> ## Documentation index
> Fetch the complete documentation index at: https://upstash.com/docs/llms.txt
> Search it with GET https://upstash.com/docs/search?q=<query>.
> Use these to discover all available pages before exploring further.

# Notify

You can notify all the workflow runs waiting for a specific event ID.
There are two ways to send a notify request.

## Notify within Workflow

Notifies other workflows waiting for a specific event from within a workflow.

<CodeGroup>
```typescript TypeScript
import { serve } from "@upstash/workflow/nextjs";

export const { POST } = serve<string>(async (context) => {
  const { orderId, processingResult } = context.requestPayload;

  await context.run("process-order", async () => {
    // ...
  })

  const { notifyResponse } = await context.notify(
    "notify-processing-complete",
    `order-${orderId}`,
    {
      orderId,
      status: "completed",
      result: processingResult,
      completedAt: new Date().toISOString()
    }
  );

});
```

</CodeGroup>

## External Notification

You can also notify workflows from external systems using the Workflow Client:

<CodeGroup>
```typescript TypeScript
import { Client } from "@upstash/workflow";

const client = new Client({ token: "<WORKFLOW_TOKEN>" });

await client.notify({
  eventId: "order-completed-123",
  eventData: {
    orderId: "123",
    status: "completed",
    deliveryTime: "2 days",
    trackingNumber: "TRK123456"
  }
});
```

</CodeGroup>

## Lookback Functionality

By default, if you call `notify` before a workflow reaches its `waitForEvent` step, the notification will be lost (race condition). To prevent this, you can provide a `workflowRunId` parameter which enables **lookback** - the notification will be stored and delivered even if sent before the wait step.

This is particularly useful when:
- You trigger a workflow and immediately want to send it an event
- You have concurrent operations where timing is unpredictable
- You want to eliminate race conditions in your event-driven workflows

<CodeGroup>
```typescript TypeScript
import { Client } from "@upstash/workflow";

const client = new Client({ token: "<WORKFLOW_TOKEN>" });

// Trigger a workflow and get its run ID
const { workflowRunId } = await client.trigger({
  url: "https://your-app.com/api/process-order",
  body: { orderId: "123" }
});

// Immediately notify it with lookback enabled
// The notification will be delivered even if the workflow
// hasn't reached waitForEvent yet
await client.notify({
  eventId: "payment-verified",
  eventData: { verified: true, amount: 100 },
  workflowRunId: workflowRunId, // Enables lookback
});
```

</CodeGroup>

The same also applies to `context.notify` by passing the `workflowRunId` as the fourth argument:

<CodeGroup>
```typescript TypeScript
import { serve } from "@upstash/workflow/nextjs";

export const { POST } = serve<string>(async (context) => {
  const { orderId, processingResult } = context.requestPayload;

  await context.run("process-order", async () => {
    // ...
  })

  const { notifyResponse } = await context.notify(
    "notify-processing-complete",
    `order-${orderId}`,
    {
      orderId,
      status: "completed",
      result: processingResult,
      completedAt: new Date().toISOString()
    },
    "targetWorkflowRunId" // Enables lookback
  );

});
```
</CodeGroup>

<Note>
When using lookback with `workflowRunId`, the notification is targeted to a specific workflow run rather than all waiters with that event ID.
</Note>