> ## Documentation Index
> Fetch the complete documentation index at: https://docs.postmetric.io/llms.txt
> Use this file to discover all available pages before exploring further.

# User identification

> Identify users to track authenticated sessions

Identify users to link their sessions and track authenticated user behavior.

## Why identify users?

* **Link sessions** - Connect multiple sessions from the same user
* **Track user journey** - See the complete user journey across sessions
* **Revenue attribution** - Link payments to users via email
* **User analytics** - See behavior by user segment

## How it works

When a user logs in, identify them with PostMetric:

```javascript theme={null}
postMetric("identify", {
  userId: "user_123",
  email: "user@example.com",
  name: "John Doe",
});
```

This links all future sessions to this user.

## Implementation

### When user logs in

```javascript theme={null}
async function handleLogin(email, password) {
  const user = await login(email, password);

  // Identify user with PostMetric
  if (window.postMetric) {
    window.postMetric("identify", {
      userId: user.id,
      email: user.email,
      name: user.name,
    });
  }

  return user;
}
```

### React example

```tsx theme={null}
function LoginButton() {
  const handleLogin = async () => {
    const user = await login();

    if (typeof window !== "undefined" && window.postMetric) {
      window.postMetric("identify", {
        userId: user.id,
        email: user.email,
        name: user.name,
      });
    }
  };

  return <button onClick={handleLogin}>Login</button>;
}
```

### Vue example

```vue theme={null}
<script>
export default {
  methods: {
    async handleLogin() {
      const user = await this.login();

      if (window.postMetric) {
        window.postMetric("identify", {
          userId: user.id,
          email: user.email,
          name: user.name,
        });
      }
    },
  },
};
</script>
```

## Server-side identification

You can also identify users server-side using the API:

```bash theme={null}
curl -X POST https://your-domain.com/api/identify \
  -H "Content-Type: application/json" \
  -d '{
    "site": "YOUR_TRACKING_CODE",
    "userId": "user_123",
    "email": "user@example.com",
    "name": "John Doe",
    "visitorId": "visitor_456",
    "sessionId": "session_789"
  }'
```

## Revenue attribution

When user identification is enabled, payments can be linked to users via email matching:

1. User is identified with email `user@example.com`
2. Payment is made with customer email `user@example.com`
3. Payment is automatically linked to the user

## View user data

Once users are identified, you can:

* See user journey across sessions
* Track user lifetime value
* Analyze behavior by user segment
* Link revenue to specific users

## Privacy

User identification is optional. You can use PostMetric without identifying users. When users are identified, their data is stored securely and only accessible to you.

## Next steps

<Card title="Connect payment providers" icon="credit-card" href="/get-started/connect-payment-providers">
  Learn how to link revenue to identified users
</Card>