What are the Four Golden Signals defined by Google SRE?

Google's SRE book defines four signals that, when monitored together, give a comprehensive picture of a user-facing service's health:

1. Latency — The time it takes to serve a request. Critically, you must distinguish latency of successful requests from latency of failed requests. A 500 error that returns in 1 ms will skew your latency distribution favorably but hides the real problem.

2. Traffic — A measure of demand placed on the system. For web services this is requests per second; for audio streaming it might be bits per second; for key-value stores it might be transactions per second.

3. Errors — The rate of requests that fail, either explicitly (HTTP 500), implicitly (HTTP 200 with wrong content), or by policy (any request over 1 second is considered an error).

4. Saturation — How full the service is. This is the resource most constrained — CPU, memory, I/O, or queue depth. Saturation often predicts impending failure before errors or latency degrade. At 100% saturation, the service is overloaded.

The Four Golden Signals are broader than RED: they include Saturation, which RED omits, making them better for evaluating whether a service has headroom or is approaching its limits.