[AGL/documentation.git] / docs / 4_APIs_and_Services / 4.3_Application_Framework_Binder / 2_How_to_write_a_binding / 2_How_to_write_a_binding.md

---
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title: How to write a binding ?
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# Overview of the bindings

The ***binder*** serves files through HTTP protocol and offers 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.

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 for 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** (**A**pplication **P**rogramming
**I**nterface).

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 results.

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.

<!-- pagebreak -->

## Sample binding: tuto-1

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

```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:

```bash
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:

```bash
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**:

```bash
$ 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):

```bash
$ 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 requests made to the binding

### Getting declarations for the binding

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

```C
  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**:

```bash
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**.

```C
 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.

```C
  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.

<!-- pagebreak -->

## 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**:

```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:

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

Running:

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

Testing:

```bash
$ 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 another terminal:

```bash
$ 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:

```bash
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"
}
```