1 # Guide for developing with events
3 Signaling agents are services that send events to any clients that
4 are subscribed to receive it.
5 The sent events carry any data.
7 To have a good understanding of how to:
9 - write a signaling agent.
10 - actions of subscribing.
11 - actions of unsubscribing.
12 - actions of producing.
13 - actions of sending and receiving.
15 Events must be described and explained.
19 The basis of a signaling agent is shown in the following figure:
21 ![scenario of using events](pictures/signaling-basis.svg)
23 This figure shows the main role of the signaling framework for the events
26 For people not familiar with the framework, a signaling agent and
27 a “binding” are similar.
29 ### Subscribing and unsubscribing
31 - Subscribing is the action that makes a client able to receive
32 data from a signaling agent.
36 1. Create resources for generating the data.
37 1. Deliver the data to the client.
39 These two aspects are not handled by the same piece of software.
41 1. Generating the data is the responsibility of the developer of the signaling agent
42 1. Delivering the data is handled by the framework.
44 When a client subscribes for data, the agent must:
46 1. Check that the subscription request is correct.
47 1. Establish the computation chain of the required data (if not already done).
48 1. Create a named event for the computed data (if not already done).
49 1. Ask the framework to establish the subscription to the event for the request.
50 1. Optionally give indications about the event in the reply to the client.
52 The first two steps do not involve the framework.
53 They are linked to the business logic of the binding.
54 The request can be any description of the requested data
55 and the computing stream can be of any nature,
56 this is specific to the binding.
58 As said before, the framework uses and integrates **libsystemd** and its event
60 Within the framework, **libsystemd** is the standard API/library for
61 bindings expecting to setup and handle I/O, timer or signal events.
63 Steps 3 and 4 are bound to the framework.
65 The agent must create an object for handling the propagation of produced
67 That object is called “event” in the framework.
68 An event has a name that allows clients to distinguish it from other
71 Events are created using the ***afb\_api\_make\_event*** function
72 that takes the api that creates the event and the name of the event.
76 event = afb_api_make_event(api, name);
79 Once created, the event can be used either to push data to its
80 subscribers or to broadcast data to any listener.
82 The event must be used to establish the subscription for the requesting
84 This is done using the ***afb\_req\_subscribe*** function
85 that takes the current request object and event and associates them
90 rc = afb_req_subscribe(req, event);
93 When successful, this function make the connection between the event and
94 the client that emitted the request.
95 The client becomes a subscriber of the event until it unsubscribes or disconnects.
96 The ***afb\_req\_subscribe*** function will fail:
98 - if the client connection is weak.
99 - if the request comes from a HTTP link.
101 To receive signals, the client must be connected.
103 The AGL framework allows connections using WebSocket.
105 The name of the event is either a well known name or an ad hoc name
106 forged for the use case.
108 Let's see a basic example:
110 - client A expects to receive the speed in km/h every second.
111 - client B expects the speed in mph twice a second.
113 In that case, there are two different events because it is not the same
114 unit and it is not the same frequency.
115 Having two different events allows to associate clients to the correct event.
116 But this doesn't tell any word about the name of these events.
117 The designer of the signaling agent has two options for naming:
119 1. names can be the same (“speed” for example) with sent data
120 self describing itself or having a specific tag (requiring from
121 clients awareness about requesting both kinds of speed isn't safe).
122 1. names of the event include the variations (by example:
123 “speed-km/h-1Hz” and “speed-mph-2Hz”) and, in that case, sent data
124 can self describe itself or not.
126 In both cases, the signaling agent might have to send the name of the
127 event and/or an associated tag to its client in the reply of the
129 This is part of the step 5 above.
131 The framework only uses the event (not its name) for:
137 When the requested data is already generated and the event used for
138 pushing it already exists, the signaling agent must not instantiate a
139 new processing chain and must not create a new event object for pushing
141 The signaling agent must reuse the existing chain and event.
143 Unsubscribing is made by the signaling agent on a request of its client.
144 The ***afb\_req\_unsubscribe*** function tells the framework to
145 remove the requesting client from the event's list of subscribers.
149 afb_req_unsubscribe(req, event);
152 Subscription count does not matter to the framework:
154 - Subscribing the same client several times has the same effect that subscribing only one time.
156 Thus, when unsubscribing is invoked, it becomes immediately effective.
158 #### More on naming events
160 - Within the AGL framework, a signaling agent is a binding that has an API prefix.
162 This prefix is meant to be unique and to identify the binding API.
163 The names of the events that this signaling agent creates are
164 automatically prefixed by the framework, using the API prefix of the
167 Thus, if a signaling agent of API prefix ***api*** creates an event
168 of name ***event*** and pushes data to that event, the subscribers
169 will receive an event of name ***api/event***.
171 ### Generating and pushing signals and data
173 - This of the responsibility of the designer of the signaling agent to establish the processing chain for generating events.
175 In many cases, this can be achieved using I/O or timer or signal events inserted in the main loop.
176 For this case, the AGL framework uses **libsystemd** and
177 provide a way to integrates to the main loop of this library using
178 afb\_api\_get\_event\_loop.
182 sdev = afb_api_get_event_loop(api);
183 rc = sd_event_add_io(sdev, &source, fd, EPOLLIN, myfunction, NULL);
186 In some other cases, the events are coming from D-Bus.
187 In that case, the framework also uses **libsystemd** internally to access D-Bus.
188 It provides two methods to get the available D-Bus objects, already existing and
189 bound to the main **libsystemd** event loop.
190 Use either ***afb\_api\_get\_system\_bus*** or
191 ***afb\_api\_get\_user\_bus*** to get the required instance.
192 Then use functions of **libsystemd** to handle D-Bus.
194 In some rare cases, the generation of the data requires to start a new
197 When a data is generated and ready to be pushed, the signaling agent
198 should call the function ***afb\_event\_push***.
202 rc = afb_event_push(event, JSON);
204 stop_generating(event);
205 afb_event_unref(event);
209 The function ***afb\_event\_push*** pushes json data to all the subscribers.
210 It then returns the count of subscribers.
211 When the count is zero, there is no subscriber listening for the event.
212 The example above shows that in that case, the signaling agent stops to
213 generate data for the event and tells that it doesn't use it anymore by calling
214 **afb\_event\_unref**.
216 This is one possible option.
217 Other valuable options are:
219 - do nothing and continue to generate and push the event.
220 - just stop to generate and push the data but keep the event existing.
222 ### Receiving the signals
224 Understanding what a client expects when it receives signals, events or
225 data shall be the most important topic of the designer of a signaling
227 The good point here is that because JSON[^1] is the exchange
228 format, structured data can be sent in a flexible way.
230 The good design is to allow as much as possible the client to describe
231 what is needed with the goal to optimize the processing to the
234 ### The exceptional case of wide broadcast
236 Some data or events have so much importance that they can be widely
237 broadcasted to alert any listening client.
238 Examples of such an alert are:
240 - system is entering/leaving “power safe” mode
241 - system is shutting down
242 - the car starts/stops moving
245 An event can be broadcasted using one of the two following methods:
247 - ***afb\_api\_broadcast\_event***
248 - ***afb\_event\_broadcast***
253 afb_api_broadcast_event(api, name, json);
259 event = afb_api_make_event(api, name);
261 afb_event_broadcast(event, json);
264 As for other events, the name of events broadcasted using
265 ***afb\_api\_broadcast\_event*** are automatically prefixed by
266 the framework with API prefix.
268 ## Reference of functions
270 See the [references for functions of class afb_event](reference-v3/func-event.html)
272 ### Function onevent (field of afbBindingExport)
274 Binding can designate an event handling function using the field **onevent**
275 of the structure **afb_binding_t**.
277 This function is called when an event is broadcasted or when an event that the
278 api subscribed to (through call or subcall mechanism) is pushed.
279 That behavior allows a service to react to an event and do what it is to do if
280 this is relevant for it.
281 (ie: car back camera detects imminent collision and broadcast it, then
282 appropriate service enable parking brake.).
286 The apis functions allow to declare event handling callbacks. These callbacks are
287 called on reception of an event matching a pattern and a receive in more that
288 the event name and its companion JSON data, a user defiend closure and the api
289 that is used to create it.