version: 1
Date: 25 May 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.
There is two kinds of plugins: application plugins and service plugins.
Application plugins are intended to be instanciated for each application: when an application using that plugin is started, its binder starts a new instance of the plugin.
It means that the application plugins mainly have only one context to manage for one client.
Service plugins are intended to be instanciated only one time only and connected to many clients.
So either it does not manage context at all or otherwise, if it manages context, it should be able to manage one context per client.
In details, it may be useful to have service plugins at a user level.
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. Let summarize it:
The function board_of_req retrieves the context stored for the plugin: the board.
The macro INFO sends a message of kind INFO to the logging system. The global variable named afbitf used represents the interface to afb-daemon.
The function describe creates a json_object representing the board.
The function afb_req_success sends the reply, attaching to it the object description.
For any implementation, the request is received by a structure of type struct afb_req.
Note that this is a PLAIN structure, not a pointer to a structure.
The definition of struct afb_req is:
/*
* Describes the request by plugins from afb-daemon
*/
struct afb_req {
const struct afb_req_itf *itf; /* the interfacing functions */
void *closure; /* the closure for functions */
};
It contains two pointers: one, itf, points to the functions needed to handle the internal request represented by the second pointer, closure.
The structure must never be used directly. Insted, use the intended functions provided by afb-daemon and described here.
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);
}
The function afb_req_context ensure an existing context for the session of the request. Its two last arguments are functions. Here, 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;
}
The second argument if the function that creates the context. For the plugin tic-tac-toe it is the function get_new_board. 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 third argument if the function that frees the context. For the plugin tic-tac-toe it is the function release_board. The function release_board decrease the the count of use of the board given as argument. If the use count decrease to zero, the board data are freed.
The definition of the other functions for dealing with contexts are:
/*
* Gets the pointer stored by the plugin for the session of 'req'.
* When the plugin has not yet recorded a pointer, NULL is returned.
*/
void *afb_req_context_get(struct afb_req req);
/*
* Stores for the plugin the pointer 'context' to the session of 'req'.
* The function 'free_context' will be called when the session is closed
* or if plugin stores an other pointer.
*/
void afb_req_context_set(struct afb_req req, void *context, void (*free_context)(void*));
/*
* Frees the pointer stored by the plugin for the session of 'req'
* and sets it to NULL.
*
* Shortcut for: afb_req_context_set(req, NULL, NULL)
*/
static inline void afb_req_context_clear(struct afb_req req)
{
afb_req_context_set(req, NULL, NULL);
}
Two kinds of replies can be made: successful replies and failure replies.
Sending a reply to a request must be done at most one time.
The two functions to send a reply of kind “success” are afb_req_success and afb_req_success_f.
/*
* Sends a reply of kind success to the request 'req'.
* The status of the reply is automatically set to "success".
* Its send the object 'obj' (can be NULL) with an
* informationnal comment 'info (can also be NULL).
*/
void afb_req_success(struct afb_req req, struct json_object *obj, const char *info);
/*
* Same as 'afb_req_success' but the 'info' is a formatting
* string followed by arguments.
*/
void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
The two functions to send a reply of kind “failure” are afb_req_fail and afb_req_fail_f.
/*
* Sends a reply of kind failure to the request 'req'.
* The status of the reply is set to 'status' and an
* informationnal comment 'info' (can also be NULL) can be added.
*
* Note that calling afb_req_fail("success", info) is equivalent
* to call afb_req_success(NULL, info). Thus even if possible it
* is strongly recommanded to NEVER use "success" for status.
*/
void afb_req_fail(struct afb_req req, const char *status, const char *info);
/*
* Same as 'afb_req_fail' but the 'info' is a formatting
* string followed by arguments.
*/
void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
Many verbs expect arguments. Afb-daemon let plugins retrieve their arguments by name not by position.
Arguments are given by the requests either through HTTP or through WebSockets.
For example, the verb join of the plugin tic-tac-toe expects one argument: the boardid to join. Here is an extract:
/*
* Join a board
*/
static void join(struct afb_req req)
{
struct board *board, *new_board;
const char *id;
/* retrieves the context for the session */
board = board_of_req(req);
INFO(afbitf, "method 'join' called for boardid %d", board->id);
/* retrieves the argument */
id = afb_req_value(req, "boardid");
if (id == NULL)
goto bad_request;
...
The function afb_req_value search in the request req for an argument whose name is given. When no argument of the given name was passed, afb_req_value returns NULL.
The search is case sensitive. So the name boardid is not the same name than BoardId. But this must not be assumed so two expected names of argument should not differ only by case.
The function afb_req_value is defined as below:
/*
* Gets from the request 'req' the string value of the argument of 'name'.
* Returns NULL if when there is no argument of 'name'.
* Returns the value of the argument of 'name' otherwise.
*
* Shortcut for: afb_req_get(req, name).value
*/
static inline const char *afb_req_value(struct afb_req req, const char *name)
{
return afb_req_get(req, name).value;
}
It is defined as a shortcut to call the function afb_req_get. That function is defined as below:
/*
* Gets from the request 'req' the argument of 'name'.
* Returns a PLAIN structure of type 'struct afb_arg'.
* When the argument of 'name' is not found, all fields of result are set to NULL.
* When the argument of 'name' is found, the fields are filled,
* in particular, the field 'result.name' is set to 'name'.
*
* There is a special name value: the empty string.
* The argument of name "" is defined only if the request was made using
* an HTTP POST of Content-Type "application/json". In that case, the
* argument of name "" receives the value of the body of the HTTP request.
*/
struct afb_arg afb_req_get(struct afb_req req, const char *name);
That function takes 2 parameters: the request and the name of the argument to retrieve. It returns a PLAIN structure of type struct afb_arg.
There is a special name that is defined when the request is of type HTTP/POST with a Content-Type being application/json. This name is “” (the empty string). In that case, the value of this argument of empty name is the string received as a body of the post and is supposed to be a JSON string.
The definition of struct afb_arg is:
/*
* Describes an argument (or parameter) of a request
*/
struct afb_arg {
const char *name; /* name of the argument or NULL if invalid */
const char *value; /* string representation of the value of the argument */
/* original filename of the argument if path != NULL */
const char *path; /* if not NULL, path of the received file for the argument */
/* when the request is finalized this file is removed */
};
The structure returns the data arguments that are known for the request. This data include a field named path. This path can be accessed using the function afb_req_path defined as below:
/*
* Gets from the request 'req' the path for file attached to the argument of 'name'.
* Returns NULL if when there is no argument of 'name' or when there is no file.
* Returns the path of the argument of 'name' otherwise.
*
* Shortcut for: afb_req_get(req, name).path
*/
static inline const char *afb_req_path(struct afb_req req, const char *name)
{
return afb_req_get(req, name).path;
}
The path is only defined for HTTP/POST requests that send file.
As it is explained just above, clients can send files using HTTP/POST requests.
Received files are attached to a arguments. For example, the following HTTP fragment (from test/sample-post.html) will send an HTTP/POST request to the method post/upload-image with 2 arguments named file and hidden.
<h2>Sample Post File</h2>
<form enctype="multipart/form-data">
<input type="file" name="file" />
<input type="hidden" name="hidden" value="bollobollo" />
<br>
<button formmethod="POST" formaction="api/post/upload-image">Post File</button>
</form>
In that case, the argument named file has its value and its path defined and not NULL.
The value is the name of the file as it was set by the HTTP client and is generally the filename on the client side.
The path is the path of the file saved on the temporary local storage area of the application. This is a randomly generated and unic filename not linked in any way with the original filename on the client.
The plugin can use the file at the given path the way that it wants: read, write, remove, copy, rename… But when the reply is sent and the query is terminated, the file at this path is destroyed if it still exist.
Plugins can get all the arguments as one single object. This feature is provided by the function afb_req_json that is defined as below:
/*
* Gets from the request 'req' the json object hashing the arguments.
* The returned object must not be released using 'json_object_put'.
*/
struct json_object *afb_req_json(struct afb_req req);
It returns a json object. This object depends on how the request was made:
For HTTP requests, this is an object whose keys are the names of the arguments and whose values are either a string for common arguments or an object like { “file”: “…”, “path”: “…” }
For WebSockets requests, the returned object is the object given by the client transparently transported.
In fact, for Websockets requests, the function afb_req_value can be seen as a shortcut to json_object_get_string(json_object_object_get(afb_req_json(req), name))
Afb-daemon provides a pkg-config configuration file.