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Reflex

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reflex /ˈriːflɛks/

  1. an action that is performed without conscious thought as a response to a stimulus. "the system is equipped with ninja-like reflexes"
  2. a thing which is determined by and reproduces the essential features or qualities of something else. "business logic is no more than a reflex of user actions"

Reflex provides an API for building distributed event notification streams.

Note reflex is used in production at Luno, but still undergoing active development, breaking changes are on the way.

Overview

logic := func(ctx context.Context, f fate.Fate, e *reflex.Event) error {
	log.Printf("Consumed event: %#v", e)
	return f.Tempt()
}
consumer := reflex.NewConsumer("logging-consumer", logic)

spec := reflex.NewSpec(streamFunc, cursorStore, consumer)

for {
	err := reflex.Run(context.Background(), spec)
	log.Printf("Consume error: %v", err)
}

Consumer encapsulates the business logic triggered on each event. It has of a name used for cursor persistence and metrics.

StreamFunc is the event source providing event streams from an offset (cursor).

CursorStore provides cursor persistence; GetCursor on start and SetCursor on successfully consumed events.

Spec combines all three above elements required to stream and consume reflex events. It is passed to reflex.Run which streams events from the source to the consumer updating the cursor on success.

See StreamFunc and CursorStore implementations below.

Characteristics

Events are primarily state change notifications

  • Event.ID is the unique event identifier. It is used as the cursor.
  • Event.Type is an enum indicating the type of state change; eg. UserCreated, TradeCompleted,EmailUpdated.
  • Event.ForeignID points to the associated (mutable) entity.
  • Event.Timestamp provides the time the event occurred.
  • Event.Metadata provides custom unstructured event metadata.
  • Event persistence implementations must provide exactly-once or at-least-once semantics.
  • It is not designed as "Event Sourcing" but rather "Event Notification" see this video.

The event store providing the StreamFunc API is inspired by Kafka partitions

  • It stores events as immutable ordered log.
  • It must be queryable by offset (cursor, event ID).
  • It must support independent concurrent stream queries with different offsets.
  • It must retain events for reasonable amount of time (days).
  • It should be responsive, new events should stream within milliseconds.

Errors always result in the consumer getting stuck (failing fast)

  • On any error, the cursor will not be updated and the reflex.Run function will return.
  • In the case of transient errors (network, io, shutdown, etc), merely re-calling Run will succeed (at some point).
  • In the case of logic or data errors, it is up to the user to either fix the bug or catch and ignore the error.

It is designed for micro-services

  • gRPC implementations are provided for StreamFunc.
  • This allows peer-to-peer event streaming without a central event bus.
  • It allows encapsulating events behind a API; #microservices_own_their_own_data

It is composable

  • CursorStore and StreamFunc are decoupled and data source/store agnostic.
  • This results in multiple types of Specs, including:
    • rsql CursorStore with gRPC StreamFunc (remove service events and local mysql cursors)
    • rsql CursorStore and rsql StreamFunc (local mysql events and cursors)
    • remote CursorStore with gRPC StreamFunc (remove service events and remote cursors)

Sources and implementations

The github.com/luno/reflex package provides the main framework API with types and interfaces.

The github.com/luno/reflex/rpatterns package provides patterns for common reflex use-cases.

The following packages provide reflex.StreamFunc event stream source implementations:

The following packages provide reflex.CursorStore cursor store implementations: