Overview

An evaluation context is the set of attributes an SDK passes to Datadog when it evaluates a flag. Datadog Feature Flags uses OpenFeature’s evaluation context: a flat map of attributes describing the subject being evaluated, such as a user, session, or device. Targeting rules and percentage rollouts read these attributes to decide which variant a subject receives.

Without an evaluation context, the SDK can still evaluate Boolean on/off flags. It cannot match targeting rules that filter on subject attributes, or produce a consistent rollout assignment for that subject.

The targeting key

The targetingKey is the primary identifier in an evaluation context. It’s typically a user ID, session ID, or device ID. Datadog uses the targetingKey for deterministic randomization, so the same subject consistently receives the same variant for a flag.

Use a stable, consistent identifier for the same subject across sessions. For logged-out or anonymous subjects, use a persistent identifier, such as a UUID stored in local storage or SharedPreferences, instead of omitting the targetingKey or regenerating it each session.

Context attributes

Beyond the targetingKey, an evaluation context can include any number of additional attributes, such as user_role, country, or tier. Reference these attributes in targeting rule filters to control who sees each variant.

Example evaluation context

const evaluationContext = {
  targetingKey: 'user-123',
  user_id: 'user-123',
  user_role: 'admin',
  country: 'US',
  tier: 'premium',
};

from openfeature.evaluation_context import EvaluationContext

eval_ctx = EvaluationContext(
    targeting_key="user-123",
    attributes={
        "user_id": "user-123",
        "user_role": "admin",
        "country": "US",
        "tier": "premium",
    },
)

evalCtx := openfeature.NewEvaluationContext(
    "user-123",
    map[string]interface{}{
        "user_id":   "user-123",
        "user_role": "admin",
        "country":   "US",
        "tier":      "premium",
    },
)

Client-side vs. server-side context

Client and server SDKs set the evaluation context differently:

• Client-side SDKs hold a single global evaluation context for the SDK instance. Set it once during initialization, then call OpenFeature.setContext() to update it when subject attributes change, such as after a user logs in. All subsequent flag evaluations use the updated context.
• Server-side SDKs don’t hold a global context. Build an evaluation context for each incoming request, based on the current user or session, and pass it explicitly to every flag evaluation call for that request. Reuse the same context object across evaluations within a request, and only rebuild it if subject attributes change.

Further reading

Additional helpful documentation, links, and articles:

Targeting Rules and FiltersDOCUMENTATION Traffic Splitting and RandomizationDOCUMENTATION Client-Side SDKsDOCUMENTATION Server-Side SDKsDOCUMENTATION