PubSub

A PubSub serves as an asynchronous message hub, allowing publishers to send messages that can be received by all current subscribers.

Unlike a Queue, where each value is delivered to only one consumer, a PubSub broadcasts each published message to all subscribers. This makes PubSub ideal for scenarios requiring message broadcasting rather than load distribution.

Basic Operations

A PubSub<A> stores messages of type A and provides two fundamental operations:

API	Description
PubSub.publish	Sends a message of type A to the PubSub, returning an effect indicating if the message was successfully published.
PubSub.subscribe	Creates a scoped effect that allows subscription to the PubSub, automatically unsubscribing when the scope ends. Subscribers receive messages through a Dequeue which holds published messages.

Example (Publishing a Message to Multiple Subscribers)

import { Effect, PubSub, Queue } from "effect"

const program = Effect.scoped(
  Effect.gen(function* () {
    const pubsub = yield* PubSub.bounded<string>(2)

    // Two subscribers
    const dequeue1 = yield* PubSub.subscribe(pubsub)
    const dequeue2 = yield* PubSub.subscribe(pubsub)

    // Publish a message to the pubsub
    yield* PubSub.publish(pubsub, "Hello from a PubSub!")

    // Each subscriber receives the message
    console.log("Subscriber 1: " + (yield* Queue.take(dequeue1)))
    console.log("Subscriber 2: " + (yield* Queue.take(dequeue2)))
  })
)

Effect.runFork(program)
/*
Output:
Subscriber 1: Hello from a PubSub!
Subscriber 2: Hello from a PubSub!
*/

Subscribe Before Publishing

A subscriber only receives messages published while it is actively subscribed. To ensure a subscriber receives a particular message, establish the subscription before publishing the message.

Creating a PubSub

Bounded PubSub

A bounded PubSub applies back pressure to publishers when it reaches capacity, suspending additional publishing until space becomes available.

Back pressure ensures that all subscribers receive all messages while they are subscribed. However, it can lead to slower message delivery if a subscriber is slow.

Example (Bounded PubSub Creation)

import { PubSub } from "effect"

// Creates a bounded PubSub with a capacity of 2
const boundedPubSub = PubSub.bounded<string>(2)

Dropping PubSub

A dropping PubSub discards new values when full. The PubSub.publish operation returns false if the message is dropped.

In a dropping pubsub, publishers can continue to publish new values, but subscribers are not guaranteed to receive all messages.

Example (Dropping PubSub Creation)

import { PubSub } from "effect"

// Creates a dropping PubSub with a capacity of 2
const droppingPubSub = PubSub.dropping<string>(2)

Sliding PubSub

A sliding PubSub removes the oldest message to make space for new ones, ensuring that publishing never blocks.

A sliding pubsub prevents slow subscribers from impacting the message delivery rate. However, there’s still a risk that slow subscribers may miss some messages.

Example (Sliding PubSub Creation)

import { PubSub } from "effect"

// Creates a sliding PubSub with a capacity of 2
const slidingPubSub = PubSub.sliding<string>(2)

Unbounded PubSub

An unbounded PubSub has no capacity limit, so publishing always succeeds immediately.

Unbounded pubsubs guarantee that all subscribers receive all messages without slowing down message delivery. However, they can grow indefinitely if messages are published faster than they are consumed.

Generally, it’s recommended to use bounded, dropping, or sliding pubsubs unless you have specific use cases for unbounded pubsubs.

Example

import { PubSub } from "effect"

// Creates an unbounded PubSub with unlimited capacity
const unboundedPubSub = PubSub.unbounded<string>()

Operators On PubSubs

publishAll

The PubSub.publishAll function lets you publish multiple values to the pubsub at once.

Example (Publishing Multiple Messages)

import { Effect, PubSub, Queue } from "effect"

const program = Effect.scoped(
  Effect.gen(function* () {
    const pubsub = yield* PubSub.bounded<string>(2)
    const dequeue = yield* PubSub.subscribe(pubsub)
    yield* PubSub.publishAll(pubsub, ["Message 1", "Message 2"])
    console.log(yield* Queue.takeAll(dequeue))
  })
)

Effect.runFork(program)
/*
Output:
{ _id: 'Chunk', values: [ 'Message 1', 'Message 2' ] }
*/

capacity / size

You can check the capacity and current size of a pubsub using PubSub.capacity and PubSub.size, respectively.

Note that PubSub.capacity returns a number because the capacity is set at pubsub creation and never changes. In contrast, PubSub.size returns an effect that determines the current size of the pubsub since the number of messages in the pubsub can change over time.

Example (Retrieving PubSub Capacity and Size)

import { Effect, PubSub } from "effect"

const program = Effect.gen(function* () {
  const pubsub = yield* PubSub.bounded<number>(2)
  console.log(`capacity: ${PubSub.capacity(pubsub)}`)
  console.log(`size: ${yield* PubSub.size(pubsub)}`)
})

Effect.runFork(program)
/*
Output:
capacity: 2
size: 0
*/

Shutting Down a PubSub

To shut down a pubsub, use PubSub.shutdown. You can also verify if it has been shut down with PubSub.isShutdown, or wait for the shutdown to complete with PubSub.awaitShutdown. Shutting down a pubsub also terminates all associated queues, ensuring that the shutdown signal is effectively communicated.

PubSub as an Enqueue

PubSub operators mirror those of Queue with the main difference being that PubSub.publish and PubSub.subscribe are used in place of Queue.offer and Queue.take. If you’re already familiar with using a Queue, you’ll find PubSub straightforward.

Essentially, a PubSub can be seen as a Enqueue that only allows writes:

import type { Queue } from "effect"

interface PubSub<A> extends Queue.Enqueue<A> {}

Here, the Enqueue type refers to a queue that only accepts enqueues (or writes). Any value enqueued here is published to the pubsub, and operations like shutdown will also affect the pubsub.

This design makes PubSub highly flexible, letting you use it anywhere you need a Enqueue that only accepts published values.