Migration from binding V1 to binding V2

The binding interface evolved from version 1 to version 2 for the following reasons:

  • integration of the security requirements within the bindings
  • simplification of the API (after developer feedbacks)
  • removal of obscure features, cleanup

The binder can run bindings v1 and/or v2 in any combination.
Thus moving from v1 to v2 is not enforced, there is no real need.

More, it is possible to write a dual binding:

  • a binding that implements the version 1 and the version 2.

However, IT IS HIGHLY RECOMMENDED TO SWITCH TO ONLY VERSION 2:

  • any new development SHOULD start using binding V2
  • existing bindings SHOULD migrate to the version 2

This guide covers the migration of bindings from version 1 to version 2.

It also explains some of the rationale taken when migrating from version 1 to version 2.

In the future, if binding api evolves to fresh versions (3, 4, …) it might be necessarily to write bindings implementing more than just one version.
For example:

  • a binding being v2 AND v3 will resolve the issue of running on older and newer version of AGL.

This should always be possible even if more complicated.

Important things to known when migrating

One of the most important change when migrating from v1 to v2 is that many functions use an hidden common variable.
This affects the functions of the following classes:

  • functions of class daemon:
    • functions starting with afb_daemon_…
    • functions for logging: ERROR, WARNING, NOTICE, INFO, DEBUG
  • functions of class service:
    • functions starting with afb_service_…
  • callback functions:
    • the register function (that is removed)
    • the service init function
    • the onevent function

For these functions, the first parameter is now implicit.

Let takes an example.
For binding v1 you had to write:

        afb_daemon_broadcast_event(afbitf->daemon, reason, description);

For binding v2, you simply write:

        afb_daemon_broadcast_event(reason, description);

This simplification is possible because the header files included for the bindings now provide a common variable for storing the daemon and service data.

As a programmer, you shouldn’t care much about that hidden variable.
It simplifies the job, that’s all and that is the reason of the change.

An other important difference is between the version 1 and the version 2 is on how the binding’s API is documented.
The version 2 emphasis the OpenAPI v3 description of the API.
For this reason, to avoid duplication of descriptions, only one description is expected:

  • The OpenAPI one.

Task list for the migration

This task list is:

  1. Enforce use of binding v2 by setting AFB_BINDING_VERSION
  2. Rewrite the main structure and the list of exported verbs
  3. Adapt the init and callback functions
  4. Removes the first parameter of functions of classes daemon and service
  5. Consider where to emit logs for requests
  6. Take care of store/unstore changes
  7. Consider use of synchronous (sub)call requests
  8. Optionally, removes explicit struct

The remaining chapters explain these task with more details.

Enforce use of binding v2 by setting AFB_BINDING_VERSION

By defining AFB_BINDING_VERSION to 2 you switch to version 2.
This is done as below.

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

After that you will get many errors when compiling.

Rewrite the main structures and the list of exported verbs

The structures describing the *binding changed from version 1 to version 2.

The structure for describing verbs changed to include security requirements.

In version 1 it was:

struct afb_verb_desc_v1
{
       const char *name;                       /* name of the verb */
       enum afb_session_flags_v1 session;      /* authorization 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 */
};

In version 2 it becomes:

struct afb_verb_v2
{
        const char *verb;                       /* name of the verb */
        void (*callback)(struct afb_req req);   /* callback function implementing the verb */
        const struct afb_auth *auth;            /* required authorization */
        uint32_t session;                       /* authorization and session requirements of the verb */
};

The migration of instances of that structure requires the following actions:

  • rename field name to verb
  • remove field info
  • adapt field session if needed
  • set field auth to NULL

Example:

 { .name= "new",   .session= AFB_SESSION_NONE, .callback= new,   .info= "Starts a new game" }

Becomes

 { .verb = "new",   .session = AFB_SESSION_NONE, .callback = new, .auth = NULL }

The field auth can be set to a value describing the requested authorization.

The main describing structure also changed.

In version 1 it was:

struct afb_binding_desc_v1
{
       const char *info;                       /* textual information about the binding */
       const char *prefix;                     /* required prefix name for the binding */
       const struct afb_verb_desc_v1 *verbs;   /* array of descriptions of verbs terminated by a NULL name */
};

In version 2 it becomes:

struct afb_binding_v2
{
        const char *api;                        /* api name for the binding */
        const char *specification;              /* textual specification of the binding */
        const struct afb_verb_v2 *verbs;        /* array of descriptions of verbs terminated by a NULL name */
        int (*preinit)();                       /* callback at load of the binding */
        int (*init)();                          /* callback for starting the service */
        void (*onevent)(const char *event, struct json_object *object); /* callback for handling events */
        unsigned noconcurrency: 1;              /* avoids concurrent requests to verbs */
};

The migration of instances of that structure requires the following actions:

  • declare, explore, name the structure as const struct afb_binding_v2 afbBindingV2
  • rename the field prefix to api
  • remove the field info
  • setup the fields preinit, init, onevent according to the next section
  • set the field noconcurrency to the right value:
    • to 1 if you want to avoid concurrent calls to verbs.
    • to 0 if you allow concurrent calls to verbs.

Example:

static const struct afb_binding plugin_desc = {
        .type = AFB_BINDING_VERSION_1,
        .v1 = {
                .info = "Minimal Hello World Sample",
                .prefix = "hello",
                .verbs = verbs
        }

Becomes:

const struct afb_binding_v2 afbBindingV2 = {
        .api = "hello",
        .specification = NULL,
        .verbs = verbs,
        .preinit = preinit,
        .init = init
};

The binder now relies only on the exported names to deduce the type of the binding.
This make the main structure more simple.

Adapt the init and callback functions

The bindings version 1 defined 3 exported functions:

  • afbBindingV1Register
  • afbBindingV1ServiceInit
  • afbBindingV1ServiceEvent

These function should not be exported any more and there definition changed.

The function afbBindingV1Register is no more used to describe the binding.
When a binding has to take actions when it is loaded, it must set the field preinit of the structure afbBindingV2.
This field, this preinit, might be used to check features at load.
When it returns a negative number, the binder stops before initializing any binding.

The function afbBindingV1ServiceInit is replaced by the field init of the structure afbBindingV2.
The init function should return 0 in case of success or a negative error code in case of problem.
It is called during initialization of services.

The function afbBindingV1ServiceEventis replaced by the field onevent of the structure afbBindingV2.

The two functions afbBindingV1Register and afbBindingV1ServiceInit, were taking as parameter the binder interface and the service interface respectively.
These interfaces are now managed hiddenly for the binding by the binder.
So the variable that bindings version used to store the binder interface and the service interface are no more needed and can be removed.

Example:

const struct afb_binding_interface *interface;
struct afb_service service;

static const struct afb_binding plugin_desc = {
        .type = AFB_BINDING_VERSION_1,
        .v1 = {
                .info = "Minimal Hello World Sample",
                .prefix = "hello",
                .verbs = verbs
        }
}

const struct afb_binding *afbBindingV1Register (const struct afb_binding_interface *itf)
{
        interface = itf;
        NOTICE(interface, "binding register");
        return &plugin_desc;
}

int afbBindingV1ServiceInit(struct afb_service svc)
{
        service = svc;
        NOTICE(interface, "binding init");
        return 0;
}

void afbBindingV1ServiceEvent(const char *event, struct json_object *object)
{
        NOTICE(interface, "onevent %s", event);
}

Becomes:

static int preinit()
{
        AFB_NOTICE("binding preinit (was register)");
        return 0;
}

static int init()
{
        AFB_NOTICE("binding init");
        return 0;
}

static void onevent(const char *event, struct json_object *object)
{
        AFB_NOTICE("onevent %s", event);
}

const struct afb_binding_v2 afbBindingV2 = {
        .api = "hello",
        .specification = NULL,
        .verbs = verbs,
        .preinit = preinit,
        .init = init,
        .onevent = onevent
};

The two functions afbBindingV1Register and afbBindingV1ServiceInit, were taking as parameter the binder interface and the service interface respectively.
These interfaces are now managed hiddenly for the binding by the binder.
So the variable that bindings version used to store the binder interface and the service interface are no more needed and can be removed.

On the above example the following lines were removed:

const struct afb_binding_interface *interface;
struct afb_service service;

        interface = itf;

        service = svc;

Removes the first parameter of functions of classes daemon and service

As explained before, many functions loose there first arguments, this are the functions of the following classes:

  • functions of class daemon:
    • functions starting with afb_daemon_…
    • functions for logging: ERROR, WARNING, NOTICE, INFO, DEBUG
  • functions of class service:
    • functions starting with afb_service_…
  • callback functions:
    • the register function (that is removed)
    • the service init function
    • the onevent function

For these functions, the first parameter is now implicit.

Example:

        afb_daemon_broadcast_event(afbitf->daemon, reason, description);

Becomes:

        afb_daemon_broadcast_event(reason, description);

Also, to avoid possible conflicts, we introduced prefixed logging functions:
the macros

  • ERROR
  • WARNING
  • NOTICE
  • INFO
  • DEBUG

have now a prefixed version:

  • AFB_ERROR
  • AFB_WARNING
  • AFB_NOTICE
  • AFB_INFO
  • AFB_DEBUG

It is now recommended to use the prefixed version.

Example:

        NOTICE(interface, "hello plugin comes to live");

Become:

        NOTICE("hello plugin comes to live");

or, better:

        AFB_NOTICE("hello plugin comes to live");

To remove definition of the un-prefixed versions of logging macros:

  • ERROR
  • WARNING
  • NOTICE
  • INFO
  • DEBUG

and just define

  • AFB_BINDING_PRAGMA_NO_VERBOSE_UNPREFIX

before to include afb/afb-binding.h.

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

Consider where to emit logs for requests

The bindings v2 now allows to emit log messages associated to requests.
This feature is valuable when debugging because it allows to return side information associated to a request.

The defined macros for logging to requests are:

  • AFB_REQ_ERROR
  • AFB_REQ_WARNING
  • AFB_REQ_NOTICE
  • AFB_REQ_INFO
  • AFB_REQ_DEBUG

We encourage the use of these new logging facilities everywhere it makes sense.

Example:

        INFO(afbitf, "method 'new' called for boardid %d", board->id);

Might become:

        AFB_REQ_INFO(req, "method 'new' called for boardid %d", board->id);

Take care of store/unstore change

For efficiency, the version 2 redefined how storing/un-storing of requests works.
Storing request is needed for asynchronous handling of requests.

For bindings version, the signature of the functions were:

struct afb_req *afb_req_store(struct afb_req req);
struct afb_req afb_req_unstore(struct afb_req *req);

For version 2 it becomes

struct afb_stored_req *afb_req_store(struct afb_req req);
struct afb_req afb_req_unstore(struct afb_stored_req *sreq);

Where the structure struct afb_stored_req is opaque.

It should require few code change.

Also check the following chapter that explain that asynchronous (sub)calls can be replaced by synchronous one, avoiding the need to store/unstore requests.

Consider use of synchronous (sub)call requests

Bindings can emit requests for themselves (calls) or for their clients (subcalls).
With bindings version 2 comes also synchronous requests for both cases.

So when migrating to bindings version 2, a developer can consider to replace the asynchronous requests (with asynchronous call back) by synchronous ones.

See functions afb_service_call_sync and afb_req_subcall_sync.

Optionally, removes explicit struct

The new definitions now includes typedefs for common structures, as shown on below sample:

typedef struct afb_daemon               afb_daemon;
typedef struct afb_event                afb_event;
typedef struct afb_arg                  afb_arg;
typedef struct afb_req                  afb_req;
typedef struct afb_service              afb_service;

So you can remove the keyword struct if it bores you.

Example:

static void verb(struct afb_req req)
{
        ...
}

Might become:

static void verb(afb_req req)
{
        ...
}

Example of migration

The first binding that migrated from v1 to v2 was the sample HelloWorld.
Here is shown the differences between the version 1 and the version 2.

diff --git a/bindings/samples/HelloWorld.c b/bindings/samples/HelloWorld.c
index c6fa779..505aee3 100644
--- a/bindings/samples/HelloWorld.c
+++ b/bindings/samples/HelloWorld.c
@@ -21,9 +21,9 @@

 #include <json-c/json.h>

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

-const struct afb_binding_interface *interface;
 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

 struct event
@@ -79,7 +80,7 @@ static int event_add(const char *tag, const char *name)
         strcpy(e->tag, tag);

         /* make the event */
-    e->event = afb_daemon_make_event(interface->daemon, name);
+    e->event = afb_daemon_make_event(name);
         if (!e->event.closure) { free(e); return -1; }

         /* link */
@@ -140,7 +141,7 @@ static void pingBug (struct afb_req request)
 static void pingEvent(struct afb_req request)
 {
         json_object *query = afb_req_json(request);
-    afb_daemon_broadcast_event(interface->daemon, "event", json_object_get(query));
+    afb_daemon_broadcast_event("event", json_object_get(query));
         ping(request, json_object_get(query), "event");
 }

@@ -288,38 +289,43 @@ static void exitnow (struct afb_req request)
         exit(0);
 }

+static int preinit()
+{
+    AFB_NOTICE("hello binding comes to live");
+    return 0;
+}
+
+static int init()
+{
+    AFB_NOTICE("hello binding starting");
+    return 0;
+}
+
 // NOTE: this sample does not use session to keep test a basic as possible
 //       in real application most APIs should be protected with AFB_SESSION_CHECK
-static const struct afb_verb_desc_v1 verbs[]= {
-  {"ping"     , AFB_SESSION_NONE, pingSample  , "Ping Application Framework"},
-  {"pingfail" , AFB_SESSION_NONE, pingFail    , "Fails"},
-  {"pingnull" , AFB_SESSION_NONE, pingNull    , "Return NULL"},
-  {"pingbug"  , AFB_SESSION_NONE, pingBug     , "Do a Memory Violation"},
-  {"pingJson" , AFB_SESSION_NONE, pingJson    , "Return a JSON object"},
-  {"pingevent", AFB_SESSION_NONE, pingEvent   , "Send an event"},
-  {"subcall",   AFB_SESSION_NONE, subcall     , "Call api/verb(args)"},
-  {"subcallsync",   AFB_SESSION_NONE, subcallsync     , "Call api/verb(args)"},
-  {"eventadd",  AFB_SESSION_NONE, eventadd    , "adds the event of 'name' for the 'tag'"},
-  {"eventdel",  AFB_SESSION_NONE, eventdel    , "deletes the event of 'tag'"},
-  {"eventsub",  AFB_SESSION_NONE, eventsub    , "subscribes to the event of 'tag'"},
-  {"eventunsub",AFB_SESSION_NONE, eventunsub  , "unsubscribes to the event of 'tag'"},
-  {"eventpush", AFB_SESSION_NONE, eventpush   , "pushs the event of 'tag' with the 'data'"},
-  {"exit",      AFB_SESSION_NONE, exitnow     , "exits from afb-daemon"},
-  {NULL}
+static const struct afb_verb_v2 verbs[]= {
+  { "ping"     ,    pingSample , NULL, AFB_SESSION_NONE },
+  { "pingfail" ,    pingFail   , NULL, AFB_SESSION_NONE },
+  { "pingnull" ,    pingNull   , NULL, AFB_SESSION_NONE },
+  { "pingbug"  ,    pingBug    , NULL, AFB_SESSION_NONE },
+  { "pingJson" ,    pingJson   , NULL, AFB_SESSION_NONE },
+  { "pingevent",    pingEvent  , NULL, AFB_SESSION_NONE },
+  { "subcall",      subcall    , NULL, AFB_SESSION_NONE },
+  { "subcallsync",  subcallsync, NULL, AFB_SESSION_NONE },
+  { "eventadd",     eventadd   , NULL, AFB_SESSION_NONE },
+  { "eventdel",     eventdel   , NULL, AFB_SESSION_NONE },
+  { "eventsub",     eventsub   , NULL, AFB_SESSION_NONE },
+  { "eventunsub",   eventunsub , NULL, AFB_SESSION_NONE },
+  { "eventpush",    eventpush  , NULL, AFB_SESSION_NONE },
+  { "exit",         exitnow    , NULL, AFB_SESSION_NONE },
+  { NULL}
 };

-static const struct afb_binding plugin_desc = {
-    .type = AFB_BINDING_VERSION_1,
-    .v1 = {
-        .info = "Minimal Hello World Sample",
-        .prefix = "hello",
-        .verbs = verbs
-    }
+const struct afb_binding_v2 afbBindingV2 = {
+    .api = "hello",
+    .specification = NULL,
+    .verbs = verbs,
+    .preinit = preinit,
+    .init = init
 };

-const struct afb_binding *afbBindingV1Register (const struct afb_binding_interface *itf)
-{
-    interface = itf;
-    NOTICE(interface, "hello plugin comes to live");
-    return &plugin_desc;
-}