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Publish and handle events in your SST App


Sometimes you might want to return a request to the user right away but run some tasks asynchronously. For example, you might want to process an order but send a confirmation email later.

SST allows you publish and subscribe to various events that occur within your application.

  • When your API gets invoked, emit an event
  • Your API then returns right away
  • A handler will be triggered asynchronously to process the event.

Let's look at it in detail.

Get started

Start by creating a new SST app by running the following command in your terminal.

npx create-sst@latest

The event bus

Your app will already have an EventBus configured. This construct receives the events and routes them to the correct subscribers.

const bus = new EventBus(stack, "bus", {
defaults: {
retries: 10,

This configuration will retry all subscriber errors up to 10 times (with exponential backoff). You can also configure this on a per subscriber basis.

The template also creates an event.ts which creates an event function that can be used to define events.

import { createEventBuilder, ZodValidator } from "sst/node/event-bus";

export const event = createEventBuilder({
bus: "bus",
validator: ZodValidator

Define events

In your application you can define events. This definition provides validation using zod as well as typesafety to subscribers. Here is a todo.created event.

import { event } from "./event";

export const Events = {
Created: event("todo.created", z.object({
id: z.string(),

Define subscriber

Subscribers receive events when they are published. The evt object in the call back will be properly typed.

Create the handler

import { EventHandler } from "sst/node/event-bus";
import { Todo } from "@my-sst-app/core/todo";

export const handler = EventHandler(Todo.Events.Created, async (evt) => {
console.log("Todo created", evt);

Setup subscription

We're exploring ways of eliminating this step.

bus.subscribe("todo.created", {
handler: "packages/functions/src/events/todo-created.handler",

Publish event

You can now publish events and you should see your subscriber should trigger.

export async function create() {
const id = crypto.randomUUID();

// write to database

await Events.Created.publish({