ServerTimingHeader

ServerTimingHeader refers to the HTTP response header used to expose server-side timing information to a client. It is designed to help developers understand how time is spent during request handling by reporting the durations of different server components, such as application logic, database queries, or caching layers. When present, the header allows performance data to be observed alongside other network metrics without requiring additional client-side instrumentation.

The header value is a comma-separated list of metric tokens. Each token starts with a metric name and may be followed by optional parameters separated by semicolons. The most common parameters are dur, which specifies the duration in milliseconds, and desc, which provides a human-readable description. Some implementations also support a shared parameter to indicate that a metric is shared across resources. For example: Server-Timing: app;dur=123.4, db;dur=67.2;desc="database query".

In practice, servers instrument components and attach one or more tokens to the Server-Timing header to convey precise timing data to the client. Browsers and developer tools can display this information in performance panels, aiding debugging and optimization. Middleware or framework-level support is often available to generate the header automatically, with the instrumentation hooks configured per request.

Security and privacy considerations should be taken into account, since exposing internal timings can reveal implementation details. Organizations may choose to gate or redact certain metrics, especially in production or public endpoints, and can disable the header generation when necessary.

See also: performance instrumentation, HTTP headers, browser developer tools, profiling.