Logging and monitoring

import contextvars, json, logging, sys
 
request_id = contextvars.ContextVar("request_id", default="-")
 
class JsonFormatter(logging.Formatter):
    """One JSON object per line: easy for log tools to search, filter and chart."""
    def format(self, record):
        entry = {"level": record.levelname, "msg": record.getMessage(), "request_id": request_id.get()}
        entry.update(getattr(record, "fields", {}))
        return json.dumps(entry)
 
log = logging.getLogger("rag-app")
handler = logging.StreamHandler(sys.stdout)
handler.setFormatter(JsonFormatter())
log.addHandler(handler)
log.setLevel(logging.INFO)
 
def handle(question: str, user_id: str, fake_latency_ms: int, fake_error: bool = False):
    request_id.set(f"req-{len(question):04d}")       # in real code: str(uuid.uuid4()) or the trace ID
    log.info("request received", extra={"fields": {"user": user_id, "question_chars": len(question)}})
    if fake_error:
        log.error("llm call failed", extra={"fields": {"provider": "anthropic", "status": 529, "attempt": 3}})
        return
    log.info("llm call done", extra={"fields": {"model": "claude-haiku-5-5", "input_tokens": 1830,
             "output_tokens": 212, "latency_ms": fake_latency_ms, "cost_usd": 0.000289}})
 
handle("How many days of leave?", "u-123", 840)
handle("Expense limit?", "u-456", 0, fake_error=True)
# Never log raw secrets or full personal data; log IDs, sizes, counts and redacted text instead.

{"level": "INFO", "msg": "request received", "request_id": "req-0023", "user": "u-123", "question_chars": 23}
{"level": "INFO", "msg": "llm call done", "request_id": "req-0023", "model": "claude-haiku-5-5", "input_tokens": 1830, "output_tokens": 212, "latency_ms": 840, "cost_usd": 0.000289}
{"level": "INFO", "msg": "request received", "request_id": "req-0014", "user": "u-456", "question_chars": 14}
{"level": "ERROR", "msg": "llm call failed", "request_id": "req-0014", "provider": "anthropic", "status": 529, "attempt": 3}