--- /dev/null
+<html>
+<head>
+ <link rel="stylesheet" type="text/css" href="doc.css">
+ <meta charset="UTF-8">
+</head>
+<body>
+<a name="HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON"></a>
+<h1>HOWTO WRITE a PLUGIN for AFB-DAEMON</h1>
+
+<pre><code>version: 1
+Date: 24 mai 2016
+Author: José Bollo
+</code></pre>
+
+<p><ul>
+ <li><a href="#HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON">HOWTO WRITE a PLUGIN for AFB-DAEMON</a>
+ <ul>
+ <li><a href="#Summary">Summary</a>
+ <ul>
+ <li><a href="#Nature.of.a.plugin">Nature of a plugin</a></li>
+ <li><a href="#Live.cycle.of.a.plugin.within.afb-daemon">Live cycle of a plugin within afb-daemon</a></li>
+ <li><a href="#Content.of.a.plugin">Content of a plugin</a>
+ <ul>
+ <li><a href="#The.name.of.the.plugin">The name of the plugin</a></li>
+ <li><a href="#Names.of.verbs">Names of verbs</a></li>
+ <li><a href="#The.initialisation.function">The initialisation function</a></li>
+ <li><a href="#Functions.implementing.verbs">Functions implementing verbs</a>
+</li>
+ </ul>
+ </li>
+ </ul>
+ </li>
+ <li><a href="#The.Tic-Tac-Toe.example">The Tic-Tac-Toe example</a></li>
+ <li><a href="#Choosing.names">Choosing names</a>
+ <ul>
+ <li><a href="#Names.for.API..plugin.">Names for API (plugin)</a></li>
+ <li><a href="#Names.for.verbs">Names for verbs</a></li>
+ <li><a href="#Names.for.arguments">Names for arguments</a></li>
+ <li><a href="#Forging.names.widely.available">Forging names widely available</a></li>
+ </ul>
+ </li>
+ <li><a href="#Options.to.set.when.compiling.plugins">Options to set when compiling plugins</a></li>
+ <li><a href="#Header.files.to.include">Header files to include</a></li>
+ <li><a href="#Writing.a.synchronous.verb.implementation">Writing a synchronous verb implementation</a>
+ <ul>
+ <li><a href="#The.incoming.request">The incoming request</a></li>
+ <li><a href="#Associating.an.object.to.the.session.for.the.plugin">Associating an object to the session for the plugin</a></li>
+ <li><a href="#Sending.the.reply.to.a.request">Sending the reply to a request</a></li>
+ </ul>
+ </li>
+ <li><a href="#Getting.argument.of.invocation">Getting argument of invocation</a></li>
+ <li><a href="#How.to.build.a.plugin">How to build a plugin</a></li>
+ </ul>
+ </li>
+</ul></p>
+
+<a name="Summary"></a>
+<h2>Summary</h2>
+
+<p>The binder afb-daemon serves files through
+the HTTP protocol and offers access to API’s through
+HTTP or WebSocket protocol.</p>
+
+<p>The plugins are used to add API’s to afb-daemon.
+This part describes how to write a plugin for afb-daemon.
+Excepting this summary, this part is intended to be read
+by developpers.</p>
+
+<p>Before going into details, through a tiny example,
+a short overview plugins basis is needed.</p>
+
+<a name="Nature.of.a.plugin"></a>
+<h3>Nature of a plugin</h3>
+
+<p>A plugin is a separate piece of code made of a shared library.
+The plugin is loaded and activated by afb-daemon when afb-daemon
+starts.</p>
+
+<p>Technically, a plugin is not linked to any library of afb-daemon.</p>
+
+<a name="Live.cycle.of.a.plugin.within.afb-daemon"></a>
+<h3>Live cycle of a plugin within afb-daemon</h3>
+
+<p>The plugins are loaded and activated when afb-daemon starts.</p>
+
+<p>At start, the plugin initialise itself.
+If it fails to initialise then afb-daemon stops.</p>
+
+<p>Conversely, if it success to initialize, it must declare
+a name, that must be unique, and a list of API’s verbs.</p>
+
+<p>When initialized, the functions implementing the API’s verbs
+of the plugin are activated on call.</p>
+
+<p>At the end, nothing special is done by afb-daemon.
+Consequently, developpers of plugins should use ‘atexit’
+or ‘on_exit’ during initialisation if they need to
+perform specific actions when stopping.</p>
+
+<a name="Content.of.a.plugin"></a>
+<h3>Content of a plugin</h3>
+
+<p>For afb-daemon, a plugin contains 2 different
+things: names and functions.</p>
+
+<p>There is two kind of names:
+ - the name of the plugin,
+ - the names of the verbs.</p>
+
+<p>There is two kind of functions:
+ - the initialisation function
+ - functions implementing verbs</p>
+
+<p>Afb-daemon translates the name of the method that is
+invoked to a pair of API and verb names. For example,
+the method named <strong>foo/bar</strong> translated to the API
+name <strong>foo</strong> and the verb name <strong>bar</strong>.
+To serve it, afb-daemon search the plugin that record
+the name <strong>foo</strong> and if it also recorded the verb <strong>bar</strong>,
+it calls the implementation function declared for this verb.</p>
+
+<p>Afb-daemon make no distinction between lower case
+and upper case when searching for a method.
+Thus, The names <strong>TicTacToe/Board</strong> and <strong>tictactoe/borad</strong>
+are equals.</p>
+
+<a name="The.name.of.the.plugin"></a>
+<h4>The name of the plugin</h4>
+
+<p>The name of the plugin is also known as the name
+of the API that defines the plugin.</p>
+
+<p>This name is also known as the prefix.</p>
+
+<p>The name of a plugin MUST be unique within afb-daemon.</p>
+
+<p>For example, when a client of afb-daemon
+calls a method named <strong>foo/bar</strong>. Afb-daemon
+extracts the prefix <strong>foo</strong> and the suffix <strong>bar</strong>.
+<strong>foo</strong> is the API name and must match a plugin name,
+the plugin that implements the verb <strong>bar</strong>.</p>
+
+<a name="Names.of.verbs"></a>
+<h4>Names of verbs</h4>
+
+<p>Each plugin exposes a set of verbs that can be called
+by client of afb-daemon.</p>
+
+<p>The name of a verb MUST be unique within a plugin.</p>
+
+<p>Plugins link verbs to functions that are called
+when clients emit requests for that verb.</p>
+
+<p>For example, when a client of afb-daemon
+calls a method named <strong>foo/bar</strong>.</p>
+
+<a name="The.initialisation.function"></a>
+<h4>The initialisation function</h4>
+
+<p>The initialisation function serves several purposes.</p>
+
+<ol>
+<li><p>It allows afb-daemon to check the version
+of the plugin using the name of the initialisation
+functions that it found. Currently, the initialisation
+function is named <strong>pluginAfbV1Register</strong>. It identifies
+the first version of plugins.</p></li>
+<li><p>It allows the plugin to initialise itself.</p></li>
+<li><p>It serves to the plugin to declare names, descriptions,
+requirements and implmentations of the verbs that it exposes.</p></li>
+</ol>
+
+
+<a name="Functions.implementing.verbs"></a>
+<h4>Functions implementing verbs</h4>
+
+<p>When a method is called, afb-daemon constructs a request
+object and pass it to the implementation function for verb
+within the plugin of the API.</p>
+
+<p>An implementation function receives a request object that
+is used to get arguments of the request, to send
+answer, to store session data.</p>
+
+<p>A plugin MUST send an answer to the request.</p>
+
+<p>But it is not mandatory to send the answer
+before to return from the implementing function.
+This behaviour is important for implementing
+asynchronous actions.</p>
+
+<p>Implementation functions that always reply to the request
+before returning are named <em>synchronous implementations</em>.
+Those that don’t always reply to the request before
+returning are named <em>asynchronous implementations</em>.</p>
+
+<p>Asynchronous implementations typically initiate an
+asynchronous action and record to send the reply
+on completion of this action.</p>
+
+<a name="The.Tic-Tac-Toe.example"></a>
+<h2>The Tic-Tac-Toe example</h2>
+
+<p>This part explains how to write an afb-plugin.
+For the sake of being practical we will use many
+examples from the tic-tac-toe example.
+This plugin example is in <em>plugins/samples/tic-tac-toe.c</em>.</p>
+
+<p>This plugin is named <strong><em>tictactoe</em></strong>.</p>
+
+<a name="Choosing.names"></a>
+<h2>Choosing names</h2>
+
+<p>The designer of a plugin must defines names for its plugin
+(or its API) and for the verbs of its API. He also
+must defines names for arguments given by name.</p>
+
+<p>While forging names, the designer should take into account
+the rules for making valid names and some rules that make
+the names easy to use across plaforms.</p>
+
+<p>The names and strings used ALL are UTF-8 encoded.</p>
+
+<a name="Names.for.API..plugin."></a>
+<h3>Names for API (plugin)</h3>
+
+<p>The names of the API are checked.
+All characters are authorised except:</p>
+
+<ul>
+<li>the control characters (\u0000 .. \u001f)</li>
+<li>the characters of the set { ‘ ’, ‘“’, ‘#’, ‘%’, ‘&’,
+‘’‘, ’/‘, ’?‘, ’`‘, ’\x7f' }</li>
+</ul>
+
+
+<p>In other words the set of forbidden characters is
+{ \u0000..\u0020, \u0022, \u0023, \u0025..\u0027,
+ \u002f, \u003f, \u0060, \u007f }.</p>
+
+<p>Afb-daemon make no distinction between lower case
+and upper case when searching for an API by its name.</p>
+
+<a name="Names.for.verbs"></a>
+<h3>Names for verbs</h3>
+
+<p>The names of the verbs are not checked.</p>
+
+<p>However, the validity rules for verb’s names are the
+same as for API’s names except that the dot (.) character
+is forbidden.</p>
+
+<p>Afb-daemon make no distinction between lower case
+and upper case when searching for an API by its name.</p>
+
+<a name="Names.for.arguments"></a>
+<h3>Names for arguments</h3>
+
+<p>The names for arguments are not restricted and can be
+anything.</p>
+
+<p>The arguments are searched with the case sensitive
+string comparison. Thus the names “index” and “Index”
+are not the same.</p>
+
+<a name="Forging.names.widely.available"></a>
+<h3>Forging names widely available</h3>
+
+<p>The key names of javascript object can be almost
+anything using the arrayed notation:</p>
+
+<pre><code>object[key] = value
+</code></pre>
+
+<p>That is not the case with the dot notation:</p>
+
+<pre><code>object.key = value
+</code></pre>
+
+<p>Using the dot notation, the key must be a valid javascript
+identifier.</p>
+
+<p>For this reason, the chosen names should better be
+valid javascript identifier.</p>
+
+<p>It is also a good practice, even for arguments, to not
+rely on the case sensitivity and to avoid the use of
+names different only by the case.</p>
+
+<a name="Options.to.set.when.compiling.plugins"></a>
+<h2>Options to set when compiling plugins</h2>
+
+<p>Afb-daemon provides a configuration file for <em>pkg-config</em>.
+Typing the command</p>
+
+<pre><code>pkg-config --cflags afb-daemon
+</code></pre>
+
+<p>will print the flags to use for compiling, like this:</p>
+
+<pre><code>$ pkg-config --cflags afb-daemon
+-I/opt/local/include -I/usr/include/json-c
+</code></pre>
+
+<p>For linking, you should use</p>
+
+<pre><code>$ pkg-config --libs afb-daemon
+-ljson-c
+</code></pre>
+
+<p>As you see, afb-daemon automatically includes dependency to json-c.
+This is done through the <strong>Requires</strong> keyword of pkg-config.</p>
+
+<p>If this behaviour is a problem, let us know.</p>
+
+<a name="Header.files.to.include"></a>
+<h2>Header files to include</h2>
+
+<p>The plugin <em>tictactoe</em> has the following lines for its includes:</p>
+
+<pre><code>#define _GNU_SOURCE
+#include <stdio.h>
+#include <string.h>
+#include <json-c/json.h>
+#include <afb/afb-plugin.h>
+</code></pre>
+
+<p>The header <em>afb/afb-plugin.h</em> includes all the features that a plugin
+needs except two foreign header that must be included by the plugin
+if it needs it:</p>
+
+<ul>
+<li><em>json-c/json.h</em>: this header must be include to handle json objects;</li>
+<li><em>systemd/sd-event.h</em>: this must be include to access the main loop;</li>
+<li><em>systemd/sd-bus.h</em>: this may be include to use dbus connections.</li>
+</ul>
+
+
+<p>The <em>tictactoe</em> plugin does not use systemd features so it is not included.</p>
+
+<p>When including <em>afb/afb-plugin.h</em>, the macro <strong>_GNU_SOURCE</strong> must be
+defined.</p>
+
+<a name="Writing.a.synchronous.verb.implementation"></a>
+<h2>Writing a synchronous verb implementation</h2>
+
+<p>The verb <strong>tictactoe/board</strong> is a synchronous implementation.
+Here is its listing:</p>
+
+<pre><code>/*
+ * get the board
+ */
+static void board(struct afb_req req)
+{
+ struct board *board;
+ struct json_object *description;
+
+ /* retrieves the context for the session */
+ board = board_of_req(req);
+ INFO(afbitf, "method 'board' called for boardid %d", board->id);
+
+ /* describe the board */
+ description = describe(board);
+
+ /* send the board's description */
+ afb_req_success(req, description, NULL);
+}
+</code></pre>
+
+<p>This examples show many aspects of writing a synchronous
+verb implementation.</p>
+
+<a name="The.incoming.request"></a>
+<h3>The incoming request</h3>
+
+<p>For any implementation, the request is received by a structure of type
+<strong>struct afb_req</strong>.</p>
+
+<p><strong><em>Important: note that this is a PLAIN structure, not a pointer to a structure.</em></strong></p>
+
+<p>This structure, here named <em>req</em>, is used</p>
+
+<p><em>req</em> is used to get arguments of the request, to send
+answer, to store session data.</p>
+
+<p>This object and its interface is defined and documented
+in the file names <em>afb/afb-req-itf.h</em></p>
+
+<p>The above example uses 2 times the request object <em>req</em>.</p>
+
+<p>The first time, it is used for retrieving the board attached to
+the session of the request.</p>
+
+<p>The second time, it is used to send the reply: an object that
+describes the current board.</p>
+
+<a name="Associating.an.object.to.the.session.for.the.plugin"></a>
+<h3>Associating an object to the session for the plugin</h3>
+
+<p>When the plugin <em>tic-tac-toe</em> receives a request, it musts regain
+the board that describes the game associated to the session.</p>
+
+<p>For a plugin, having data associated to a session is a common case.
+This data is called the context of the plugin for the session.
+For the plugin <em>tic-tac-toe</em>, the context is the board.</p>
+
+<p>The requests <em>afb_req</em> offer four functions for
+storing and retrieving the context associated to the session.</p>
+
+<p>These functions are:</p>
+
+<ul>
+<li><p><strong>afb_req_context_get</strong>:
+retrieves the context data stored for the plugin.</p></li>
+<li><p><strong>afb_req_context_set</strong>:
+store the context data of the plugin.</p></li>
+<li><p><strong>afb_req_context</strong>:
+retrieves the context data of the plugin,
+if needed, creates the context and store it.</p></li>
+<li><p><strong>afb_req_context_clear</strong>:
+reset the stored data.</p></li>
+</ul>
+
+
+<p>The plugin <em>tictactoe</em> use a convenient function to retrieve
+its context: the board. This function is <em>board_of_req</em>:</p>
+
+<pre><code>/*
+ * retrieves the board of the request
+ */
+static inline struct board *board_of_req(struct afb_req req)
+{
+ return afb_req_context(req, (void*)get_new_board, (void*)release_board);
+}
+</code></pre>
+
+<p>This function is very simple because it merely wraps
+a call to the function <strong>afb_req_context</strong>, providing
+all needed arguments.
+The casts are required to avoid a warning when compiling.</p>
+
+<p>Here is the definition of the function <strong>afb_req_context</strong></p>
+
+<pre><code>/*
+ * Gets the pointer stored by the plugin for the session of 'req'.
+ * If the stored pointer is NULL, indicating that no pointer was
+ * already stored, afb_req_context creates a new context by calling
+ * the function 'create_context' and stores it with the freeing function
+ * 'free_context'.
+ */
+static inline void *afb_req_context(struct afb_req req, void *(*create_context)(), void (*free_context)(void*))
+{
+ void *result = afb_req_context_get(req);
+ if (result == NULL) {
+ result = create_context();
+ afb_req_context_set(req, result, free_context);
+ }
+ return result;
+}
+</code></pre>
+
+<p>This powerful function ensures that the context exists and is
+stored for the session.</p>
+
+<p>The function <strong>get_new_board</strong> creates a new board and set its
+count of use to 1. The boards are counting their count of use
+to free there ressources when no more used.</p>
+
+<p>The function <strong>release_board</strong></p>
+
+<a name="Sending.the.reply.to.a.request"></a>
+<h3>Sending the reply to a request</h3>
+
+<a name="Getting.argument.of.invocation"></a>
+<h2>Getting argument of invocation</h2>
+
+<a name="How.to.build.a.plugin"></a>
+<h2>How to build a plugin</h2>
+
+<p>Afb-daemon provides a The packaging of afb-daemon</p>
+</body>
+</html>
Code Review / src / app-framework-binder.git / blobdiff