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.

To have a good understanding of how to:

  • 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:

scenario of using events

This figure shows the main role of the signaling framework for the events propagation.

For people not familiar with the framework, a signaling agent and a “binding” are similar.

Subscribing and unsubscribing

  • 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.
  2. 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
  2. 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.

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 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 events.

Events are created using the afb_daemon_make_event function that takes the name of the event.
Example:

    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 that takes the current request object and event and associates them together.
Example:

    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 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.
  • 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.

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.

The framework only uses the event (not its name) for:

  • 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.

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.
Example:

    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.

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 automatically prefixed by the framework, using the API prefix of the binding.

Thus, if a signaling agent of API prefix api creates an event of name event and pushes data to that event, the subscribers will receive an event of name api/event.

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 provide a way to integrates to the main loop of this library using afb_daemon_get_event_loop.
Example:

    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 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:

    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.
  • 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 format, structured data can be sent in a flexible way.

The good design is to allow as much as possible the client to describe what is needed with the goal to optimize the processing to the requirements only.

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:

  • 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
  • afb_event_broadcast

Example 1:

afb_daemon_broadcast_event(name, json);

Example 2:

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

Function afb_event afb_daemon_make_event

The function afb_daemon_make_event that is defined as below:

/*
 * 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:

/*
 * 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:

/*
 * 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:

/*
 * 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

The function afb_req_unsubscribe is defined as below:

/*
 * 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);

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:

/*
 * 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:

/*
 * 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);

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.
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.).