PubSub

In this guide, we’ll explore the concept of a PubSub, which is an asynchronous message hub. It allows publishers to send messages to the pubsub, and subscribers can receive those messages.

Unlike a Queue, where each value offered to the queue can be taken by one taker, each value published to a pubsub can be received by all subscribers.

Whereas a Queue represents the optimal solution to the problem of how to distribute values, a PubSub represents the optimal solution to the problem of how to broadcast them.

Basic Operations

The core operations of a PubSub are PubSub.publish and PubSub.subscribe:

• The publish operation sends a message of type A to the pubsub. It returns an effect that indicates whether the message was successfully published.
• The subscribe operation returns a scoped effect that allows you to subscribe to the pubsub. It automatically unsubscribes when the scope is closed. Within the scope, you gain access to a Dequeue, which is essentially a Queue for dequeuing messages published to the pubsub.

Let’s look at an example to understand how to use a pubsub:

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

const program = PubSub.bounded<string>(2).pipe(
  Effect.andThen((pubsub) =>
    Effect.scoped(
      Effect.gen(function* () {
        const dequeue1 = yield* PubSub.subscribe(pubsub)
        const dequeue2 = yield* PubSub.subscribe(pubsub)
        yield* PubSub.publish(pubsub, "Hello from a PubSub!")
        yield* Queue.take(dequeue1).pipe(Effect.andThen(Console.log))
        yield* Queue.take(dequeue2).pipe(Effect.andThen(Console.log))
      })
    )
  )
)

Effect.runPromise(program)
/*
Output:
Hello from a PubSub!
Hello from a PubSub!
*/

It’s important to note that a subscriber will only receive messages published to the pubsub while it’s subscribed. To ensure that a specific message reaches a subscriber, make sure that the subscription has been established before publishing the message.

Creating PubSubs

You can create a pubsub using various constructors provided by the PubSub module:

Bounded PubSub

A bounded pubsub applies back pressure to publishers when it’s at capacity, meaning publishers will block if the pubsub is full.

import { PubSub } from "effect"

const boundedPubSub = PubSub.bounded<string>(2)

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.

Dropping PubSub

A dropping pubsub simply discards values if it’s full. The PubSub.publish function will return false when the pubsub is full.

import { PubSub } from "effect"

const droppingPubSub = PubSub.dropping<string>(2)

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

Sliding PubSub

A sliding pubsub drops the oldest value when it’s full, ensuring that publishing always succeeds immediately.

import { PubSub } from "effect"

const slidingPubSub = PubSub.sliding<string>(2)

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.

Unbounded PubSub

An unbounded pubsub can never be full, and publishing always succeeds immediately.

import { PubSub } from "effect"

const unboundedPubSub = PubSub.unbounded<string>()

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.

Operators On PubSubs

PubSubs support various operations similar to those available on queues.

Publishing Multiple Values

You can use the PubSub.publishAll operator to publish multiple values to the pubsub at once:

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

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

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

Checking Size

You can determine the capacity and current size of the pubsub using PubSub.capacity and PubSub.size:

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

const program = PubSub.bounded<number>(2).pipe(
  Effect.tap((pubsub) =>
    Console.log(`capacity: ${PubSub.capacity(pubsub)}`)
  ),
  Effect.tap((pubsub) =>
    PubSub.size(pubsub).pipe(
      Effect.andThen((size) => Console.log(`size: ${size}`))
    )
  )
)

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

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

Shutting Down a PubSub

You can shut down a pubsub using PubSub.shutdown, check if it’s shut down with PubSub.isShutdown, or await its shutdown with PubSub.awaitShutdown. Shutting down a pubsub also shuts down all associated queues, ensuring the proper propagation of the shutdown signal.

PubSub as an Enqueue

As you can see, the operators on PubSub are identical to the ones on Queue with the exception of PubSub.publish and PubSub.subscribe replacing Queue.offer and Queue.take. So if you know how to use a Queue, you already know how to use a PubSub.

In fact, a PubSub can be viewed as a Queue that can only be written to:

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

Here, the Enqueue type represents a queue that can only be enqueued. Enqueuing to the queue publishes a value to the pubsub, and actions like shutting down the queue also shut down the pubsub.

This versatility allows you to use a PubSub wherever you currently use a Queue that you only write to.