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.

Versions of the bindings

Since introduction of the binder, the way how bindings are written evolved a little. While changing, attention was made to ensure binary compatibility between the different versions.

Actually it exists 3 ways of writing bindings. You can either write:

  • a binding version 1 (not more supported);
  • a binding version 2 (not recommended);
  • a binding version 3 (RECOMMENDED).

A binder loads and runs any of these version in any combination.
This document explain how to write bindings version 3.

Sample binding: tuto-1

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

  1 #define AFB_BINDING_VERSION 3
  2 #include <afb/afb-binding.h>
  3
  4 void hello(afb_req_t req)
  5 {
  6         AFB_REQ_DEBUG(req, "hello world");
  7         afb_req_reply(req, NULL, NULL, "hello world");
  8 }
  9
 10 const afb_verb_t verbs[] = {
 11         { .verb="hello", .callback=hello },
 12         { .verb=NULL }
 13 };
 14
 15 const afb_binding_t afbBindingExport = {
 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)

Note: the variable environment variable PKG_CONFIG_PATH might be necessary tuned to get pkg-config working properly

Running:

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

At this point, afb-daemon has started, it loaded the binding tuto-1.so and now listen at localhost on the port 3333.

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 3
  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 3 (earlier versions 1 and 2 are deprecated).

If you don’t define it, an error is reported and the compilation aborts.

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 a draft version of C++ api.
To get it include the file <afb/afb-binding> (without .h).

Declaring the API of the binding

Lines 10 to 18 show the declaration of the binding.

The binder knows that this is a binding because it finds the exported symbol afbBindingExport that is expected to be a structure of type afb_binding_t.

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

The structure afbBindingExport 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 of type afb_verb_t, 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).

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_t req)
  5 {
  6         AFB_REQ_DEBUG(req, "hello world");
  7         afb_req_reply(req, NULL, 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.

At the line 7, the callback for tuto-1/hello replies to the request req. Parameters of the reply are:

  1. The first parameter is the replied request
  2. The second parameter is a json object (here NULL)
  3. The third parameter is the error string indication (here NULL: no error)
  4. The fourth parameter is an informative string (that can be NULL) that can be used to provide meta data.

The 3 last parameters are sent back to the client as the reply content.

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 3
      5 #include <afb/afb-binding.h>
      6 
      7 afb_event_t event_login, event_logout;
      8 
      9 void login(afb_req_t 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_reply(req, NULL, "bad-request", NULL);
     19         } else if (afb_req_context_get(req)) {
     20                 AFB_REQ_ERROR(req, "login, bad state, logout first");
     21                 afb_req_reply(req, NULL, "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_reply(req, NULL, "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_reply(req, NULL, NULL, NULL);
     31                 afb_event_push(event_login, json_object_new_string(usr));
     32         }
     33 }
     34 
     35 void action(afb_req_t 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_reply(req, json_object_get(args), NULL, NULL);
     55 }
     56 
     57 void logout(afb_req_t 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_reply(req, NULL, NULL, NULL);
     67 }
     68 
     69 int preinit(afb_api_t api)
     70 {
     71         AFB_API_NOTICE(api, "preinit");
     72         return 0;
     73 }
     74 
     75 int init(afb_api_t api)
     76 {
     77         AFB_API_NOTICE(api, "init");
     78         event_login = afb_api_make_event(api, "login");
     79         event_logout = afb_api_make_event(api, "logout");
     80         if (afb_event_is_valid(event_login) && afb_event_is_valid(event_logout))
     81                 return 0;
     82         AFB_API_ERROR(api, "Can't create events");
     83         return -1;
     84 }
     85 
     86 const afb_verb_t 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_t afbBindingExport = {
     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"
}