version: 1
Date: 24 mai 2016
Author: José Bollo
The binder afb-daemon serves files through the HTTP protocol and offers access to API’s through HTTP or WebSocket protocol.
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.
Before going into details, through a tiny example, a short overview plugins basis is needed.
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.
Technically, a plugin is not linked to any library of afb-daemon.
The plugins are loaded and activated when afb-daemon starts.
At start, the plugin initialise itself. If it fails to initialise then afb-daemon stops.
Conversely, if it success to initialize, it must declare a name, that must be unique, and a list of API’s verbs.
When initialized, the functions implementing the API’s verbs of the plugin are activated on call.
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.
For afb-daemon, a plugin contains 2 different things: names and functions.
There is two kind of names: - the name of the plugin, - the names of the verbs.
There is two kind of functions: - the initialisation function - functions implementing verbs
Afb-daemon translates the name of the method that is invoked to a pair of API and verb names. For example, the method named foo/bar translated to the API name foo and the verb name bar. To serve it, afb-daemon search the plugin that record the name foo and if it also recorded the verb bar, it calls the implementation function declared for this verb.
Afb-daemon make no distinction between lower case and upper case when searching for a method. Thus, The names TicTacToe/Board and tictactoe/borad are equals.
The name of the plugin is also known as the name of the API that defines the plugin.
This name is also known as the prefix.
The name of a plugin MUST be unique within afb-daemon.
For example, when a client of afb-daemon calls a method named foo/bar. Afb-daemon extracts the prefix foo and the suffix bar. foo is the API name and must match a plugin name, the plugin that implements the verb bar.
Each plugin exposes a set of verbs that can be called by client of afb-daemon.
The name of a verb MUST be unique within a plugin.
Plugins link verbs to functions that are called when clients emit requests for that verb.
For example, when a client of afb-daemon calls a method named foo/bar.
The initialisation function serves several purposes.
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 pluginAfbV1Register. It identifies the first version of plugins.
It allows the plugin to initialise itself.
It serves to the plugin to declare names, descriptions, requirements and implmentations of the verbs that it exposes.
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.
An implementation function receives a request object that is used to get arguments of the request, to send answer, to store session data.
A plugin MUST send an answer to the request.
But it is not mandatory to send the answer before to return from the implementing function. This behaviour is important for implementing asynchronous actions.
Implementation functions that always reply to the request before returning are named synchronous implementations. Those that don’t always reply to the request before returning are named asynchronous implementations.
Asynchronous implementations typically initiate an asynchronous action and record to send the reply on completion of this action.
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 plugins/samples/tic-tac-toe.c.
This plugin is named tictactoe.
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.
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.
The names and strings used ALL are UTF-8 encoded.
The names of the API are checked. All characters are authorised except:
In other words the set of forbidden characters is { \u0000..\u0020, \u0022, \u0023, \u0025..\u0027, \u002f, \u003f, \u0060, \u007f }.
Afb-daemon make no distinction between lower case and upper case when searching for an API by its name.
The names of the verbs are not checked.
However, the validity rules for verb’s names are the same as for API’s names except that the dot (.) character is forbidden.
Afb-daemon make no distinction between lower case and upper case when searching for an API by its name.
The names for arguments are not restricted and can be anything.
The arguments are searched with the case sensitive string comparison. Thus the names “index” and “Index” are not the same.
The key names of javascript object can be almost anything using the arrayed notation:
object[key] = value
That is not the case with the dot notation:
object.key = value
Using the dot notation, the key must be a valid javascript identifier.
For this reason, the chosen names should better be valid javascript identifier.
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.
Afb-daemon provides a configuration file for pkg-config. Typing the command
pkg-config --cflags afb-daemon
will print the flags to use for compiling, like this:
$ pkg-config --cflags afb-daemon
-I/opt/local/include -I/usr/include/json-c
For linking, you should use
$ pkg-config --libs afb-daemon
-ljson-c
As you see, afb-daemon automatically includes dependency to json-c. This is done through the Requires keyword of pkg-config.
If this behaviour is a problem, let us know.
The plugin tictactoe has the following lines for its includes:
#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <json-c/json.h>
#include <afb/afb-plugin.h>
The header afb/afb-plugin.h includes all the features that a plugin needs except two foreign header that must be included by the plugin if it needs it:
The tictactoe plugin does not use systemd features so it is not included.
When including afb/afb-plugin.h, the macro _GNU_SOURCE must be defined.
The verb tictactoe/board is a synchronous implementation. Here is its listing:
/*
* 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);
}
This examples show many aspects of writing a synchronous verb implementation.
For any implementation, the request is received by a structure of type struct afb_req.
Important: note that this is a PLAIN structure, not a pointer to a structure.
This structure, here named req, is used
req is used to get arguments of the request, to send answer, to store session data.
This object and its interface is defined and documented in the file names afb/afb-req-itf.h
The above example uses 2 times the request object req.
The first time, it is used for retrieving the board attached to the session of the request.
The second time, it is used to send the reply: an object that describes the current board.
When the plugin tic-tac-toe receives a request, it musts regain the board that describes the game associated to the session.
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 tic-tac-toe, the context is the board.
The requests afb_req offer four functions for storing and retrieving the context associated to the session.
These functions are:
afb_req_context_get: retrieves the context data stored for the plugin.
afb_req_context_set: store the context data of the plugin.
afb_req_context: retrieves the context data of the plugin, if needed, creates the context and store it.
afb_req_context_clear: reset the stored data.
The plugin tictactoe use a convenient function to retrieve its context: the board. This function is board_of_req:
/*
* 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);
}
This function is very simple because it merely wraps a call to the function afb_req_context, providing all needed arguments. The casts are required to avoid a warning when compiling.
Here is the definition of the function afb_req_context
/*
* 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;
}
This powerful function ensures that the context exists and is stored for the session.
The function get_new_board 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.
The function release_board
Afb-daemon provides a The packaging of afb-daemon