Instant-relay NPM

instant-relay

instant-relay is a library of primitives for backpressure-aware, intra-process asynchronous communication. It provides foundational building blocks that may be combined to form any desired topology.

How to use

instant-relay exports three primitive classes: BusToOne, BusToMany and Subscriber.

import { BusToOne, ButToMany, Subscriber } from 'instant-relay';

The `Bus` abstract class

A bus is a strongly-typed communication channel which multiple consumers may subscribe to and which multiple producers may publish to. Each bus is typed according to the kinds of messages that may be published to it and to the kind of values that may be returned by subscribers as a result of processing each message.

All bus classes extends the abstract Bus class. Bus instances are backpressure-aware; the Bus.prototype.publish() method returns a promise that resolves when it is safe to publish additional messages.

Buses may be terminated via the Bus.prototype.destroy() method.

The `BusToOne` class

The BusToOne class implements a bus that dispatches published messages to only one of its subscribers, selected according to the specified strategy.

The promise returned by the BusToOne.prototype.publish() method resolves to the value returned by the (one) subscriber that processed the message.

const number_bus = new BusToOne<number, string>();
const as_string: string = await number_bus.publish(Math.random());

Subscriber selector functions can be passed to the BusToOne constructor via the selector option:

const selector: BusToOne.Selector = { pick: subs => subs[0] };
new BusToOne<number, string>({ selector });

Available strategies:

default	class	description
	`BusToOne.FirstSelector`	returns the first subscriber in the array
X	`BusToOne.RoundRobinSelector`	cycles through subscriber
	`BusToOne.RandomSelector`	returns a randomly-selected subscriber
	`BusToOne.LowestLagSelector`	returns the subscriber with the lowest lag

The concurrency constructor option can be used to set the max. amount of concurrent dispatches to subscribers.

The `BusToMany` class

The BusToMany class implements a bus that dispatches published messages to some or all of its subscribers, selected according to the specified strategy.

The promised returned by the BusToMany.prototype.publish() method resolves to an array of all the values returned by the (multiple) subscribers that processed the message.

const number_bus = new BusToOne<number, string>();
const as_string: string[] = await number_bus.publish(Math.random());

Subscriber selector functions can be passed to the BusToMany constructor via the selector option:

const selector: BusToMany.Selector = { pick: subs => subs.slice(1) };
new BusToMany<number, string>({ selector });

Available strategies:

default	class	description
X	`BusToMany.AllSelector`	returns all subscribers in the array

The concurrency constructor option can be used to set the max. amount of concurrent dispatches to subscribers.

The `Subscriber` class

The Subscriber class may be used to create subscribers to one or more Bus instances out of a single handler function. The TS compiler will automatically infer the type of the message passed to the handler function based on the provided Bus instances.

const number_bus = new BusToOne<number>();
const boolean_bus = new BusToOne<boolean>();

const subscriber = Subscriber.create([number_bus, boolean_bus], async (message) => {
  // This will make the TS compiler throw an error
  // as the type of message is `number | boolean`
  // and must be narrowed further before treating
  // it as a number.
  message + 2;
});

Subscribers may be terminated via the Subscriber.prototype.destroy() method.

The concurrency constructor option can be used to set the max. amount of messages being processed at any one time.