> ## Documentation Index > Fetch the complete documentation index at: https://docs.datalyr.com/llms.txt > Use this file to discover all available pages before exploring further. # Visitor Identification > How DATALYR identifies and tracks visitors across sessions DATALYR automatically identifies visitors to build complete customer journeys, even across multiple sessions and devices. ## How Visitor Identification Works Every visitor gets a unique anonymous ID that persists across sessions. When users log in or provide their email, DATALYR links their anonymous ID to their user ID for cross-device tracking. ### Anonymous ID **What it is:** A unique identifier automatically generated for every visitor (`anon_abc123...`). **How it's stored:** * Primary: Root domain cookie (`__dl_visitor_id`) * Backup: localStorage (`dl_anonymous_id`) **Persistence:** * Lasts 1 year by default * Works across subdomains (e.g., [www.example.com](http://www.example.com) and app.example.com) * Survives browser restarts * Does NOT survive browser data clearing **Generation:** ```javascript theme={null} // Automatically generated on first visit anon_f47ac10b-58cc-4372-a567-0e02b2c3d479 ``` ### User ID **What it is:** Your internal identifier for logged-in users (user\_123, email address, database ID, etc.). **How it's set:** ```javascript theme={null} // Web datalyr.identify('user_123', { email: 'user@example.com', name: 'John Doe', plan: 'pro' }); // iOS await DatalyrSDK.shared.identify("user_123", properties: [ "email": "user@example.com" ]) // React Native await Datalyr.identify('user_123', { email: 'user@example.com' }); ``` **Persistence:** * Stored in localStorage (`dl_user_id`) * Survives page reloads and browser restarts * Cleared on logout (call `datalyr.reset()`) ### Distinct ID **What it is:** The primary identifier DATALYR uses for each visitor. **Logic:** ``` Distinct ID = User ID (if logged in) || Anonymous ID ``` **Example Flow:** ``` Visit 1: anonymous_id = anon_abc123 distinct_id = anon_abc123 User logs in: anonymous_id = anon_abc123 (unchanged) user_id = user_456 distinct_id = user_456 Visit 2 (same browser): anonymous_id = anon_abc123 user_id = user_456 distinct_id = user_456 ``` ## Cross-Subdomain Tracking DATALYR automatically tracks visitors across subdomains using root domain cookies. ### How It Works When a visitor first arrives at **[www.example.com](http://www.example.com)**: 1. DATALYR generates `anon_abc123` 2. Sets cookie on root domain (`.example.com`) 3. Cookie is accessible on all subdomains When the same visitor goes to **app.example.com**: 1. DATALYR reads cookie from root domain 2. Uses same `anon_abc123` ID 3. Journey continues seamlessly ### Supported Domains **Standard TLDs:** * `.com`, `.net`, `.org` → Cookie set on `.example.com` **Country-Specific TLDs:** * `.co.uk`, `.com.au`, `.co.nz` → Cookie set on `.example.co.uk` **Localhost:** * `localhost` → Cookie set on `localhost` **IP Addresses:** * `192.168.1.1` → Cookie set on exact IP ## Cross-Device Tracking Link visitors across devices using user identification. ### Example Journey **Device 1 (Mobile):** ``` Day 1, 10:00 AM - Mobile phone - Clicks Meta ad (fbclid captured) - anonymous_id: anon_mobile_123 - Browses products - Does NOT purchase ``` **Device 2 (Desktop):** ``` Day 1, 8:00 PM - Desktop computer - Types URL directly (direct traffic) - anonymous_id: anon_desktop_456 - Logs in as user_789 - Completes purchase ``` **Without Identify:** * Purchase attributed to "direct" (wrong!) * Ad spend wasted **With Identify:** ```javascript theme={null} // User logs in on mobile datalyr.identify('user_789'); // anonymous_id: anon_mobile_123 → user_id: user_789 // User logs in on desktop datalyr.identify('user_789'); // anonymous_id: anon_desktop_456 → user_id: user_789 // Purchase attributed correctly to Meta ad! ``` DATALYR links both devices to `user_789` and attributes the purchase to the original Facebook ad. ## Session Tracking Sessions track continuous user activity with 30-minute timeout. ### Session ID **Format:** ``` sess_1727634645123_a1b2c3d4 ``` **Components:** * `sess_`: Prefix * `1727634645123`: Timestamp (milliseconds) * `a1b2c3d4`: Random identifier **Storage:** * Stored in localStorage (`dl_session_data`) * Includes start time, last activity, page count, event count ### Session Lifecycle **New Session Starts When:** * First visit to site * 30 minutes of inactivity * Browser is closed and reopened (after timeout) **Session Continues When:** * User navigates between pages * User is active (clicks, scrolls, etc.) * Time since last activity is less than 30 minutes **Session Ends When:** * 30 minutes of inactivity * User closes browser (eventually) * `datalyr.reset()` is called ### Session Data Each session stores: ```json theme={null} { "id": "sess_1727634645123_a1b2c3d4", "startTime": 1727634645123, "lastActivity": 1727638245123, "pageCount": 5, "eventCount": 23 } ``` ## Identity Resolution DATALYR automatically links anonymous IDs to user IDs when you call `identify()`. ### How It Works 1. **Pre-Login:** ``` anonymous_id: anon_abc123 user_id: null distinct_id: anon_abc123 ``` 2. **User Identifies (Login/Signup):** ```javascript theme={null} datalyr.identify('user_456'); ``` 3. **Post-Login:** ``` anonymous_id: anon_abc123 (unchanged) user_id: user_456 distinct_id: user_456 ``` 4. **Identity Link Event Sent:** ```json theme={null} { "event_name": "$identify", "anonymous_id": "anon_abc123", "user_id": "user_456", "previous_id": null, "identified_at": "2025-09-29T10:30:45.123Z" } ``` 5. **All Past Events Linked:** * DATALYR retroactively links all `anon_abc123` events to `user_456` * Complete journey from anonymous → identified * Attribution preserved across identification ## Cookie Storage ### Primary Cookie **Name:** `__dl_visitor_id` **Value:** Anonymous ID (`anon_...`) **Attributes:** * Domain: Root domain (e.g., `.example.com`) * Path: `/` * Max-Age: 31536000 (1 year) * SameSite: `Lax` * Secure: Yes (on HTTPS sites) **Example:** ``` __dl_visitor_id=anon_f47ac10b-58cc-4372-a567-0e02b2c3d479; domain=.example.com; path=/; max-age=31536000; SameSite=Lax; Secure ``` ### localStorage Backup DATALYR also stores IDs in localStorage as a backup: **Keys:** * `dl_anonymous_id`: Anonymous ID * `dl_user_id`: User ID (after identify) * `dl_session_data`: Current session info **Why Both Cookie and localStorage?** * Cookie: Works across subdomains * localStorage: Survives when cookies are blocked or deleted * Fallback strategy ensures tracking continues ## Privacy and Compliance ### Opt-Out Users can opt out of tracking: ```javascript theme={null} // User opts out datalyr.optOut(); ``` This sets a cookie: ``` __dl_opt_out=true ``` All tracking stops. To opt back in: ```javascript theme={null} datalyr.optIn(); ``` ### Do Not Track DATALYR respects browser Do Not Track (DNT) settings by default: **Respected Signals:** * `navigator.doNotTrack === '1'` * `navigator.globalPrivacyControl === true` * `window.globalPrivacyControl === true` To disable DNT respect: ```javascript theme={null} datalyr.init({ workspaceId: 'ws_123', respectDoNotTrack: false // Don't respect DNT }); ``` ### GDPR Compliance Clear user data on request: ```javascript theme={null} // Remove user ID (keeps anonymous_id for analytics) datalyr.reset(); // OR fully opt out datalyr.optOut(); ``` ## Troubleshooting **Visitor ID changes on every page load?** Check that: * Cookies are enabled in browser * Third-party cookie blockers are disabled * You're not in incognito/private mode * Cookie domain is set correctly **Cross-subdomain tracking not working?** Verify: * Both subdomains are on same root domain * Cookie is set on root domain (check with DevTools) * No cookie blockers interfering **User ID not persisting?** Check: * `identify()` was called with a non-empty userId * localStorage is available (not disabled) * `reset()` wasn't called unintentionally **Anonymous ID lost after browser restart?** This means: * Cookies were cleared by user * Cookie didn't save properly (check domain) * localStorage also cleared ## Next Steps Learn how to identify users Understand event tracking Track across multiple domains How attribution works ## Need Help? Questions about visitor identification? Check our [troubleshooting guide](/troubleshooting/tracking-not-working) or contact support.