version: 1
-Date: 27 mai 2016
+Date: 30 mai 2016
Author: José Bollo
The binder afb-daemon serves files through -the HTTP protocol and offers access to API’s through +
Afb-daemon binders serve files through HTTP protocol +and offers to developers the capability to expose application API methods 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.
+Binder plugins are used to add API to afb-daemon. +This part describes how to write a plugin for afb-daemon.
-Before going into details, through a tiny example, -a short overview plugins basis is needed.
+Excepting this summary, this document target developers.
+ +Before moving further through an example, here after +a short overview of binder plugins fundamentals.
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.
+A plugin is an independent piece of software. A plugin is self contain and exposes application logic as sharable library. +A plugin is intended to be dynamically loaded by afb-daemon to expose application API.
-Technically, a plugin is not linked to any library of afb-daemon.
+Technically, a binder plugin does not reference and is not linked with any afb-daemon library.
- -There is two kinds of plugins: application plugins and service -plugins.
+Application binder supports two kinds of plugins: application plugins and service plugins. +Technically both class of plugin are equivalent are use the same coding convention. Only sharing mode and security context diverge.
- -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.
+Application-plugins implements the glue in between application’s UI and services. Every AGL application +has a corresponding binder that typically activates one or many plugins to interface the application logic with lower platform services. +When an application is started by the AGL application framework, a dedicate binder is started that loads/activates application plugin(s). +API expose by application-plugin are executed within corresponding application security context.
-It means that the application plugins mainly have only one -context to manage for one client.
+Application plugins generally handle a unique context for a unique client. As the application framework start +a dedicated instance of afb_daemon for each AGL application, if a given plugin is used within multiple application each of those +application get a new and private instance of eventually “shared” plugin.
- -Service plugins are intended to be instanciated only one time -only and connected to many clients.
+Service-plugins enable API activation within corresponding service security context and not within calling application context. +Service-plugins are intended to run as a unique instance. Service-plugins can be shared in between multiple 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.
+Service-plugins can either be stateless or manage client context. When managing context each client get a private context.
-In details, it may be useful to have service plugins at a user -level.
+Sharing may either be global to the platform (ie: GPS service) or dedicated to a given user (ie: user preferences)
- -The plugins are loaded and activated when afb-daemon starts.
+Application and service plugins are loaded and activated each time a new afb-daemon is started.
-At start, the plugin initialise itself. -If it fails to initialise then afb-daemon stops.
+At launch time, every loaded plugin initialise itself. +If a single plugin initialisation fail corresponding instance of afb-daemon self aborts.
-Conversely, if it success to initialize, it must declare -a name, that must be unique, and a list of API’s verbs.
+Conversely, when a plugin initialisation succeeds, it should register +its unique name as well as the list of verbs attached to the methods it exposes.
-When initialized, the functions implementing the API’s verbs -of the plugin are activated on call.
+When initialised, on request from application clients to the right API/verb, plugin methods +are activated by the afb-daemon attached to the application or service.
-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.
+At exit time, no special action is enforced by afb-daemon. When a specific actions is required at afb-daemon stop, +developers should use ‘atexit/on_exit’ during plugin initialisation sequence to register a custom exit function.
- -For afb-daemon, a plugin contains 2 different -things: names and functions.
+Afb-daemon’s plugin register two classes of objects: names and functions.
-There is two kind of names: - - the name of the plugin, - - the names of the verbs.
+Plugins declare categories of names: + - A unique plugin name to access all API expose by this plugin, + - One name for each methods/verbs provided by this plugin.
-There is two kind of functions: - - the initialisation function - - functions implementing verbs
+Plugins declare two categories of functions: + - function use for the initialisation + - functions implementing exposed API methods
-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 parses URI requests to extract the API(plugin name) and the VERB(method to activate). +As an example, URI foo/bar translates to plugin named foo and method named bar. +To serve such a request, afb-daemon looks for an active plugin named foo and then within this plugin for a method named bar. +When find afb-daemon calls corresponding method with attached parameter if any.
-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.
+Afb-daemon ignores letter case when parsing URI. Thus TicTacToe/Board and tictactoe/board are equivalent.
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.
+The name of a given plugin is also known as the name +of the API prefix that defines the plugin.
-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.
+The name of a plugin SHOULD be unique within a given afb-daemon instance.
- -For example, when a client of afb-daemon calls a URI named foo/bar. Afb-daemon +extracts the prefix foo and the suffix bar. foo must match a plugin name and bar a VERB attached to some method.
-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.
+Each plugin exposes a set of methods that can be called +by the clients of a given afb-daemon.
-Plugins link verbs to functions that are called -when clients emit requests for that verb.
+VERB’s name attached to a given plugin (API) MUST be unique within a plugin.
-For example, when a client of afb-daemon -calls a method named foo/bar.
+Plugins static declaration link VERBS to corresponding methods. +When clients emit requests on a given API/VERB corresponding method is called by afb-daemon.
- -The initialisation function serves several purposes.
+Plugin’s 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.
It allows afb-daemon to control plugin version depending on initialisation function name. +As today, the only supported initialisation function is pluginAfbV1Register. This identifies +version “one” of plugins.
It allows plugins to initialise itself.
It enables names declarations: descriptions, requirements and implementations of exposed API/VERB.
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.
+When an API/VERB is called, afb-daemon constructs a request object. Then it +passes this request object to the implementation function corresponding to requested method, this +within attached API plugin.
An implementation function receives a request object that -is used to get arguments of the request, to send -answer, to store session data.
+is used to: get arguments of the request, send +answer, store session data. -A plugin MUST send an answer to the request.
+A plugin MUST set an answer to every received requests.
-But it is not mandatory to send the answer -before to return from the implementing function. -This behaviour is important for implementing -asynchronous actions.
+Nevertheless it is not mandatory to set the answer +before returning from API/VERB implementing function. +This behaviour is important for 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.
+API/VERB implementation that set an answer before returning are called synchronous implementations. +Those that do not systematically set an answer before returning are called asynchronous implementations.
-Asynchronous implementations typically initiate an -asynchronous action and record to send the reply -on completion of this action.
+Asynchronous implementations typically launch asynchronous actions. They record some context at +request time and provide answer to the request only at completion of asynchronous actions.
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. +For the sake of being practical it uses many +examples based on tic-tac-toe. This plugin example is in plugins/samples/tic-tac-toe.c.
This plugin is named tictactoe.
@@ -274,7 +248,7 @@ Typing the commandpkg-config --cflags afb-daemon
will print the flags to use for compiling, like this:
+Print flags use for compilation:
$ pkg-config --cflags afb-daemon
-I/opt/local/include -I/usr/include/json-c
@@ -286,27 +260,24 @@ Typing the command
-ljson-c
As you see, afb-daemon automatically includes dependency to json-c. -This is done through the Requires keyword of pkg-config -because almost all plugin will use json-c.
- -If this behaviour is a problem, let us know.
+Afb-daemon automatically includes dependency to json-c. +This is activated through Requires keyword in pkg-config. +While almost every plugin replies on json-c this is not a must have dependency.
-Internally, afb-daemon uses libsystemd for its event loop -and for its binding to D-Bus. -Plugins developpers are encouraged to also use this library. -But it is a matter of choice. -Thus there is no dependency to libsystemd.
+Internally, afb-daemon relies on libsystemd for its event loop, as well +as for its binding to D-Bus. +Plugins developers are encouraged to leverage libsystemd when possible. +Nevertheless there is no hard dependency to libsystemd if ever +you rather not use it, feel free to do so.
-+Afb-daemon provides no library for plugins. -The functions that the plugin need to have are given -to the plugin at runtime through pointer using read-only -memory.
Afb-daemon plugin are fully self contain. They do not enforce dependency on any libraries from the application framework. +Afb-daemon dependencies requirer to run AGL plugins are given at runtime through pointers leveraging read-only +memory feature.
The plugin tictactoe has the following lines for its includes:
+Plugin tictactoe has following includes:
#define _GNU_SOURCE
#include <stdio.h>
@@ -315,39 +286,39 @@ memory.
#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:
+Header afb/afb-plugin.h is the only hard dependency, it includes all features +that a plugin MUST HAVE. Outside of includes used to support application logic, +common external headers used within plugins are:
The tictactoe plugin does not use systemd features so it is not included.
+The tictactoe plugin does not leverage systemd features, also only json.h +is used on top of mandatory afb/afb-plugin.h.
-When including afb/afb-plugin.h, the macro _GNU_SOURCE must be +
When including afb/afb-plugin.h, the macro _GNU_SOURCE MUST be defined.
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.
+Designers of plugins should define a unique name for every API plugin +as well as for methods VERBs. They should also define names for request +arguments passed as name/value pair in URI.
-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.
+While forging names, designers should respect few rules to +ensure that created names are valid and easy to use across platforms.
-The names and strings used ALL are UTF-8 encoded.
+All names and strings are UTF-8 encoded.
The names of the API are checked. +
Plugin API name are checked. All characters are authorised except:
Afb-daemon make no distinction between lower case -and upper case when searching for an API by its name.
+Afb-daemon makes no distinction between lower case +and upper case when searching for API/VERB.
- -The names of the verbs are not checked.
+The names of methods VERBs are totally free and not checked.
-However, the validity rules for verb’s names are the -same as for API’s names except that the dot (.) character +
However, the validity rules for method’s VERB name are the +same as for Plugin API name 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.
+Afb-daemon makes no case distinction when searching for an API by name.
The names for arguments are not restricted and can be -anything.
+Argument’s name are not restricted and can be everything you wish.
-The arguments are searched with the case sensitive -string comparison. Thus the names “index” and “Index” -are not the same.
+Warning arguments search is case sensitive and “index” and “Index” +are not two different arguments.
object[key] = value
That is not the case with the dot notation:
+Nevertheless this is not the case with javascript dot notation:
object.key = value
Using the dot notation, the key must be a valid javascript -identifier.
+identifier and dash(-) as well as few other reserved characters cannot be used. -For this reason, the chosen names should better be -valid javascript identifier.
+For this reason, we advise developper to chose name compatible with both javascript and HTML notation.
-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.
+It is a good practice, even for arguments not to rely on case sensitivity. +This may reduce headache strength at debug time, especially with interpreted language like +javascript that may not warn you that a variable was not defined.
- -The verb tictactoe/board is a synchronous implementation. +
The method tictactoe/board is a synchronous implementation. Here is its listing:
/*
@@ -436,8 +403,8 @@ static void board(struct afb_req req)
}
This examples show many aspects of writing a synchronous -verb implementation. Let summarize it:
+This example shows many aspects of a synchronous +method implementation. Let summarise it:
The function board_of_req retrieves the context stored @@ -471,12 +438,12 @@ struct afb_req { }; -
It contains two pointers: one, itf, points to the functions needed -to handle the internal request represented by the second pointer, closure.
+It contains two pointers: first one itf, points to functions used +to handle internal request. Second one closure point onto function closure.
+Instead developer should use the intended functions provided +by afb-daemon as described here after.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.
@@ -484,39 +451,36 @@ 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 above example uses twice req object request.
-The first time, it is used for retrieving the board attached to -the session of the request.
+The first time, to retrieve 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.
+The second time, 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.
+When tic-tac-toe plugin receives a request, it musts get +the board describing 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.
+For a plugin, having data associated to a session is common. +This data is called “plugin context” for the session. +Within tic-tac-toe plugin the context is the board.
-The requests afb_req offer four functions for -storing and retrieving the context associated to the session.
+Requests afb_req offer four functions for storing and retrieving session associated context.
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 function afb_req_context ensure an existing context +
The function afb_req_context ensures 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.
+Its two last arguments are functions to allocate and free context. +Note function type casts to avoid compilation warnings.Here is the definition of the function afb_req_context
@@ -558,18 +522,16 @@ static inline void *afb_req_context(struct afb_req req, void *(*create_context)(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.
+For plugin tic-tac-toe (function get_new_board). +The function get_new_board creates a new board and set usage its count to 1. +The boards are checking usage count to free resources when not 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 third argument is a function that frees context resources. +For plugin tic-tac-toe (function release_board). +The function release_board decrease usage count of the board passed in argument. +When usage count falls to zero, data board are freed.
-The definition of the other functions for dealing with contexts are:
+Definition of other functions dealing with contexts:
/*
* Gets the pointer stored by the plugin for the session of 'req'.
@@ -596,63 +558,81 @@ static inline void afb_req_context_clear(struct afb_req req)
}
Two kinds of replies can be made: successful replies and -failure replies.
+Two kinds of replies: successful or failure.
-+Sending a reply to a request must be done at most one time.
-Sending a reply to a request MUST be done once and only once.
The two functions to send a reply of kind “success” are -afb_req_success and afb_req_success_f.
+It exists two functions for “success” replies: 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).
+ *
+ * For conveniency, the function calls 'json_object_put' for 'obj'.
+ * Thus, in the case where 'obj' should remain available after
+ * the function returns, the function 'json_object_get' shall be used.
*/
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.
+ *
+ * For conveniency, the function calls 'json_object_put' for 'obj'.
+ * Thus, in the case where 'obj' should remain available after
+ * the function returns, the function 'json_object_get' shall be used.
*/
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.
+It exists two functions for “failure” replies: 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.
+ * informational 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.
+ * is strongly recommended to NEVER use "success" for status.
+ *
+ * For conveniency, the function calls 'json_object_put' for 'obj'.
+ * Thus, in the case where 'obj' should remain available after
+ * the function returns, the function 'json_object_get' shall be used.
*/
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.
+ *
+ * For conveniency, the function calls 'json_object_put' for 'obj'.
+ * Thus, in the case where 'obj' should remain available after
+ * the function returns, the function 'json_object_get' shall be used.
*/
void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
+For conveniency, these functions automatically call json_object_put to release obj. +Because obj usage count is null after being passed to a reply function, it SHOULD not be used anymore. +If exceptionally obj needs to remain usable after reply function then using json_object_get on obj +to increase usage count and cancels the effect the json_object_put is possible.
Many verbs expect arguments. Afb-daemon let plugins -retrieve their arguments by name not by position.
+Many methods expect arguments. Afb-daemon’s plugins +retrieve arguments by name and not by position.
-Arguments are given by the requests either through HTTP -or through WebSockets.
+Arguments are passed by requests through either HTTP +or WebSockets.
-For example, the verb join of the plugin tic-tac-toe +
For example, the method join of plugin tic-tac-toe expects one argument: the boardid to join. Here is an extract:
/*
@@ -674,18 +654,17 @@ static void join(struct afb_req req)
...
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 function afb_req_value searches in the request req +for argument name passed in the second argument. When argument name +is not 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 search is case sensitive and boardid is not equivalent to BoardId. +Nevertheless having argument names that only differ by name case is not a good idea.
The function afb_req_value is defined as below:
+The function afb_req_value is defined here after:
/*
* Gets from the request 'req' the string value of the argument of 'name'.
@@ -701,7 +680,7 @@ static inline const char *afb_req_value(struct afb_req req, const char *name)
It is defined as a shortcut to call the function afb_req_get. -That function is defined as below:
+That function is defined here after:/*
* Gets from the request 'req' the argument of 'name'.
@@ -744,8 +723,7 @@ struct afb_arg {
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:
+can be accessed using the function afb_req_path defined here after:
/*
* Gets from the request 'req' the path for file attached to the argument of 'name'.
@@ -765,10 +743,9 @@ static inline const char *afb_req_path(struct afb_req req, const char *name)
Arguments for received files
-As it is explained just above, clients can send files using
-HTTP/POST requests.
+As it is explained above, clients can send files using HTTP/POST requests.
-Received files are attached to a arguments. For example, the
+
Received files are attached to “file” argument name. 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
@@ -783,28 +760,25 @@ will send an HTTP/POST request to the method
</form>
-In that case, the argument named file has its value and its
-path defined and not NULL.
+Argument named file should have both its value and path defined.
-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 value is the name of the file as it was set by the HTTP client.
+Generally it is the filename on 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 path is the effective path of saved file on the temporary local storage
+area of the application. This is a randomly generated and unique filename.
+It is not linked with the original filename as used on client side.
-The plugin can use the file at the given path the way that it wants:
+
After success the plugin can use the uploaded file directly from local storage path with no restriction:
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.
+Nevertheless when request reply is set and query terminated, the uploaded temporary file at
+path is destroyed.
Arguments as a JSON object
-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:
+Plugins may also request every arguments of a given call as one single object.
+This feature is provided by the function afb_req_json defined here after:
/*
* Gets from the request 'req' the json object hashing the arguments.
@@ -813,15 +787,12 @@ that is defined as below:
struct json_object *afb_req_json(struct afb_req req);
-It returns a json object. This object depends on how the request was
-made:
+It returns a json object. This object depends on how the request was built:
-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.
For HTTP requests, this json object uses key names mapped on argument name.
+Values are either string for common arguments or object ie: { “file”: “…”, “path”: “…” }
For WebSockets requests, returned directly the object as provided by the client.
@@ -829,36 +800,34 @@ given by the client transparently transported.
can be seen as a shortcut to
json_object_get_string(json_object_object_get(afb_req_json(req), name))
-
-Initialisation of the plugin and declaration of verbs
+
+Initialisation of the plugin and declaration of methods
-To be active, the verbs of the plugin should be declared to
-afb-daemon. And even more, the plugin itself must be recorded.
+To be active, plugin’s methods should be declared to
+afb-daemon. Furthermore, the plugin itself must be recorded.
-The mechanism for doing this is very simple: when afb-need starts,
-it loads the plugins that are listed in its argument or configuration.
+The registration mechanism is very basic: when afb-need starts,
+it loads all plugins listed in: command line or configuration file.
Loading a plugin follows the following steps:
-It loads the plugin using dlopen.
It searchs for the symbol named pluginAfbV1Register using dlsym.
+
Afb-daemon loads the plugin with dlopen.
Afb-daemon searches for a symbol named pluginAfbV1Register using dlsym.
This symbol is assumed to be the exported initialisation function of the plugin.
It build an interface object for the plugin.
It calls the found function pluginAfbV1Register and pass it the pointer
-to its interface.
The function pluginAfbV1Register setup the plugin, initialize it.
The function pluginAfbV1Register returns the pointer to a structure
-that describes the plugin: its version, its name (prefix or API name), and the
-list of its verbs.
Afb-daemon builds an interface object for the plugin.
Afb-daemon calls the found function pluginAfbV1Register with interface pointer
+as parameter.
Function pluginAfbV1Register setups the plugin and initialises it.
Function pluginAfbV1Register returns the pointer to a structure
+describing the plugin: version, name (prefix or API name), and list of methods.
Afb-daemon checks that the returned version and name can be managed.
-If it can manage it, the plugin and its verbs are recorded and can be used
-when afb-daemon finishes it initialisation.
-Here is the listing of the function pluginAfbV1Register of the plugin
-tic-tac-toe:
+Here after the code used for pluginAfbV1Register from plugin tic-tac-toe:
/*
* activation function for registering the plugin called by afb-daemon
@@ -870,11 +839,12 @@ const struct AFB_plugin *pluginAfbV1Register(const struct AFB_interface *itf)
}
-This is a very small function because the tic-tac-toe plugin doesn’t have initialisation step.
-It merely record the daemon’s interface and returns its descritption.
+It is a very minimal initialisation function because tic-tac-toe plugin doesn’t
+have any application related initialisation step. It merely record daemon’s interface
+and returns its description.
-The variable afbitf is a variable global to the plugin. It records the
-interface to afb-daemon and is used for logging and pushing events.
+
The variable afbitf is a plugin global variable. It keeps the
+interface to afb-daemon that should be used for logging and pushing events.
Here is its declaration:
/*
@@ -883,12 +853,12 @@ Here is its declaration:
const struct AFB_interface *afbitf;
-The description of the plugin is defined as below.
+The description of the plugin is defined here after.
/*
- * array of the verbs exported to afb-daemon
+ * array of the methods exported to afb-daemon
*/
-static const struct AFB_verb_desc_v1 plugin_verbs[] = {
+static const struct AFB_method_desc_v1 plugin_methods[] = {
/* VERB'S NAME SESSION MANAGEMENT FUNCTION TO CALL SHORT DESCRIPTION */
{ .name= "new", .session= AFB_SESSION_NONE, .callback= new, .info= "Starts a new game" },
{ .name= "play", .session= AFB_SESSION_NONE, .callback= play, .info= "Asks the server to play" },
@@ -911,40 +881,39 @@ static const struct AFB_plugin plugin_description =
.v1= { /* fills the v1 field of the union when AFB_PLUGIN_VERSION_1 */
.prefix= "tictactoe", /* the API name (or plugin name or prefix) */
.info= "Sample tac-tac-toe game", /* short description of of the plugin */
- .verbs = plugin_verbs /* the array describing the verbs of the API */
+ .methods = plugin_methods /* the array describing the methods of the API */
}
};
The structure plugin_description describes the plugin.
-It declares the type and version of the plugin, its name, a description
-and a list of its verbs.
+It declares the type and version of the plugin, its name, a short description
+and its methods list.
-The list of verbs is an array of structures describing the verbs and terminated by a marker:
-a verb whose name is NULL.
+The list of methods is an array of structures describing the methods and terminated by a NULL marker.
-The description of the verbs for this version is made of 4 fields:
+In version one of afb-damon plugin, a method description contains 4 fields:
-the name of the verbs,
the name of the method,
the session management flags,
the implementation function to be call for the verb,
the implementation function to be call for the method,
a short description.
-The structure describing verbs is defined as follows:
+The structure describing methods is defined as follows:
/*
- * Description of one verb of the API provided by the plugin
+ * Description of one method of the API provided by the plugin
* This enumeration is valid for plugins of type 1
*/
-struct AFB_verb_desc_v1
+struct AFB_method_desc_v1
{
- const char *name; /* name of the verb */
- enum AFB_session_v1 session; /* authorisation and session requirements of the verb */
- void (*callback)(struct afb_req req); /* callback function implementing the verb */
- const char *info; /* textual description of the verb */
+ const char *name; /* name of the method */
+ enum AFB_session_v1 session; /* authorisation and session requirements of the method */
+ void (*callback)(struct afb_req req); /* callback function implementing the method */
+ const char *info; /* textual description of the method */
};
@@ -957,7 +926,7 @@ enumeration but the wrapper of constant definitions that can be mixed using bitw
-Constant name
+Constant name
Meaning
@@ -1030,22 +999,22 @@ enumeration but the wrapper of constant definitions that can be mixed using bitw
-If any of this flags is set, afb-daemon requires the token authentification
-as if the flag AFB_SESSION_CHECK had been set.
+If any of this flag is set, afb-daemon requires an authentication token
+as if AFB_SESSION_CHECK flag was also set.
-The special value AFB_SESSION_NONE is zero and can be used to avoid any check.
+The special value AFB_SESSION_NONE is zero and can be used to bypass token check.
Note that AFB_SESSION_CREATE and AFB_SESSION_CLOSE might be removed in later versions.
Sending messages to the log system
-Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.
+Afb-daemon provides 4 levels of verbosity and 5 methods for logging messages.
The verbosity is managed. Options allow the change the verbosity of afb-daemon
and the verbosity of the plugins can be set plugin by plugin.
-The verbs for logging messages are defined as macros that test the
+
The methods for logging messages are defined as macros that test the
verbosity level and that call the real logging function only if the
message must be output. This avoid evaluation of arguments of the
formatting messages if the message must not be output.
@@ -1053,14 +1022,14 @@ formatting messages if the message must not be output.
Verbs for logging messages
-The 5 logging verbs are:
+The 5 logging methods are:
Macro
Verbosity
- Meaning
+ Meaning
syslog level
@@ -1068,13 +1037,13 @@ formatting messages if the message must not be output.
ERROR
0
- Error conditions
+ Error conditions
3
WARNING
1
- Warning conditions
+ Warning conditions
4
@@ -1086,34 +1055,34 @@ formatting messages if the message must not be output.
INFO
2
- Informational
+ Informational
6
DEBUG
3
- Debug-level messages
+ Debug-level messages
7
-You can note that the 2 verbs WARNING and INFO have the same level
+
You can note that the 2 methods WARNING and INFO have the same level
of verbosity. But they don’t have the same syslog level. It means that
they are output with a different level on the logging system.
-All of these verbs have the same signature:
+All of these methods have the same signature:
void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
The first argument afbitf is the interface to afb daemon that the
-plugin received at its initialisation when pluginAfbV1Register was called.
+plugin received at initialisation time when pluginAfbV1Register is called.
The second argument message is a formatting string compatible with printf/sprintf.
-The remaining arguments are arguments of the formating message like for printf.
+The remaining arguments are arguments of the formating message like with printf.
Managing verbosity
@@ -1200,17 +1169,17 @@ The prefixes are:
-The message is issued to the standard error.
-The final destination of the message depends on how the systemd service
-was configured through the variable StandardError: It can be
+
The message is pushed to standard error.
+The final destination of the message depends on how systemd service
+was configured through its variable StandardError. It can be
journal, syslog or kmsg. (See man sd-daemon).
Sending events
Since version 0.5, plugins can broadcast events to any potential listener.
-This kind of bradcast is not targeted. Event targeted will come in a future
-version of afb-daemon.
+As today only unattended even are supported. Targeted events are expected for next
+coming version.
The plugin tic-tac-toe broadcasts events when the board changes.
This is done in the function changed:
@@ -1234,66 +1203,149 @@ static void changed(struct board *board, const char *reason)
The description of the changed board is pushed via the daemon interface.
-Within the plugin tic-tac-toe, the reason indicates the origin of
-the change. For the function afb_daemon_broadcast_event, the second
-parameter is the name of the broadcasted event. The third argument is the
+
Within plugin tic-tac-toe, reason indicates the origin of
+the change. In function afb_daemon_broadcast_event the second
+parameter is the name of broadcasted event. The third argument is the
object that is transmitted with the event.
-The function afb_daemon_broadcast_event is defined as below:
+Function afb_daemon_broadcast_event is defined here after:
/*
* Broadcasts widely the event of 'name' with the data 'object'.
* 'object' can be NULL.
* 'daemon' MUST be the daemon given in interface when activating the plugin.
+ *
+ * For conveniency, the function calls 'json_object_put' for 'object'.
+ * Thus, in the case where 'object' should remain available after
+ * the function returns, the function 'json_object_get' shall be used.
*/
void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object);
-In fact the event name received by the listener is prefixed with
-the name of the plugin. So when the change occurs after a move, the
-reason is move and then the clients receive the event tictactoe/move.
+Be aware, as with reply functions object is automatically released using
+json_object_put when using this function. Call json_object_get before
+calling afb_daemon_broadcast_event to keep object available
+after function returns.
-Note that nothing is said about the case sensitivity of event names.
+
Event name received by listeners is prefixed with plugin name.
+So when a change occurs after a move, the reason is move and every clients
+receive an event tictactoe/move.
+
+Note that nothing is said about case sensitivity of event names.
However, the event is always prefixed with the name that the plugin
declared, with the same case, followed with a slash /.
Thus it is safe to compare event using a case sensitive comparison.
-
-Writing an asynchronous verb implementation
+
+Writing an asynchronous method implementation
+
+The tic-tac-toe example allows two clients or more to share the same board.
+This is implemented by the method join that illustrated partly how to
+retrieve arguments.
+
+When two or more clients are sharing a same board, one of them can wait
+until the state of the board changes, but this could also be implemented using
+events because an even is generated each time the board changes.
+
+In this case, the reply to the wait is sent only when the board changes.
+See the diagram below:
+
+CLIENT A CLIENT B TIC-TAC-TOE
+ | | |
+ +--------------|----------------->| wait . . . . . . . .
+ | | | .
+ : : : .
+ : : : .
+ | | | .
+ | +----------------->| move . . . .
+ | | | V .
+ | |<-----------------+ success of move .
+ | | | .
+ |<-------------|------------------+ success of wait <
+
-/
- * signals a change of the board
- /
-static void changed(struct board board, const char reason)
+
Here, this is an invocation of the plugin by an other client that
+unblock the suspended wait call.
+Nevertheless in most case this should be a timer, a hardware event, a sync with
+a concurrent process or thread, …
+
+Common case of an asynchronous implementation.
+
+Here is the listing of the function wait:
+
+static void wait(struct afb_req req)
{
- struct waiter waiter, next;
- struct json_object *description;
-
-/* get the description */
-description = describe(board);
-
-waiter = board->waiters;
-board->waiters = NULL;
-while (waiter != NULL) {
- next = waiter->next;
- afb_req_success(waiter->req, json_object_get(description), reason);
- afb_req_unref(waiter->req);
- free(waiter);
- waiter = next;
+ struct board *board;
+ struct waiter *waiter;
+
+ /* retrieves the context for the session */
+ board = board_of_req(req);
+ INFO(afbitf, "method 'wait' called for boardid %d", board->id);
+
+ /* creates the waiter and enqueues it */
+ waiter = calloc(1, sizeof *waiter);
+ waiter->req = req;
+ waiter->next = board->waiters;
+ afb_req_addref(req);
+ board->waiters = waiter;
}
+
+
+After retrieving the board, the function adds a new waiter to
+waiters list and returns without setting a reply.
+
+Before returning, it increases req request’s reference count using afb_req_addref function.
+
+When a method returns without setting a reply,
+it MUST increment request’s reference count
+using afb_req_addref. If unpredictable behaviour may pop up.
+
+Later, when a board changes, it calls tic-tac-toe changed function
+with reason of change in parameter.
+
+Here is the full listing of the function changed:
+
+/*
+ * signals a change of the board
+ */
+static void changed(struct board *board, const char *reason)
+{
+ struct waiter *waiter, *next;
+ struct json_object *description;
+
+ /* get the description */
+ description = describe(board);
+
+ waiter = board->waiters;
+ board->waiters = NULL;
+ while (waiter != NULL) {
+ next = waiter->next;
+ afb_req_success(waiter->req, json_object_get(description), reason);
+ afb_req_unref(waiter->req);
+ free(waiter);
+ waiter = next;
+ }
-afb_event_sender_push(afb_daemon_get_event_sender(afbitf->daemon), reason, description);
+ afb_event_sender_push(afb_daemon_get_event_sender(afbitf->daemon), reason, description);
+}
-}
+The list of waiters is walked and a reply is sent to each waiter.
+After sending the reply, the reference count of the request
+is decremented using afb_req_unref to allow resources to be freed.
+
+The reference count MUST be decremented using afb_req_unref to free
+resources and avoid memory leaks.
+This usage count decrement should happen AFTER setting reply or
+bad things may happen.
How to build a plugin
Afb-daemon provides a pkg-config configuration file that can be
-queried by the name afb-daemon.
+queried by providing afb-daemon in command line arguments.
This configuration file provides data that should be used
-for compiling plugins. Examples:
+for plugins compilation. Examples:
$ pkg-config --cflags afb-daemon
$ pkg-config --libs afb-daemon
@@ -1332,13 +1384,13 @@ endif()