Overview of the bindings

The binder serves files through HTTP protocol and offers to developers the capability to offer application API methods through HTTP or WebSocket protocol.

The bindings are used to add API to binders.
This part describes how to write a binding for binder or in other words how to add a new API to the system.

Excepting this summary, this section target developers.

This section shortly explain how to write a binding using the C programming language.

It is convenient to install the binder on the desktop used for writing the binding.
It allows easy debug and test.

Nature of a binding

A binding is an independent piece of software compiled as a shared library and dynamically loaded by a binder.
It is intended to provide one API (Application Programming Interface).

The API is designated and accessed through its name.
It contains several verbs that implement the binding functionalities.
Each of these verbs is a method that processes requests of applications and sends result.

The binding’s methods are invoked by HTTP or websocket requests.

The methods of the bindings are noted api/verb where api is the API name of the binding and verb is the method’s name within the API.
This notation comes from HTTP invocations that rely on URL path terminated with api/verb.

The name of an API can be made of any characters except:

  • the control characters (\u0000 .. \u001f)
  • the characters of the set { ‘ ‘, ‘”’, ‘#’, ‘%’, ‘&’, ‘'’, ‘/’, ‘?’, ‘`’, ‘\x7f’ }

The names of the verbs can be any character.

The binder makes no distinctions between upper case and lower case latin letters.
So API/VERB matches Api/Verb or api/verb.

Actually it exists 2 ways of writing bindings. You can either write:

  • a binding version 1 (not recommended);
  • a binding version 2 (RECOMMENDED).

A binder loads and runs any of these version in any combination.
This document explain how to write bindings version 2.

Sample binding: tuto-1

This is the code of the binding tuto-1.c:

  1 #define AFB_BINDING_VERSION 2
  2 #include <afb/afb-binding.h>
  3
  4 void hello(afb_req req)
  5 {
  6         AFB_REQ_DEBUG(req, "hello world");
  7         afb_req_success(req, NULL, "hello world");
  8 }
  9
 10 const afb_verb_v2 verbs[] = {
 11         { .verb="hello", .callback=hello },
 12         { .verb=NULL }
 13 };
 14
 15 const afb_binding_v2 afbBindingV2 = {
 16         .api = "tuto-1",
 17         .verbs = verbs
 18 };

Compiling:

gcc -fPIC -shared tuto-1.c -o tuto-1.so $(pkg-config --cflags-only-I afb-daemon)

Running:

afb-daemon --binding tuto-1.so --port 3333 --token ''

Testing using curl:

$ curl http://localhost:3333/api/tuto-1/hello
{"jtype":"afb-reply","request":{"status":"success","info":"hello world","uuid":"1e587b54-900b-49ab-9940-46141bc2e1d6"}}

Testing using afb-client-demo (with option -H for getting a human readable output):

$ afb-client-demo -H ws://localhost:3333/api?token=x tuto-1 hello
ON-REPLY 1:tuto-1/hello: OK
{
  "jtype":"afb-reply",
  "request":{
    "status":"success",
    "info":"hello world",
    "uuid":"03a84ad1-458a-4ace-af74-b1da917391b9"
  }
}

This shows basic things:

  • The include to get for creating a binding
  • How to declare the API offered by the binding
  • How to handle request made to the binding

Getting declarations for the binding

The lines 1 and 2 show how to get the include file afb-binding.h.

  1 #define AFB_BINDING_VERSION 2
  2 #include <afb/afb-binding.h>

You must define the version of binding that you are using.
This is done line 1 where we define that this is the version 2.

If you don’t define it, a warning message is prompted by the compiler and the version is switched to version 1.
This behaviour is temporarily and enables to continue to use previously written binding without change but it will change in some future when bindings V1 will become obsoletes.

To include afb-binding.h successfully, the include search path should be set correctly if needed (not needed only if installed in /usr/include/afb directory that is the default).

Setting the include path is easy using pkg-config:

pkg-config --cflags-only-I afb-daemon

Note for C++ developers:

  • The binder currently expose only C language API.
    The file afb/afb-binding.h isn’t C++ ready.

You should use the construct extern “C” as below:

  #define AFB_BINDING_VERSION 2
  extern "C" {
  #include <afb/afb-binding.h>
  }

Future version of the binder will include a C++ interface.
Until it is available, please, use the above construct.

Declaring the API of the binding

Lines 10 to 18 show the declaration of the binding.

The binder knows that this is a binding version 2 because it finds the exported symbol afbBindingV2 that is expected to be a structure of type afb_binding_v2.

 10 const afb_verb_v2 verbs[] = {
 11         { .verb="hello", .callback=hello },
 12         { .verb=NULL }
 13 };
 14
 15 const afb_binding_v2 afbBindingV2 = {
 16         .api = "tuto-1",
 17         .verbs = verbs
 18 };

The structure afbBindingV2 actually tells that:

  • the exported API name is tuto-1 (line 16)
  • the array of verbs is the above defined one

The exported list of verb is specified by an array of structures, each describing a verb, ended with a verb NULL (line 12).

The only defined verb here (line 11) is named hello (field .verb) and the function that handle the related request is hello (field .callback).

Note that you can explicitly mark the fact that these are struct by typing the struct as below:

 10 const struct afb_verb_v2 verbs[] = {
 11         { .verb="hello", .callback=hello },
 12         { .verb=NULL }
 13 };
 14
 15 const struct afb_binding_v2 afbBindingV2 = {
 16         .api = "tuto-1",
 17         .verbs = verbs
 18 };

Handling binder’s requests

As shown above this is by default the common include directory where the AGL stuff is installed.

  4 void hello(afb_req req)
  5 {
  6         AFB_REQ_DEBUG(req, "hello world");
  7         afb_req_success(req, NULL, "hello world");
  8 }

When the binder receives a request for the verb hello of of the api tuto-1, it invoke the callback hello of the binding with the argument req that handles the client request.

The callback has to treat synchronously or asynchronously the request and should at the end emit a reply for the request.

Here, the callback for tuto-1/hello replies a successful answer (line 7) to the request req.
The second parameter (here NULL) is a json object that is sent to the client with the reply.
The third parameter is also sent with the reply and is a string called info that can be used as some meta data.

Here again, you can explicitly mark the fact that afb_req is a structure by declaring hello as below:

  4 void hello(struct afb_req req)

Sample binding: tuto-2

The second tutorial shows many important feature that can commonly be used when writing a binding:

  • initialization, getting arguments, sending replies, pushing events.

This is the code of the binding tuto-2.c:

      1 #include <string.h>
      2 #include <json-c/json.h>
      3
      4 #define AFB_BINDING_VERSION 2
      5 #include <afb/afb-binding.h>
      6
      7 afb_event event_login, event_logout;
      8
      9 void login(afb_req req)
     10 {
     11         json_object *args, *user, *passwd;
     12         char *usr;
     13
     14         args = afb_req_json(req);
     15         if (!json_object_object_get_ex(args, "user", &user)
     16          || !json_object_object_get_ex(args, "password", &passwd)) {
     17                 AFB_REQ_ERROR(req, "login, bad request: %s", json_object_get_string(args));
     18                 afb_req_fail(req, "bad-request", NULL);
     19         } else if (afb_req_context_get(req)) {
     20                 AFB_REQ_ERROR(req, "login, bad state, logout first");
     21                 afb_req_fail(req, "bad-state", NULL);
     22         } else if (strcmp(json_object_get_string(passwd), "please")) {
     23                 AFB_REQ_ERROR(req, "login, unauthorized: %s", json_object_get_string(args));
     24                 afb_req_fail(req, "unauthorized", NULL);
     25         } else {
     26                 usr = strdup(json_object_get_string(user));
     27                 AFB_REQ_NOTICE(req, "login user: %s", usr);
     28                 afb_req_session_set_LOA(req, 1);
     29                 afb_req_context_set(req, usr, free);
     30                 afb_req_success(req, NULL, NULL);
     31                 afb_event_push(event_login, json_object_new_string(usr));
     32         }
     33 }
     34
     35 void action(afb_req req)
     36 {
     37         json_object *args, *val;
     38         char *usr;
     39
     40         args = afb_req_json(req);
     41         usr = afb_req_context_get(req);
     42         AFB_REQ_NOTICE(req, "action for user %s: %s", usr, json_object_get_string(args));
     43         if (json_object_object_get_ex(args, "subscribe", &val)) {
     44                 if (json_object_get_boolean(val)) {
     45                         AFB_REQ_NOTICE(req, "user %s subscribes to events", usr);
     46                         afb_req_subscribe(req, event_login);
     47                         afb_req_subscribe(req, event_logout);
     48                 } else {
     49                         AFB_REQ_NOTICE(req, "user %s unsubscribes to events", usr);
     50                         afb_req_unsubscribe(req, event_login);
     51                         afb_req_unsubscribe(req, event_logout);
     52                 }
     53         }
     54         afb_req_success(req, json_object_get(args), NULL);
     55 }
     56
     57 void logout(afb_req req)
     58 {
     59         char *usr;
     60
     61         usr = afb_req_context_get(req);
     62         AFB_REQ_NOTICE(req, "login user %s out", usr);
     63         afb_event_push(event_logout, json_object_new_string(usr));
     64         afb_req_session_set_LOA(req, 0);
     65         afb_req_context_clear(req);
     66         afb_req_success(req, NULL, NULL);
     67 }
     68
     69 int preinit()
     70 {
     71         AFB_NOTICE("preinit");
     72         return 0;
     73 }
     74
     75 int init()
     76 {
     77         AFB_NOTICE("init");
     78         event_login = afb_daemon_make_event("login");
     79         event_logout = afb_daemon_make_event("logout");
     80         if (afb_event_is_valid(event_login) && afb_event_is_valid(event_logout))
     81                 return 0;
     82         AFB_ERROR("Can't create events");
     83         return -1;
     84 }
     85
     86 const afb_verb_v2 verbs[] = {
     87         { .verb="login", .callback=login },
     88         { .verb="action", .callback=action, .session=AFB_SESSION_LOA_1 },
     89         { .verb="logout", .callback=logout, .session=AFB_SESSION_LOA_1 },
     90         { .verb=NULL }
     91 };
     92
     93 const afb_binding_v2 afbBindingV2 = {
     94         .api = "tuto-2",
     95         .specification = NULL,
     96         .verbs = verbs,
     97         .preinit = preinit,
     98         .init = init,
     99         .noconcurrency = 0
    100 };

Compiling:

gcc -fPIC -shared tuto-2.c -o tuto-2.so $(pkg-config --cflags --libs afb-daemon)

Running:

afb-daemon --binding tuto-2.so --port 3333 --token ''

Testing:

$ afb-client-demo -H localhost:3333/api?token=toto
tuto-2 login {"help":true}
ON-REPLY 1:tuto-2/login: ERROR
{
  "jtype":"afb-reply",
  "request":{
    "status":"bad-request",
    "uuid":"e2b24a13-fc43-487e-a5f4-9266dd1e60a9"
  }
}
tuto-2 login {"user":"jose","password":"please"}
ON-REPLY 2:tuto-2/login: OK
{
  "jtype":"afb-reply",
  "request":{
    "status":"success"
  }
}
tuto-2 login {"user":"jobol","password":"please"}
ON-REPLY 3:tuto-2/login: ERROR
{
  "jtype":"afb-reply",
  "request":{
    "status":"bad-state"
  }
}
tuto-2 action {"subscribe":true}
ON-REPLY 4:tuto-2/action: OK
{
  "response":{
    "subscribe":true
  },
  "jtype":"afb-reply",
  "request":{
    "status":"success"
  }
}

In an other terminal:

$ afb-client-demo -H localhost:3333/api?token=toto
tuto-2 login {"user":"jobol","password":"please"}
ON-REPLY 1:tuto-2/login: OK
{
  "jtype":"afb-reply",
  "request":{
    "status":"success",
    "uuid":"a09f55ff-0e89-4f4e-8415-c6e0e7f439be"
  }
}
tuto-2 logout true
ON-REPLY 2:tuto-2/logout: OK
{
  "jtype":"afb-reply",
  "request":{
    "status":"success"
  }
}

It produced in the first terminal:

ON-EVENT tuto-2/login:
{
  "event":"tuto-2\/login",
  "data":"jobol",
  "jtype":"afb-event"
}
ON-EVENT tuto-2/logout:
{
  "event":"tuto-2\/logout",
  "data":"jobol",
  "jtype":"afb-event"
}