OpenTelemetry FastAPI Instrumentation

import os
from contextlib import asynccontextmanager

from fastapi import FastAPI
from opentelemetry import trace
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from opentelemetry.sdk.resources import Resource
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter

# Configure OpenTelemetry SDK
# It is recommended to initialize OpenTelemetry within FastAPI's lifespan events
# when using multi-process servers (e.g., uvicorn --workers > 1 or gunicorn).
# For simple single-process development, top-level initialization is sufficient.

def setup_tracing():
    resource = Resource.create(attributes={
        "service.name": os.environ.get("OTEL_SERVICE_NAME", "my-fastapi-app")
    })
    tracer_provider = TracerProvider(resource=resource)
    
    # Use OTLP HTTP exporter for traces
    # OTLP endpoint can be configured via environment variable OTEL_EXPORTER_OTLP_ENDPOINT
    # Default is http://localhost:4318/v1/traces for HTTP
    otlp_exporter = OTLPSpanExporter(
        endpoint=os.environ.get("OTEL_EXPORTER_OTLP_ENDPOINT", "http://localhost:4318/v1/traces")
    )
    span_processor = BatchSpanProcessor(otlp_exporter)
    tracer_provider.add_span_processor(span_processor)
    trace.set_tracer_provider(tracer_provider)

    print("OpenTelemetry tracing initialized.")

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup logic: Initialize OpenTelemetry
    setup_tracing()
    FastAPIInstrumentor.instrument_app(app)
    yield
    # Shutdown logic: Flush and shutdown exporter
    provider = trace.get_tracer_provider()
    if hasattr(provider, 'shutdown'):
        provider.shutdown()
    print("OpenTelemetry tracing shutdown.")

# Initialize FastAPI app with lifespan events
app = FastAPI(lifespan=lifespan)

@app.get("/hello")
async def read_root():
    return {"message": "Hello, World!"}

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id, "message": "Item fetched"}

# To run this application:
# 1. Save it as main.py
# 2. Run from your terminal:
#    OTEL_EXPORTER_OTLP_ENDPOINT="http://localhost:4318/v1/traces" OTEL_SERVICE_NAME="my-fastapi-app" uvicorn main:app --port 8000 --reload --lifespan on
#    Note: For production, omit --reload and manage workers via gunicorn post_fork hooks if using multiple workers.
# 3. Access in browser: http://localhost:8000/hello or http://localhost:8000/items/123