Core Concepts

Flags and Variants

A Flag represents a feature toggle or configuration. It has a key (e.g., new-checkout-flow) and a type (boolean, string, number, JSON).

Variants are the possible values a flag can take. For a boolean flag, variants are true and false.

Rules and Evaluation

Flags are evaluated based on Rules. A rule consists of Conditions and a Result.

Conditions

Conditions check against the Evaluation Context. Supported operators include:

• equals, contains
• gt, lt, gte, lte (numbers)
• in_list, not_in_list (hashed lists)

Evaluation Context

The context is a JSON object containing attributes about the user or request.

• userId: Unique identifier for the user (required for percentage rollouts and lists).
• attributes: Custom attributes (e.g., email, plan, country).

Evaluation Modes

Local Evaluation (Node.js)

The SDK fetches Flag Definitions (rules, lists) at initialization and evaluates flags locally. This ensures low latency and high availability.

Remote Evaluation (React/Client-side)

The SDK sends the context to the FlagControl API, which evaluates the flags and returns the results. This keeps sensitive rules and lists secure on the server.

Lists and Privacy

Lists allow targeting specific groups of users (e.g., beta-users). To protect user privacy, list members are hashed (with a project-specific salt) before being sent to the SDK (in Local Mode). The SDK hashes the user ID from the context and checks for existence in the hashed list.