Four Golden Signals

Observability: Four Golden Signals

# 1. Latency    -- response time (p50, p95, p99)
# 2. Traffic    -- requests per second
# 3. Errors     -- 4xx + 5xx rate
# 4. Saturation -- CPU, memory, queue depth

from prometheus_client import Counter, Histogram
import time

REQUESTS = Counter("requests_total","Requests",["endpoint","status"])
LATENCY  = Histogram("latency_seconds","Latency",["endpoint"])

@app.middleware("http")
async def metrics(req, call_next):
    start = time.time()
    resp  = await call_next(req)
    LATENCY.labels(endpoint=req.url.path).observe(time.time()-start)
    REQUESTS.labels(endpoint=req.url.path,status=resp.status_code).inc()
    return resp

# PromQL alert: error rate > 1%
# rate(requests_total{status=~"5.."}[5m]) > 0.01