Client SDK vs server SDK

Context handling differs significantly between client-side and server-side SDKs due to their different operational models.

Context handling comparison

Client-side SDKs

Client-side SDKs handle one active user or session:

• Provide context at initialization and update via identify()
• SDK fetches variations for that specific context and caches them
• When calling variation(), no need to provide context anymore
• Evaluations happen remotely via an evaluation endpoint

Server-side SDKs

Server-side SDKs evaluate for many users:

• No context required at initialization
• SDK downloads all flag rules at startup
• Pass context on every variation() call: variation(flag, context, fallback)
• SDK calculates variations against locally cached rules

Evaluation flow

Server-side evaluation

Application Request → Create Context → Call variation(flag, context, fallback)
                                     ↓
                        SDK evaluates locally using cached rules
                                     ↓
                                Return variation

Client-side evaluation

Page Load → Initialize SDK with context → SDK fetches user's variations
                                        ↓
                              Cache variations locally
                                        ↓
                         Call variation(flag) // No context needed

When to use each

Use server-side SDKs when

• Evaluating flags for multiple different users or entities
• Running backend services or APIs
• Need to keep flag rules private
• Evaluating flags in high-security contexts

Use client-side SDKs when

• Evaluating flags for a single active session
• Running in browsers or mobile apps
• Need real-time flag updates for the current user
• Implementing user-specific feature rollouts

Updating contexts

Client-side context updates

Use identify() to update the context:

const ldclient = LaunchDarkly.initialize(clientId, context);

// User logs in
ldclient.identify({
  kind: "user",
  key: "user-123",
  name: "Jane Doe",
  email: "jane@example.com"
});

The SDK will fetch new variations for the updated context.

Server-side context changes

Simply pass a different context to variation():

// Evaluate for user 1
const variation1 = ldclient.variation("flag-key", userContext1, false);

// Evaluate for user 2
const variation2 = ldclient.variation("flag-key", userContext2, false);

No SDK reconfiguration needed.

Performance considerations

Client-side SDKs

• Initial page load includes SDK initialization time
• Subsequent evaluations are instant (served from cache)
• identify() calls require network round-trip
• Consider bootstrapping to eliminate initialization delay

Server-side SDKs

• Initialization happens once at application startup
• All evaluations are local and extremely fast
• No per-request network overhead
• Consider persistent stores for daemon mode deployments