Identifying Users (User ID)

A User Identifier (User ID) is a unique string used to identify an individual user within your system. Since campaigns and feature evaluations are directly associated with users, VWO SDKs rely on the User ID you provide to ensure deterministic bucketing and consistent experience delivery.

The structure and generation of the User ID are fully customizable based on your business requirements.

Generating a User ID

You can generate a User ID using various strategies, such as:

• A client-side first-party cookie that persists across server requests
• A device identifier unique to the user’s device
• A universally unique identifier (UUID) generated within your system

The only requirement is that the identifier remains stable and unique per user.

Ensuring Consistent Behavior

To maintain consistent experiment behavior across sessions, the same User ID must be supplied on every evaluation request.

Example: A user visits your website for the first time and is assigned:

f34c3d91-a66e-4389-92fb-595fa9874725

The SDK evaluates this User ID and assigns the user to Variation-1.

If the user returns later and the same User ID is passed again:

• The SDK deterministically assigns the same variation
• The user experience remains consistent
• Metrics remain accurate

UUID Conversion and Privacy Compliance

For privacy and compliance reasons:

• The raw User ID is never stored in VWO systems.
• The SDK anonymizes the provided User ID into a VWO-generated UUID before transmission.
• Only the derived UUID is stored and used for:
  • Bucketing
  • Experiment decisioning
  • Conversion attribution
  • Cross-product stitching

This approach supports:

• GDPR compliance
• Data minimization
• Privacy-by-design principles

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Anonymization of User ID

The User IDs you provide are anonymized by our SDKs into a UUID before being sent to VWO servers.

VWO UUID Characteristics

The VWO UUID:

• Is a 32-character unique identifier
• Is generated from the provided User ID
• Ensures a 1:1 mapping:
  • One unique User ID → One unique VWO UUID
• Is the canonical identity used internally by VWO

The original User ID is used only for deterministic UUID generation and is not persisted in raw form.

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User IDs should be Unique

Ensure User IDs are unique: User IDs must be unique for a campaign. VWO SDK relies on the User ID you provide for consistent behavior across platforms. VWO buckets users and provides test metrics based on the User IDs that you provide.

Important Guidelines

• User IDs must be unique per user
• Do not use session-based or temporary identifiers
• Always pass the same stable identifier across:
  • Backend services
  • Frontend applications
  • Mobile SDKs

Correct implementation ensures:

• Deterministic bucketing
• Accurate reporting
• Cross-platform identity consistency