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Events

Publish and handle events in your SST App

Overview​

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@two

The event bus​

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

stacks/MyStack.ts
const bus = new EventBus(stack, "bus", {
  defaults: {
    retries: 10,
  },
});
info

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.

/packages/core/src/event.ts
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.

packages/core/src/todo.ts
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​

packages/functions/src/events/todo-created.ts
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.

stacks/MyStack.ts
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.

packages/core/src/todo.ts
export async function create() {
  const id = crypto.randomUUID();

  // write to database

  await Events.Created.publish({
    id,
  });
}