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ARCHITECTURE.md

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Architecture

Eventador

This struct wraps the Arc<RingBuffer> and offers public sync/async APIs to publish and subscribe to events.

RingBuffer

This is the core data structure that stores events. It contains a Sequencer that atomically assigns a publishable event to a slot in the ring. Each event slot is an EventWrapper.

Sequencer

The Sequencer has a monotonically increasing count of the number of events that have been published thus far. A publisher must complete a challenge to acquire write-access to an EventWrapper, and this challenge is determined by both the declared WaitStrategy and a CAS loop.

The sequence number assigned to the publisher is the sequence number for the event, and is also mapped to a specific location on the ring.

EventWrapper

This is a type-alias for CachePadded<Arc<EventEnvelope>>.

EventEnvelope

This structure tracks the event sequence number, and the actual event message. The envelope atomically replaces the event message inside on publishing, and updates the sequence number accordingly.

This structure also tracks the number of subscribers that are waiting to read the next event to be written, and their wake-up handles.

Subscriber

This structure has an internal Sequence counter, which is atomically incremented after it reads an event. The sequence number of the subscriber indicates what event it needs to read next.

The subscriber registers its handle with the EventEnvelope to wake it again when the event becomes readable.

An event is readable if the internal sequence number of the subscriber matches the internal sequence number of the event envelope. An event will be ignored if it is not the same type that the subscriber is subscribed to.

All subscribers walk the entirety of the ring at this point in time, though this can be optimized.

Publish

Publishing an event involves:

  1. Successfully completing the posed challenge from the Sequencer
  2. Overwriting the event in the envelope with the new one
  3. Updating the envelope's sequence number to be the same as the event's
  4. Waking all waiting subscribers

WaitStrategy

These are policies that enable the Sequencer to behave in different ways when subscribers are lagging behind publishers. As there is a bounded number of EventWrappers in the ring, the user decides how and when a publisher can overwrite an event that has not yet been read by all subscribers.