To compile this program, go to your |hpx| build directory (see :ref:`building_hpx` for information on configuring and building |hpx|) and enter: .. code-block:: shell-session $ make examples.accumulators.accumulator To run the program type: .. code-block:: shell-session $ ./bin/accumulator_client Once the program starts running, it will print the following prompt and then wait for input. An example session is given below: .. code-block:: text commands: reset, add [amount], query, help, quit > add 5 > add 10 > query 15 > add 2 > query 17 > reset > add 1 > query 1 > quit Walkthrough ----------- Now, let's take a look at the source code of the accumulator example. This example consists of two parts: an |hpx| component library (a library that exposes an |hpx| component) and a client application which uses the library. This walkthrough will cover the |hpx| component library. The code for the client application can be found here: :download:`accumulator_client.cpp <../../examples/accumulators/accumulator_client.cpp>`. An |hpx| component is represented by two C++ classes: * **A server class** - The implementation of the component's functionality. * **A client class** - A high-level interface that acts as a proxy for an instance of the component. Typically, these two classes both have the same name, but the server class usually lives in different sub-namespaces (``server``). For example, the full names of the two classes in accumulator are: * ``examples::server::accumulator`` (server class) * ``examples::accumulator`` (client class) The server class ^^^^^^^^^^^^^^^^ The following code is from :hpx-header:`examples/accumulators,server/accumulator.hpp`. All |hpx| component server classes must inherit publicly from the |hpx| component base class: :hpx:class:`hpx::components::component_base` The accumulator component inherits from :hpx:class:`hpx::components::locking_hook`. This allows the runtime system to ensure that all action invocations are serialized. That means that the system ensures that no two actions are invoked at the same time on a given component instance. This makes the component thread safe and no additional locking has to be implemented by the user. Moreover, an accumulator component is a component because it also inherits from :hpx:class:`hpx::components::component_base` (the template argument passed to locking_hook is used as its base class). The following snippet shows the corresponding code: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_server_inherit :end-before: //] Our accumulator class will need a data member to store its value in, so let's declare a data member: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_server_data_member :end-before: //] The constructor for this class simply initializes ``value_`` to 0: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_server_ctor :end-before: //] Next, let's look at the three methods of this component that we will be exposing as component actions: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_components :end-before: //] Here are the action types. These types wrap the methods we're exposing. The wrapping technique is very similar to the one used in the :ref:`examples_fibonacci` and the :ref:`examples_hello_world`: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_action_types :end-before: //] The last piece of code in the server class header is the declaration of the action type registration code: .. literalinclude:: ../../examples/accumulators/server/accumulator.hpp :language: c++ :start-after: //[accumulator_registration_declarations :end-before: //] .. note:: The code above must be placed in the global namespace. The rest of the registration code is in :hpx-header:`examples/accumulators,accumulator.cpp` .. literalinclude:: ../../examples/accumulators/accumulator.cpp :language: c++ :start-after: //[accumulator_registration_definitions :end-before: //] .. note:: The code above must be placed in the global namespace. The client class ^^^^^^^^^^^^^^^^ The following code is from :hpx-header:`examples/accumulators,accumulator.hpp` The client class is the primary interface to a component instance. Client classes are used to create components:: // Create a component on this locality. examples::accumulator c = hpx::new_(hpx::find_here()); and to invoke component actions:: c.add(hpx::launch::apply, 4); Clients, like servers, need to inherit from a base class, this time, :hpx:class:`hpx::components::client_base`: .. literalinclude:: ../../examples/accumulators/accumulator.hpp :language: c++ :start-after: //[accumulator_client_inherit :end-before: //] For readability, we typedef the base class like so: .. literalinclude:: ../../examples/accumulators/accumulator.hpp :language: c++ :start-after: //[accumulator_base_type :end-before: //] Here are examples of how to expose actions through a client class: There are a few different ways of invoking actions: * **Non-blocking**: For actions that don't have return types, or when we do not care about the result of an action, we can invoke the action using fire-and-forget semantics. This means that once we have asked |hpx| to compute the action, we forget about it completely and continue with our computation. We use :hpx:func:`hpx::post` to invoke an action in a non-blocking fashion. .. literalinclude:: ../../examples/accumulators/accumulator.hpp :language: c++ :start-after: //[accumulator_client_reset_non_blocking :end-before: //] * **Asynchronous**: Futures, as demonstrated in :ref:`examples_fibonacci_local`, :ref:`examples_fibonacci`, and the :ref:`examples_hello_world`, enable asynchronous action invocation. Here's an example from the accumulator client class: .. literalinclude:: ../../examples/accumulators/accumulator.hpp :language: c++ :start-after: //[accumulator_client_query_async :end-before: //] * **Synchronous**: To invoke an action in a fully synchronous manner, we can simply call :hpx:func:`hpx::sync` which is semantically equivalent to :hpx:func:`hpx::async`\ ``().get()`` (i.e., create a future and immediately wait on it to be ready). Here's an example from the accumulator client class: .. literalinclude:: ../../examples/accumulators/accumulator.hpp :language: c++ :start-after: //[accumulator_client_add_sync :end-before: //] Note that ``this->get_id()`` references a data member of the :hpx:class:`hpx::components::client_base` base class which identifies the server accumulator instance. :hpx:struct:`hpx::id_type` is a type which represents a global identifier in |hpx|. This type specifies the target of an action. This is the type that is returned by :hpx:func:`hpx::find_here` in which case it represents the :term:`locality` the code is running on. Template accumulator ==================== Walkthrough ----------- The server class ^^^^^^^^^^^^^^^^ The following code is from :hpx-header:`examples/accumulators,server/template_accumulator.hpp`. Similarly to the accumulator example, the component server class inherits publicly from :hpx:class:`hpx::components::component_base` and from :hpx:class:`hpx::components::locking_hook` ensuring thread-safe method invocations. .. literalinclude:: ../../examples/accumulators/server/template_accumulator.hpp :language: c++ :start-after: //[template_accumulator_server_inherit :end-before: //] The body of the template accumulator class remains mainly the same as the accumulator with the difference that it uses templates in the data types. .. literalinclude:: ../../examples/accumulators/server/template_accumulator.hpp :language: c++ :start-after: //[template_accumulator_server_body_class :end-before: //] The last piece of code in the server class header is the declaration of the action type registration code. `REGISTER_TEMPLATE_ACCUMULATOR_DECLARATION(type)` declares actions for the specified type, while `REGISTER_TEMPLATE_ACCUMULATOR(type)` registers the actions and the component for the specified type, using macros to handle boilerplate code. .. literalinclude:: ../../examples/accumulators/server/template_accumulator.hpp :language: c++ :start-after: //[template_accumulator_registration_declarations :end-before: //] .. note:: The code above must be placed in the global namespace. Finally, `HPX_REGISTER_COMPONENT_MODULE()` in file :hpx-header:`examples/accumulators,server/template_accumulator.cpp` adds the factory registration functionality. The client class ^^^^^^^^^^^^^^^^ The client class of the template accumulator can be found in :hpx-header:`examples/accumulators,template_accumulator.hpp` and is very similar to the client class of the accumulator with the only difference that it uses templates and hence can work with different types. Template function accumulator ============================= Walkthrough ----------- The server class ^^^^^^^^^^^^^^^^ The following code is from :hpx-header:`examples/accumulators,server/template_function_accumulator.hpp`. The component server class inherits publicly from :hpx:class:`hpx::components::component_base`. .. literalinclude:: ../../examples/accumulators/server/template_function_accumulator.hpp :language: c++ :start-after: //[template_func_accumulator_server_inherit :end-before: //] `typedef hpx::spinlock mutex_type` defines a `mutex_type` as `hpx::spinlock` for thread safety, while the code that follows exposes the functionality of this component. .. literalinclude:: ../../examples/accumulators/server/template_function_accumulator.hpp :language: c++ :start-after: //[template_func_accumulator_server_exposed_func :end-before: //] - `reset()`: Resets the accumulator value to `0` in a thread-safe manner using `std::lock_guard`. - `add()`: Adds a value to the accumulator, allowing any type `T` that can be cast to double. - `query()`: Returns the current value of the accumulator in a thread-safe manner. To define the actions for `reset()` and `query()` we can use the macro `HPX_DEFINE_COMPONENT_ACTION`. However, actions with template arguments require special type definitions. Therefore, we use `make_action()` to define `add()`. .. literalinclude:: ../../examples/accumulators/server/template_function_accumulator.hpp :language: c++ :start-after: //[template_func_accumulator_server_actions :end-before: //] The last piece of code in the server class header is the action registration: .. literalinclude:: ../../examples/accumulators/server/template_function_accumulator.hpp :language: c++ :start-after: //[template_func_accumulator_registration_declarations :end-before: //] .. note:: The code above must be placed in the global namespace. The rest of the registration code is in :hpx-header:`examples/accumulators,accumulator.cpp` .. literalinclude:: ../../examples/accumulators/template_function_accumulator.cpp :language: c++ :start-after: //[template_func_accumulator_registration_definitions :end-before: //] .. note:: The code above must be placed in the global namespace. The client class ^^^^^^^^^^^^^^^^ The client class of the template accumulator can be found in :hpx-header:`examples/accumulators,template_function_accumulator.hpp` and is very similar to the client class of the accumulator with the only difference that it uses templates and hence can work with different types.