langfuse-sdk-patterns

# Langfuse SDK Patterns ## Overview Production-quality patterns for the Langfuse SDK: singleton clients, the `observe` wrapper, `startActiveObservation` for nested traces, session tracking, graceful shutdown, and error-safe tracing. ## Prerequisites - Completed `langfuse-install-auth` setup - Understanding of async/await patterns - For v4+: `@langfuse/tracing`, `@langfuse/otel`, `@opentelemetry/sdk-node` ## Instructions ### Pattern 1: Singleton Client with Graceful Shutdown ```typescript // src/lib/langfuse.ts -- single file, import everywhere import { LangfuseClient } from "@langfuse/client"; import { LangfuseSpanProcessor } from "@langfuse/otel"; import { NodeSDK } from "@opentelemetry/sdk-node"; // Singleton client for prompts, datasets, scores let client: LangfuseClient | null = null; export function getLangfuseClient(): LangfuseClient { if (!client) { client = new LangfuseClient(); } return client; } // One-time OTel setup (call at app entry point) let sdk: NodeSDK | null = null; export function initTracing(): NodeSDK { if (!sdk) { sdk = new NodeSDK({ spanProcessors: [new LangfuseSpanProcessor()], }); sdk.start(); // Graceful shutdown on process exit const shutdown = async () => { await sdk?.shutdown(); process.exit(0); }; process.on("SIGTERM", shutdown); process.on("SIGINT", shutdown); } return sdk; } ``` **Legacy v3 singleton:** ```typescript import { Langfuse } from "langfuse"; let instance: L...