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SDK Concepts

FlagSync SDKs enable feature management across platforms:

Setup

Learn the prerequisites and initialization process for SDKs.

SDK Types

Explore client-side and server-side SDK differences.

Flag Evaluation

Understand how FlagSync evaluates flags for users.

FlagSync CLI

Learn how FlagSync CLI provides type safety in your application.

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SDK Best Practices

The FlagSync CLI generates TypeScript types from your feature flag config.
  • IntelliSense autocompletion (e.g., client.flag(...), useFlag(...))
  • Compile-time safety.
  • Confidence that your code matches your flags.

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User Context Best Practices

When defining the user context for flag evaluations, always provide a unique and persistent key in the FsUserContext object to ensure accurate billing and consistent flag behavior:
  • Billing Impact: FlagSync tracks monthly active users (MAUs) based on unique key values, which are limited by your pricing tier. Non-unique or non-persistent keys (e.g., hardcoding 'anonymous' or generating a new ID per request) can undercount or overcount MAUs, affecting billing accuracy.
  • Functionality Impact: During percentage rollouts, FlagSync uses the user context for deterministic hashing to ensure the same user receives consistent flag values across sessions. Non-persistent keys cause inconsistent flag evaluations, disrupting rollout behavior.
Avoid hardcoding key values like 'anonymous' or generating a new ID on every request (e.g., calling nanoid() without persistence). Instead, use a persistent identifier, such as a cookie for unauthenticated users (e.g., visitor-id), to ensure accurate MAU tracking and consistent flag evaluations.