State Machine

In Streamer, the subscribers by subscribing to the specific names on the event broker to get the expected events. Because of the nature of asynchronous communication, the event broker cannot guarantee the events that the actor subscribes to will arrive in the desired order. Some actors do rely on a specific temporal event sequence to work properly.

Why State Machine

Actors in Streamer may need to adapt their processing behavior based on the type of events they receive. A State Machine can represent different processing modes, allowing actors to switch between modes dynamically.

For instance, an actor subscribes to 2 event A and B to perform its tasks. It becomes ready for event B only after receiving and processing event A. Similarly, after processing event B, it returns to a state ready for event A, and this cycle continues.

One approach is to buffer all the events until all the events are received, put them in order, and then send these events to the actor to process. The approach is simple and straightforward, but there are some performance issues.

This isn't an optimal solution because the worker thread has to sit idle until all the events have been received. The time wasted on waiting for all the message to arrive could be spent on processing them instead when partial sequence events have become available.

What Does It Do

The state machine is responsible for controlling the state transitions of the actor. At initialization, the actor registers with the state machine and inform it about the events and order in which they need to be received. The state machine will only forward an event to the actor if it is received in right order or buffer the event otherwise. The actor then notifies the state machine about the next event it is ready to receive.