Flow

A clojure-based standard for representation of logical processes producing multiple values over time.

Goals

• composable. State management required by the protocol must be encapsulated in an immutable value allowing functional composition.
• performant. The protocol must have low overhead and not require post-bootstrap allocations in order to limit GC pressure.
• portable. The browser is a first-class target, the protocol must not rely on threads or other platform-specific features.
• versatile. The protocol must be suitable for both discrete event streaming and continuous propagation of change.

Prior art

Flow shares several goals with various existing streaming protocols, especially reactive streams, and aims to overcome its limitations.

• null handling. Reactive streams is highly opinionated against null, which makes protocol violations common in languages embracing null like clojure. Flow is agnostic about nil, so applicative code doesn't have to guard against it.
• lazy sampling. Reactive streams forces the producer to eagerly sample continuous signals, i.e transfer current state each time it changes with a keep-latest backpressure strategy. Flow's transfer mechanism makes lazy sampling possible without compromising on backpressure.
• graceful shutdown. Reactive streams explicitly forbids post-cancellation communication, thus a producer requiring a non-trivial cleanup procedure has no way to inform the consumer when its resources have been actually released. Flow doesn't make any assumptions about cancelling behavior, which allows for arbitrarily complex graceful shutdown logic.

Specification

Flow defines a protocol to perform a unidirectional transfer of values from a producer to a consumer.

• A flow is a function provided by the producer taking two arguments, a notifier and a terminator. It is called by the consumer to spawn a new instance of the process. It must not block, it must not throw, it must return an iterator.
• A notifier is a zero-argument function provided by the consumer. It is called by the producer each time it becomes ready to transfer a value, at which point it must stop signaling until the transfer is confirmed by the consumer. It must not be called after the terminator has been called. It must not block, it must not throw, its return value should be ignored.
• A terminator is a zero-argument function provided by the consumer. It is called exactly once by the producer when the process instance has no more values to transfer and all of its resources have been released. It must not block, it must not throw, its return value should be ignored.
• An iterator is an object provided by the producer, it must be callable as a zero-argument function and derefable. deref is called by the consumer to trigger the transfer of a value, at which point the producer becomes allowed to signal again. The consumer must not call deref before the producer calls the notifier. deref must not block and must return the transferred value. It may throw an exception to indicate a failure, in this case it must not call the notifier again. The consumer may call the iterator as a zero-argument function to cancel the process instance. The producer must expect this operation to happen at any time including after termination, it must be idempotent, it must not block, it must not throw, its return value should be ignored.