Event Polling

Event polling is simple, but inefficient mechanism. It should be avoided, if implementation can support push-receive.

PollService configuration

The following options are supported:

• allow_reliable = true - controls, if reliable consumer registration is allowed.
• allow_polling = true - controls, if polling is enabled.
• max_chunk_events = 100 - control maximum number events which can be returned in a single response.

DBPollService supports extra options:

• event_table - custom events table name
• consumer_table - custom consumers table name

Registration for reliable delivery

This registration must be done regardless if reliable consumer is going to poll or receive pushed events.

• registerConsumer(as, component)

  • component - 1 to 16 symbols of upper and lower letters, digits and underscores.

Registration can be repeated on each startup to simplify logic.

PollFace.register(as, ccm, '#evtpoll', endpoint );
// ...
const evtpoll = ccm.iface('#evtpoll');

// All events
evtpoll.registerConsumer(as, 'AllEvents');
evtpoll.registerConsumer(as, 'Security');

Polling call

Usually, polling is done in a loop. The initial value of last_known_id_here should be stored and read from component-specific persistent configuration. The value must advance to the last processed event only if processing is complete and cannot be rolled back by some error or unexpected shutdown.

• pollEvents( as, component, [last_id [, want]] )

  • last_id - last processed event ID, if any
  • want - array of event filters, if any
  • returns array of events

It’s important to note that event processing must be aware of possible repeats!

PollFace.register(as, ccm, '#evtpoll', endpoint );
// ...
const evtpoll = ccm.iface('#evtpoll');
let last_known_id_all_events = read_from_somewhere();
let last_known_id_filtered = read_from_somewhere();

as.loop( (as) => {
    // All events
    evtpoll.pollEvents(as, 'AllEvents', last_known_id_all_events);
    as.add((as, events) => {
        for ( let evt of events ) {
            // do repeat-safe processing here
            last_known_id_all_events = evt.id;
            store_somewhere(last_known_id_all_events);
        }
    });


    // Filtered byte type events
    evtpoll.pollEvents(as, 'Security', last_known_id_filtered, ['USR_ADD', 'USR_MOD', 'USR_LOGIN']);
    as.add((as, events) => {
        // ....
    });
} );