What are the four golden signals of monitoring?

Question

Accepted Answer

From Google's SRE book, the four golden signals are **latency, traffic, errors, and saturation**. If you can only measure four things about a user-facing system, measure these — together they catch the vast majority of issues.

## The four signals

```text
LATENCY     how long a request takes
            → split SUCCESSFUL vs FAILED latency (a fast 500 isn't "fast")
            → track percentiles (p50/p95/p99), not averages

TRAFFIC     how much demand the system is under
            → requests/sec, transactions/sec, concurrent sessions

ERRORS      rate of failing requests
            → explicit (HTTP 500) and implicit (wrong content, too slow)

SATURATION  how "full" the system is — its most constrained resource
            → CPU, memory, I/O, queue depth; a leading indicator of trouble
```

## Why these four cover most issues

Latency and errors are what users **feel directly**. Traffic explains the **load context** (a latency spike during a 10x traffic surge means something different than one at baseline). Saturation is the **leading indicator** — it rises before latency and errors do, giving you warning before users are hurt.

```text
saturation climbs → latency rises → errors appear → users complain
     ^ warning here, while there's still time to act
```

## Contrast with USE and RED

Golden signals are one of three related frameworks; they overlap heavily and pick the same vital signs from different angles.

```text
GOLDEN  latency, traffic, errors, saturation   → user-facing services (general)
RED     Rate, Errors, Duration                 → request-driven services (a subset)
USE     Utilization, Saturation, Errors        → resources/infra (per device)
```

RED is essentially golden signals minus saturation, oriented around requests; USE is resource-centric (apply it per disk, NIC, CPU). Use RED for services, USE for hardware, golden signals as the general default.

## Why it matters

Monitoring everything produces noise and blind spots; monitoring nothing leaves you flying blind. The golden signals are a proven minimal set that ties dashboards and alerts to user experience while still surfacing saturation early — so you get warning before an incident rather than a post-mortem after it.