-Guide for developing with events
-================================
+# Guide for developing with events
Signaling agents are services that send events to any clients that
-subscribed for receiving it. The sent events carry any data.
+subscribed for receiving it.
+The sent events carry any data.
-To have a good understanding of how to write a signaling agent, the
-actions of subscribing, unsubscribing, producing, sending and receiving
-events must be described and explained.
+To have a good understanding of how to:
-Overview of events
-------------------
+- write a signaling agent.
+- actions of subscribing.
+- actions of unsubscribing.
+- actions of producing.
+- actions of sending and receiving.
+
+Events must be described and explained.
+
+## Overview of events
The basis of a signaling agent is shown in the following figure:
### Subscribing and unsubscribing
-Subscribing is the action that makes a client able to receive data from a
-signaling agent. Subscription must create resources for generating the data, and
-for delivering the data to the client. These two aspects are not handled by the
-same piece of software. Generating the data is the responsibility of the
-developer of the signaling agent while delivering the data is handled by the
-framework.
+- Subscribing is the action that makes a client able to receive
+ data from a signaling agent.
+
+Subscription must :
+
+1. Create resources for generating the data.
+1. Delivering the data to the client.
+
+These two aspects are not handled by the same piece of software.
+
+1. Generating the data is the responsibility of the developer of the signaling agent
+1. Delivering the data is handled by the framework.
When a client subscribes for data, the agent must:
-1. check that the subscription request is correct;
-2. establish the computation chain of the required data, if not already
- done;
-3. create a named event for the computed data, if not already done;
-4. ask the framework to establish the subscription to the event for the
- request;
-5. optionally give indications about the event in the reply to
- the client.
+1. check that the subscription request is correct.
+1. establish the computation chain of the required data (if not already done).
+1. create a named event for the computed data (if not already done).
+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, this
-is specific to the binding.
+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. Within the framework, **libsystemd** is the standard API/library for
+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. An
-event has a name that allows clients to distinguish it from other
+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. Example:
+that takes the name of the event.
+Example:
```C
- event = afb_daemon_make_event(name);
+ event = afb_daemon_make_event(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. This is done using the ***afb\_req\_subscribe*** function
+client.
+This is done using the ***afb\_req\_subscribe*** function
that takes the current request object and event and associates them
-together. Example:
+together.
+Example:
```C
- rc = afb_req_subscribe(req, event);
+ rc = afb_req_subscribe(req, event);
```
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
-***afb\_req\_subscribe*** function will fail if the client
-connection is weak: if the request comes from a HTTP link. To receive
-signals, the client must be connected. The AGL framework allows connections
-using WebSocket.
+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.
+- if the request comes from a HTTP link.
+
+To receive signals, the client must be connected.
+
+The AGL framework allows connections using WebSocket.
The name of the event is either a well known name or an ad hoc name
forged for the use case.
-Let's see a basic example: client A expects to receive the speed in km/h
-every second while 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. The designer of the signaling
-agent has two options for naming:
-
-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).
-2. names of the event include the variations (by example:
- “speed-km/h-1Hz” and “speed-mph-2Hz”) and, in that case, sent data
- can self describe itself or not.
+Let's see a basic example:
+
+- client A expects to receive the speed in km/h every second.
+- 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.
+The designer of the signaling agent has two options for naming:
+
+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:
+ “speed-km/h-1Hz” and “speed-mph-2Hz”) and, in that case, sent data
+ can self describe itself or not.
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. This is part of the step 5 above.
+subscription.
+This is part of the step 5 above.
+
+The framework only uses the event (not its name) for:
-The framework only uses the event (not its name) for subscription,
-un-subscription and pushing.
+- subscription
+- un-subscription
+- pushing
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. The signaling agent must reuse the existing chain and event.
+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
- afb_req_unsubscribe(req, event);
+ afb_req_unsubscribe(req, event);
```
-Subscription count does not matter to the framework: subscribing the
-same client several times has the same effect that subscribing only one
-time. Thus, when unsubscribing is invoked, it becomes immediately
-effective.
+Subscription count does not matter to the framework:
+
+- Subscribing the same client several times has the same effect that subscribing only one time.
+
+Thus, when unsubscribing is invoked, it becomes immediately effective.
#### More on naming events
-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. The names of the events that this signaling agent creates are
+- 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.
+The names of the events that this signaling agent creates are
automatically prefixed by the framework, using the API prefix of the
binding.
### Generating and pushing signals and data
-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. For this case, the AGL framework uses **libsystemd** and
+- 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.
+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. Example:
+afb\_daemon\_get\_event\_loop.
+Example:
```C
- sdev = afb_daemon_get_event_loop();
- rc = sd_event_add_io(sdev, &source, fd, EPOLLIN, myfunction, NULL);
+ sdev = afb_daemon_get_event_loop();
+ 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. 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. Then
-use functions of **libsystemd** to handle 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.
+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***. Example:
+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);
- }
+ rc = afb_event_push(event, JSON);
+ if (rc == 0) {
+ stop_generating(event);
+ afb_event_drop(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: do nothing and continue to
-generate and push the event or just stop to generate and push the data
-but keep the event existing.
+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:
+
+- do nothing and continue to generate and push the event.
+- just stop to generate and push the data but keep the event existing.
### Receiving the signals
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. The good point here is that because JSON[^1] is the exchange
+agent.
+The good point here is that because JSON[^1] is the exchange
format, structured data can be sent in a flexible way.
The good design is to allow as much as possible the client to describe
### 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. Examples of such an alert
-are:
+broadcasted to alert any listening client.
+Examples of such an alert are:
-- system is entering/leaving “power safe” mode
-- system is shutting down
-- the car starts/stops moving
-- ...
+- system is entering/leaving “power safe” mode
+- system is shutting down
+- the car starts/stops moving
+- ...
An event can be broadcasted using one of the two following methods:
-***afb\_daemon\_broadcast\_event*** or
-***afb\_event\_broadcast***.
+
+- ***afb\_daemon\_broadcast\_event***
+- ***afb\_event\_broadcast***
Example 1:
```C
- afb_daemon_broadcast_event(name, json);
+afb_daemon_broadcast_event(name, json);
```
Example 2:
```C
- event = afb_daemon_make_event(name);
- . . . .
- afb_event_broadcast(event, json);
+event = afb_daemon_make_event(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).
-Reference of functions
-----------------------
+## Reference of functions
### Function afb\_event afb\_daemon\_make\_event
int afb_req_unsubscribe(struct afb_req req, struct afb_event event);
```
-The un-subscription removes the client of the request of the list of subscribers
-to the event.
+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.
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:
int afb_daemon_broadcast_event(const char *name, struct json_object *object);
```
-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".
+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".
### Function onevent (field of afbBindingV2)
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.
+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.).
+relevant for it.
+(ie: car back camera detects imminent collision and broadcast it, then
+appropriate service enable parking brake.).
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