Events and parameters

When a Workflow is triggered, it can receive an optional event. This event can include data that your Workflow can act on, including request details, user data fetched from your database (such as D1 or KV) or from a webhook, or messages from a Queue consumer.

Events are a powerful part of a Workflow, as you often want a Workflow to act on data. Because a given Workflow instance executes durably, events are a useful way to provide a Workflow with data that should be immutable (not changing) and/or represents data the Workflow needs to operate on at that point in time.

Pass data to a Workflow

You can pass parameters to a Workflow in three ways:

• As an optional argument to the create method on a Workflow binding when triggering a Workflow from a Worker.
• Via the --params flag when using the wrangler CLI to trigger a Workflow.
• Via the step.waitForEvent API, which allows a Workflow instance to wait for an event (and optional data) to be received while it is running. Workflow instances can be sent events from external services over HTTP or via the Workers API for Workflows.

You can pass any JSON-serializable object as a parameter.

Warning

A WorkflowEvent and its associated payload property are effectively immutable: any changes to an event are not persisted across the steps of a Workflow. This includes both cases when a Workflow is progressing normally, and in cases where a Workflow has to be restarted due to a failure.

Store state durably by returning it from your step.do callbacks.

JavaScript

export default {
  async fetch(req, env) {
    let someEvent = { url: req.url, createdTimestamp: Date.now() };
    // Trigger our Workflow
    // Pass our event as the second parameter to the `create` method
    // on our Workflow binding.
    let instance = await env.MY_WORKFLOW.create({
      id: await crypto.randomUUID(),
      params: someEvent,
    });

    return Response.json({
      id: instance.id,
      details: await instance.status(),
    });

    return Response.json({ result });
  },
};

TypeScript

export default {
  async fetch(req: Request, env: Env) {
    let someEvent = { url: req.url, createdTimestamp: Date.now() }
    // Trigger our Workflow
    // Pass our event as the second parameter to the `create` method
    // on our Workflow binding.
    let instance = await env.MY_WORKFLOW.create({
      id: await crypto.randomUUID(),
      params: someEvent
    });

    return Response.json({
      id: instance.id,
      details: await instance.status(),
    });

    return Response.json({ result });
  },
};

To pass parameters via the wrangler command-line interface, pass a JSON string as the second parameter to the workflows trigger sub-command:

npx wrangler@latest workflows trigger workflows-starter '{"some":"data"}'

🚀 Workflow instance "57c7913b-8e1d-4a78-a0dd-dce5a0b7aa30" has been queued successfully

Wait for events

A running Workflow can wait for an event (or events) by calling step.waitForEvent within the Workflow, which allows you to send events to the Workflow in one of two ways:

1. Via the Workers API binding: call instance.sendEvent to send events to specific workflow instances.
2. Using the REST API (HTTP API)'s Events endpoint.

Because waitForEvent is part of the WorkflowStep API, you can call it multiple times within a Workflow, and use control flow to conditionally wait for an event.

Calling waitForEvent requires you to specify an type, which is used to match the corresponding type when sending an event to a Workflow instance.

For example, to wait for billing webhook:

JavaScript

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event, step) {
    // Other steps in your Workflow
    let event = await step.waitForEvent(
      "receive invoice paid webhook from Stripe",
      { type: "stripe-webhook", timeout: "1 hour" },
    );
    // Rest of your Workflow
  }
}

TypeScript

export class MyWorkflow extends WorkflowEntrypoint<Env, Params> {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // Other steps in your Workflow
    let event = await step.waitForEvent<IncomingStripeWebhook>("receive invoice paid webhook from Stripe", { type: "stripe-webhook", timeout: "1 hour" })
    // Rest of your Workflow
  }
}

The above example:

• Calls waitForEvent with a type of stripe-webhook - the corresponding sendEvent call would thus be await instance.sendEvent({type: "stripe-webhook", payload: webhookPayload}).
• Uses a TypeScript type parameter ↗ to type the return value of step.waitForEvent as our IncomingStripeWebhook.
• Continues on with the rest of the Workflow.

Send events to running workflows

Workflow instances that are waiting on events using the waitForEvent API can be sent events using the instance.sendEvent API:

JavaScript

export default {
  async fetch(req, env) {
    const instanceId = new URL(req.url).searchParams.get("instanceId");
    const webhookPayload = await req.json();

    let instance = await env.MY_WORKFLOW.get(instanceId);
    // Send our event, with `type` matching the event type defined in
    // our step.waitForEvent call
    await instance.sendEvent({
      type: "stripe-webhook",
      payload: webhookPayload,
    });

    return Response.json({
      status: await instance.status(),
    });
  },
};

TypeScript

export default {
  async fetch(req: Request, env: Env) {
    const instanceId = new URL(req.url).searchParams.get("instanceId")
    const webhookPayload = await req.json<Payload>()

    let instance = await env.MY_WORKFLOW.get(instanceId);
    // Send our event, with `type` matching the event type defined in
    // our step.waitForEvent call
    await instance.sendEvent({type: "stripe-webhook", payload: webhookPayload})

    return Response.json({
      status: await instance.status(),
    });
  },
};

• Similar to the waitForEvent example in this guide, the type property in our waitForEvent and sendEvent fields must match.
• To send multiple events to a Workflow that has multiple waitForEvent calls, call sendEvent with the corresponding type property set.
• Events can also be sent using the REST API (HTTP API)'s Events endpoint.

TypeScript and type parameters

By default, the WorkflowEvent passed to the run method of your Workflow definition has a type that conforms to the following, with payload (your data), timestamp, and instanceId properties:

export type WorkflowEvent<T> = {
  // The data passed as the parameter when the Workflow instance was triggered
  payload: T;
  // The timestamp that the Workflow was triggered
  timestamp: Date;
  // ID of the current Workflow instance
  instanceId: string;
};

You can optionally type these events by defining your own type and passing it as a type parameter ↗ to the WorkflowEvent:

// Define a type that conforms to the events your Workflow instance is
// instantiated with
interface YourEventType {
  userEmail: string;
  createdTimestamp: number;
  metadata?: Record<string, string>;
}

When you pass your YourEventType to WorkflowEvent as a type parameter, the event.payload property now has the type YourEventType throughout your workflow definition:

src/index.ts

// Import the Workflow definition
import { WorkflowEntrypoint, WorkflowStep, WorkflowEvent} from 'cloudflare:workers';

export class MyWorkflow extends WorkflowEntrypoint {
    // Pass your type as a type parameter to WorkflowEvent
    // The 'payload' property will have the type of your parameter.
    async run(event: WorkflowEvent<YourEventType>, step: WorkflowStep) {
        let state = step.do("my first step", async () => {
          // Access your properties via event.payload
          let userEmail = event.payload.userEmail
          let createdTimestamp = event.payload.createdTimestamp
        })

        step.do("my second step", async () => { /* your code here */ )
    }
}

Warning

Providing a type parameter does not validate that the incoming event matches your type definition. In TypeScript, properties (fields) that do not exist or conform to the type you provided will be dropped. If you need to validate incoming events, we recommend a library such as zod ↗ or your own validator logic.

You can also provide a type parameter to the Workflows type when creating (triggering) a Workflow instance using the create method of the Workers API. Note that this does not propagate type information into the Workflow itself, as TypeScript types are a build-time construct. To provide the type of an incoming WorkflowEvent, refer to the TypeScript and type parameters section of the Workflows documentation.