# 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" } ```