Telemetry

Starting from factcast version 0.7.9, you can extend your server implementation to listen for internal telemetry events. This can be useful for monitoring and debugging purposes.

The telemetry events are emitted using a dedicated internal Guava EventBus.

Subscription lifecycle events

Currently, the factcast-store module emits an event on each phase of the subscription lifecycle (see org.factcast.store.internal.telemetry.PgStoreTelemetry):

• PgStoreTelemetry.Connect emitted whenever a client connects to the factcast server
• PgStoreTelemetry.Catchup emitted whenever the subscription catches up to the current state of the store
• PgStoreTelemetry.Follow emitted whenever the subscription started consuming live events
• PgStoreTelemetry.Close emitted whenever the client disconnects from the factcast server
• PgStoreTelemetry.Complete emitted whenever the subscription completed its lifecycle

Each emitted event contains a request, which holds the client’s request details.

How to listen to telemetry events

It boils down to implementing a listener that is able to consume telemetry events, through com.google.common.eventbus.Subscribe annotated methods, and registering it via the PgStoreTelemetry bean.

Here is an example:

import com.google.common.eventbus.Subscribe;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.factcast.store.internal.telemetry.PgStoreTelemetry;

@RequiredArgsConstructor
@Slf4j
public class MyTelemetryListener {

  public MyTelemetryListener(PgStoreTelemetry telemetry) {
    telemetry.register(this);
  }

  @Subscribe
  public void on(PgStoreTelemetry.Connect signal) {
    log.info("FactStreamTelemetry Connect: {}", signal.request());
  }

  @Subscribe
  public void on(PgStoreTelemetry.Close signal) {
    log.info("FactStreamTelemetry Close: {}", signal.request());
  }
}

You can check out the full example in the factcast-example-server-telemetry module. That module contains a simple example of how to listen to each subscription lifecycle event, to log the request details and maintaining a list of following subscriptions, which can be read through the actuator /info endpoint.