csw.event.client

== Event Service ==

This module implements an Event Service responsible for publishing an csw.params.events.Event or subscribing to an csw.params.events.Event. An event is published on a csw.params.events.EventKey. The event key is composed of a csw.prefix.models.Prefix depicting the source and an csw.params.events.EventName. The subscriber can subscribe to the csw.params.events.EventKey to receive all events published on the Key.

=== Example: Event Service ===

    val eventServiceFactory          = new EventServiceFactory()
    val eventService: EventService   = eventServiceFactory.make(locationService)

Using above code, you can create instance of csw.event.api.scaladsl.EventService. EventService is the factory to create publishers and subscribers.

You can choose to use defaultPublisher in case you want to share same connection for publishing different events.

=== Example: Event Publisher ===

Event Publisher provides asynchronous APIs to publish one or more Events

    val event = SystemEvent(prefix, EventName("filter_wheel"))

    val publisher    = eventService.defaultPublisher
    publisher.publish(event)

=== Example: Event Subscriber (subscribe and get API) ===

Event Subscriber provides asynchronous APIs to subscribe to one or more Event Keys. It also provides APIs to subscribe using a pattern. In addition, there are APIs to get latest events for one ore more Event Keys without making a subscription

The EventSubscriber provides various API's to subscribe to csw.params.events.EventKey. One of such a subscribe method takes an ActorRef of an arbitrary actor or a callback function to be called when an event matching the given csw.params.events.EventKey is received. In the example below, we only provide the callback argument value. You could also provide an ActorRef of some actor that should receive the Event message.

    def callback(ev: Event): Unit = {
      // ...
    }

    async {
      val subscriber    = eventService.defaultSubscriber
      subscriber.subscribeCallback(Set(EventKey(prefix, EventName("filter_wheel"))), callback)

      val event = await(subscriber.get(event.eventKey))

      event match {
        case e: ObserveEvent => assert(e.prefix == expectedPrefix)
        case _               => fail("Expected ObserveEvent")
      }
   }

csw provides two implementations for Event Service which can be accessed through csw-event-client :

1. RedisEventService - Redis as event store and publisher/subscriber functionality
2. KafkaEventService - Kafka as event store and publisher/subscriber functionality

Complete guide of usage of different API's provided by EventService is available at: https://tmtsoftware.github.io/csw/services/event.html