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# client.notify

The `notify` method notifies workflows that are waiting for a specific event.

Workflows paused at a [`context.waitForEvent`](/workflow/basics/context/waitForEvent) step with the matching `eventId` will be resumed, and the provided `eventData` will be passed back to them.

## Arguments

<ParamField body="eventId" type="string" required>
    The identifier of the event to notify.
</ParamField>

<ParamField body="eventData" type="any" optional>
    Data to deliver to the waiting workflow(s).
    This value will be returned in the `eventData` field of `context.waitForEvent`.
</ParamField>

<ParamField body="workflowRunId" type="string" optional>
    The workflow run ID to notify. When provided, enables **lookback functionality** - the notification will be stored and delivered even if `notify` is called before `waitForEvent`.

    This solves race conditions where notifications might be sent before a workflow reaches its wait step.
</ParamField>

## Response

<ResponseField name="response" type="NotifyResponse[]">
    A list of `NotifyResponse` objects, one for each workflow that was waiting on the event.
    An empty list means no workflow was waiting.

    <Expandable defaultOpen>
        <ResponseField name="messageId" type="string">
            The ID of the notification message delivered to the workflow.
            This is unique to every notification.
        </ResponseField>

        <ResponseField name="error" type="string">
            Error message if delivering the notification to this waiter failed.
        </ResponseField>

        <ResponseField name="Waiter" type="object">
  <Expandable>
   	<ResponseField name="url" type="string" required>
      URL to call upon notify
    </ResponseField>
   	<ResponseField name="deadline" type="number" required>
      Unix timestamp for when the wait will time out
    </ResponseField>
   	<ResponseField name="headers" type="Record<string, string[]>" required>
      Headers sent in case of notify
    </ResponseField>
   	<ResponseField name="timeoutUrl" type="string">
      URL to call upon timeout
    </ResponseField>
   	<ResponseField name="timeoutBody" type="unknown">
      Body used in timeout request
    </ResponseField>
   	<ResponseField name="timeoutHeaders" type="Record<string, string[]>">
      Headers sent in case of time out
    </ResponseField>
  </Expandable>
</ResponseField>

    </Expandable>
</ResponseField>

## Usage

### Basic Notification

```javascript
import { Client } from "@upstash/workflow";

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

await client.notify({
  eventId: "my-event-id",
  eventData: "my-data", // data passed to the workflow run
});
```

### Notification with Lookback

To prevent race conditions where a notification might arrive before the workflow reaches `waitForEvent`, you can provide a `workflowRunId`. This enables lookback - the notification will be stored and delivered even if sent before the wait step:

```javascript
import { Client } from "@upstash/workflow";

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

// Notify a specific workflow run with lookback support
await client.notify({
  eventId: "payment-processed",
  eventData: { amount: 100, status: "success" },
  workflowRunId: "wfr_abc123", // Enables lookback for this workflow run
});
```

This is particularly useful when you know which specific workflow run should receive the notification, such as after triggering a workflow and immediately sending it an event.