# Guide for developing with events
Signaling agents are services that send events to any clients that
-subscribed for receiving it.
+subscribed for receiving it.
The sent events carry any data.
To have a good understanding of how to:
### Subscribing and unsubscribing
-- Subscribing is the action that makes a client able to receive
+- Subscribing is the action that makes a client able to receive
data from a signaling agent.
Subscription must :
1. ask the framework to establish the subscription to the event for the request.
1. optionally give indications about the event in the reply to the client.
-The first two steps are not involving the framework.
-They are linked to the business logic of the binding.
-The request can be any description of the requested data
-and the computing stream can be of any nature,
+The first two steps are not involving the framework.
+They are linked to the business logic of the binding.
+The request can be any description of the requested data
+and the computing stream can be of any nature,
this is specific to the binding.
As said before, the framework uses and integrates **libsystemd** and its event
-loop.
+loop.
Within the framework, **libsystemd** is the standard API/library for
bindings expecting to setup and handle I/O, timer or signal events.
Steps 3 and 4 are bound to the framework.
The agent must create an object for handling the propagation of produced
-data to its clients.
-That object is called “event” in the framework.
+data to its clients.
+That object is called “event” in the framework.
An event has a name that allows clients to distinguish it from other
events.
-Events are created using the ***afb\_daemon\_make\_event*** function
-that takes the name of the event.
+Events are created using the ***afb\_api\_make\_event*** function
+that takes the api that creates the event and the name of the event.
Example:
```C
- event = afb_daemon_make_event(name);
+ event = afb_api_make_event(api, name);
```
Once created, the event can be used either to push data to its
subscribers or to broadcast data to any listener.
The event must be used to establish the subscription for the requesting
-client.
+client.
This is done using the ***afb\_req\_subscribe*** function
that takes the current request object and event and associates them
-together.
+together.
Example:
```C
```
When successful, this function make the connection between the event and
-the client that emitted the request.
-The client becomes a subscriber of the event until it unsubscribes or disconnects.
+the client that emitted the request.
+The client becomes a subscriber of the event until it unsubscribes or disconnects.
The ***afb\_req\_subscribe*** function will fail:
- if the client connection is weak.
- client B expects the speed in mph twice a second.
In that case, there are two different events because it is not the same
-unit and it is not the same frequency.
-Having two different events allows to associate clients to the correct event.
-But this doesn't tell any word about the name of these events.
+unit and it is not the same frequency.
+Having two different events allows to associate clients to the correct event.
+But this doesn't tell any word about the name of these events.
The designer of the signaling agent has two options for naming:
-1. names can be the same (“speed” for example) with sent data
+1. names can be the same (“speed” for example) with sent data
self describing itself or having a specific tag (requiring from
clients awareness about requesting both kinds of speed isn't safe).
1. names of the event include the variations (by example:
In both cases, the signaling agent might have to send the name of the
event and/or an associated tag to its client in the reply of the
-subscription.
+subscription.
This is part of the step 5 above.
The framework only uses the event (not its name) for:
When the requested data is already generated and the event used for
pushing it already exists, the signaling agent must not instantiate a
new processing chain and must not create a new event object for pushing
-data.
+data.
The signaling agent must reuse the existing chain and event.
-Unsubscribing is made by the signaling agent on a request of its client.
+Unsubscribing is made by the signaling agent on a request of its client.
The ***afb\_req\_unsubscribe*** function tells the framework to
-remove the requesting client from the event's list of subscribers.
+remove the requesting client from the event's list of subscribers.
Example:
```C
Subscription count does not matter to the framework:
-- Subscribing the same client several times has the same effect that subscribing only one time.
+- Subscribing the same client several times has the same effect that subscribing only one time.
Thus, when unsubscribing is invoked, it becomes immediately effective.
- Within the AGL framework, a signaling agent is a binding that has an API prefix.
-This prefix is meant to be unique and to identify the binding API.
+This prefix is meant to be unique and to identify the binding API.
The names of the events that this signaling agent creates are
automatically prefixed by the framework, using the API prefix of the
binding.
- This of the responsibility of the designer of the signaling agent to establish the processing chain for generating events.
-In many cases, this can be achieved using I/O or timer or signal events inserted in the main loop.
+In many cases, this can be achieved using I/O or timer or signal events inserted in the main loop.
For this case, the AGL framework uses **libsystemd** and
provide a way to integrates to the main loop of this library using
-afb\_daemon\_get\_event\_loop.
+afb\_api\_get\_event\_loop.
Example:
```C
- sdev = afb_daemon_get_event_loop();
+ sdev = afb_api_get_event_loop(api);
rc = sd_event_add_io(sdev, &source, fd, EPOLLIN, myfunction, NULL);
```
-In some other cases, the events are coming from D-Bus.
-In that case, the framework also uses **libsystemd** internally to access D-Bus.
+In some other cases, the events are coming from D-Bus.
+In that case, the framework also uses **libsystemd** internally to access D-Bus.
It provides two methods to get the available D-Bus objects, already existing and
-bound to the main **libsystemd** event loop.
-Use either ***afb\_daemon\_get\_system\_bus*** or
-***afb\_daemon\_get\_user\_bus*** to get the required instance.
+bound to the main **libsystemd** event loop.
+Use either ***afb\_api\_get\_system\_bus*** or
+***afb\_api\_get\_user\_bus*** to get the required instance.
Then use functions of **libsystemd** to handle D-Bus.
In some rare cases, the generation of the data requires to start a new
thread.
When a data is generated and ready to be pushed, the signaling agent
-should call the function ***afb\_event\_push***.
+should call the function ***afb\_event\_push***.
Example:
```C
rc = afb_event_push(event, JSON);
if (rc == 0) {
stop_generating(event);
- afb_event_drop(event);
+ afb_event_unref(event);
}
```
-The function ***afb\_event\_push*** pushes json data to all the subscribers.
-It then returns the count of subscribers.
-When the count is zero, there is no subscriber listening for the event.
-The example above shows that in that case, the signaling agent stops to generate data for the event and delete the event using afb\_event\_drop.
-This is one possible option.
-Other valuable options are:
+The function ***afb\_event\_push*** pushes json data to all the subscribers.
+It then returns the count of subscribers.
+When the count is zero, there is no subscriber listening for the event.
+The example above shows that in that case, the signaling agent stops to
+generate data for the event and tells that it doesn't use it anymore by calling
+**afb\_event\_unref**.
+
+This is one possible option.
+Other valuable options are:
- do nothing and continue to generate and push the event.
- just stop to generate and push the data but keep the event existing.
Understanding what a client expects when it receives signals, events or
data shall be the most important topic of the designer of a signaling
-agent.
+agent.
The good point here is that because JSON[^1] is the exchange
format, structured data can be sent in a flexible way.
### The exceptional case of wide broadcast
Some data or events have so much importance that they can be widely
-broadcasted to alert any listening client.
+broadcasted to alert any listening client.
Examples of such an alert are:
- system is entering/leaving “power safe” mode
An event can be broadcasted using one of the two following methods:
-- ***afb\_daemon\_broadcast\_event***
+- ***afb\_api\_broadcast\_event***
- ***afb\_event\_broadcast***
Example 1:
```C
-afb_daemon_broadcast_event(name, json);
+afb_api_broadcast_event(api, name, json);
```
Example 2:
```C
-event = afb_daemon_make_event(name);
+event = afb_api_make_event(api, name);
. . . .
afb_event_broadcast(event, json);
```
As for other events, the name of events broadcasted using
-***afb\_daemon\_broadcast\_event*** are automatically prefixed by
-the framework with API prefix of the binding (signaling agent).
+***afb\_api\_broadcast\_event*** are automatically prefixed by
+the framework with API prefix.
## Reference of functions
-### Function afb\_event afb\_daemon\_make\_event
-
-The function ***afb\_daemon\_make\_event*** that is defined as below:
-
-```C
-/*
- * Creates an event of 'name' and returns it.
- */
-struct afb_event afb_daemon_make_event(const char *name);
-```
-
-The correct way to create the event at initialization is to call the function
-***afb\_daemon\_make\_event*** within the initialization
-function referenced by the field ***init*** of the structure ***afbBindingV2***.
-
-### Function afb\_event\_push
-
-The function ***afb\_event\_push*** is defined as below:
-
-```C
-/*
- * Pushes the 'event' with the data 'object' to its observers.
- * 'object' can be NULL.
- *
- * For convenience, the function calls 'json_object_put' for object'.
- * Thus, in the case where 'object' should remain available after
- * the function returns, the function 'json_object_get' shall be used.
- *
- * Returns the count of clients that received the event.
- */
-int afb_event_push(struct afb_event event, struct json_object *object);
-```
-
-As the function ***afb\_event\_push*** returns 0 when there is no
-more subscriber, a binding can remove such unexpected event using the
-function ***afb\_event\_drop***.
-
-### Function afb\_event\_drop
-
-The function ***afb\_event\_drop*** is defined as below:
-
-```C
-/*
- * Drops the data associated to the event
- * After calling this function, the event
- * MUST NOT BE USED ANYMORE.
- */
-void afb_event_drop(struct afb_event event);
-```
-
-### Function afb\_req\_subscribe
-
-The function ***afb\_req\_subscribe*** is defined as below:
-
-```C
-/*
- * Establishes for the client link identified by 'req' a subscription
- * to the 'event'.
- * Returns 0 in case of successful subscription or -1 in case of error.
- */
-int afb_req_subscribe(struct afb_req req, struct afb_event event);
-```
-
-The subscription adds the client of the request to the list of subscribers
-to the event.
-
-### Function afb\_req\_unsubscribe
+See the [references for functions of class afb_event](reference-v3/func-event.md)
-The function ***afb\_req\_unsubscribe*** is defined as
-below:
-
-```C
-/*
- * Revokes the subscription established to the 'event' for the client
- * link identified by 'req'.
- * Returns 0 in case of successful un-subscription or -1 in case of error.
- */
-int afb_req_unsubscribe(struct afb_req req, struct afb_event event);
-```
+### Function onevent (field of afbBindingExport)
-The un-subscription removes the client of the request of the
-list of subscribers to the event.
-When the list of subscribers to the event becomes empty,
-the function ***afb\_event\_push*** will return zero.
-
-### Function afb\_event\_broadcast
-
-The function ***afb\_event\_broadcast*** is defined as below:
-
-```C
-/*
- * Broadcasts widely the 'event' with the data 'object'.
- * 'object' can be NULL.
- *
- * For convenience, the function calls 'json_object_put' for 'object'.
- * Thus, in the case where 'object' should remain available after
- * the function returns, the function 'json_object_get' shall be used.
- *
- * Returns the count of clients that received the event.
- */
-int afb_event_broadcast(struct afb_event event, struct json_object *object);
-```
-
-This uses an existing event (created with ***afb\_daemon\_make\_event***)
-for broadcasting an event having its name.
-
-### Function afb\_daemon\_broadcast\_event
-
-The function ***afb\_daemon\_broadcast\_event*** is defined as below:
-
-```C
-/*
- * Broadcasts widely the event of 'name' with the data 'object'.
- * 'object' can be NULL.
- *
- * For convenience, the function calls 'json_object_put' for 'object'.
- * Thus, in the case where 'object' should remain available after
- * the function returns, the function 'json_object_get' shall be used.
- *
- * Returns the count of clients that received the event.
- */
-int afb_daemon_broadcast_event(const char *name, struct json_object *object);
-```
+Binding can designate an event handling function using the field **onevent**
+of the structure **afb_binding_t**.
-The name is given here explicitly.
-The name is automatically prefixed with the name of the binding.
-For example, a binding of prefix "xxx" would broadcast the event "xxx/name".
+This function is called when an event is broadcasted or when an event that the
+api subscribed to (through call or subcall mechanism) is pushed.
+That behavior allows a service to react to an event and do what it is to do if
+this is relevant for it.
+(ie: car back camera detects imminent collision and broadcast it, then
+appropriate service enable parking brake.).
-### Function onevent (field of afbBindingV2)
+### Event handlers
-Binding can designate an event handling function using the field **onevent**
-of the structure **afbBindingV2**.
-This function is called when an event is broadcasted or when an event the
-binding subscribed to is pushed.
-That allow a service to react to an event and do what it is to do if this is
-relevant for it.
-(ie: car back camera detects imminent collision and broadcast it, then
-appropriate service enable parking brake.).
+The apis functions allow to declare event handling callbacks. These callback are
+called on reception of an event matching a pattern and a receive in more that
+the event name and its companion JSON data, a user defiend closure and the api
+that is used to create it.
Code Review / src / app-framework-binder.git / blobdiff