# Session

Learn about event sessions.

A session represents a period of continuous user activity, capturing a chronological view of events as a user navigates through your applications over time.

These are the properties common to all sessions:

- `sessionId`: `string`

  A unique identifier for the session in UUID format.

  This ID is assigned to the session based on the session expiration policy defined in the application.

- `userId`: `string`

  A unique identifier for the user in UUID format.

  This ID is consistent across sessions based on the [anonymity scope](/explanation/application/anonymity-scope) defined in the application. It is not the same ID as the one you specify when identifying a user.

- `parentId`: `string` (optional)

  The ID of the session that superseded this session.

  Usually set when a user is identified, causing the current anonymous session to end and a new identified session to begin.

- `externalUserId`: `string` (optional)

  The external user ID used to identify the user on the application side.

  Always `null` for anonymous users.

- `window`: `object`

  The time window covering the first and last event of the session.

  - `start`: `integer`

    The session start timestamp in milliseconds since epoch.

  - `end`: `integer`

    The session end timestamp in milliseconds since epoch.

- `closeTime`: `integer`

  The time from which the session is closed for new events.

  May be extended if new events arrive before the session is fully closed.

- `referrer`: `string` (optional)

  The URL of the content that linked to the page that started the session.

- `landingPageUrl`: `string` (optional)

  The URL of the page that started the session.

- `campaign`: [`Campaign`](/reference/event/data-types/campaign) (optional)

  The marketing campaign that drove the user to the application, based on UTM parameters.

- `location`: [`Location`](/reference/event/data-types/location) (optional)

  The geographic location detected for the user, such as country, region, and city.

- `client`: [`Client`](/reference/event/data-types/client) (optional)

  The device and software used to access the application, including device, operating system, and browser.

- `attributes`: `object`

  The [attributes](#attributes) associated with the session.

- `statistics`: `object` (optional)

  The aggregated statistics of the session.

  - `pageViews`: `integer`

    The total number of page views.

  - `tabViews`: `integer`

    The total number of tab views.

  - `orders`: `integer`

    The total number of orders placed.

## Attributes

In addition to the standard properties, sessions support custom attributes. Use them to track information specific to a single visit, such as the selected plan, referral code, or experiment group.

> **Best practice**
>
> Use descriptive camel case names like `plan`, `useCase`, and `referralCode` for custom attributes to improve readability and consistency with standard attributes.

### Restrictions

The following restrictions apply to session attributes:

- Sessions can have up to 10 custom attributes.
- Attribute names must be strings up to 20 characters, starting with a letter or underscore, followed by letters, digits, or underscores.
- Attributes can be primitives (strings, numbers, booleans, null) or collections (lists, maps).
- Strings can be up to 100 characters long.
- Lists can contain up to 10 elements, including primitives, lists of primitives, or maps of primitives.
- Maps can contain up to 10 elements, including primitives, lists of primitives, or maps of primitives.
- Map keys must be strings up to 50 characters long.

Attribute names are case-insensitive, so `referralCode` and `referralcode` are treated as the same attribute and will override each other.

## Examples

Below are examples of event contexts:

**Web context**

```json
{
  "sessionId": "12345678-1234-1234-1234-123456789012",
  "userId": "87654321-4321-4321-4321-210987654321",
  "parentId": null,
  "externalUserId": "user-123",
  "window": {
    "start": 1440990000000,
    "end": 1441000000000
  },
  "closeTime": 1441001000000,
  "referrer": "https://www.google.com/",
  "landingPageUrl": "https://www.example.com/winter-sale",
  "campaign": {
    "name": "winter-sale",
    "source": "google",
    "medium": "search",
    "content": "shoes",
    "term": "black shoes"
  },
  "location": {
    "continent": "NA",
    "country": "US",
    "region": {
      "name": "California",
      "code": "CA"
    },
    "city": "Los Angeles",
    "source": "IP"
  },
  "client": {
    "device": {
      "name": "MacBook Pro",
      "vendor": "Apple",
      "category": "desktop",
      "operatingSystem": {
        "name": "iOS",
        "version": "10.0.18363"
      }
    },
    "browser": {
      "name": "Chrome",
      "version": "78.0.3904.108",
      "type": "web"
    }
  },
  "attributes": {
    "plan": "starter"
  },
  "statistics": {
    "pageViews": 12,
    "tabViews": 3,
    "orders": 1
  }
}
```

## Explore

- [Analytics](/reference/analytics): Learn how to analyze your dashboards.
- [CQL](/reference/cql/expressions/events/overview): Learn how to create event-based audiences.