Key-Value Pairs

Okay, time to finally put the structure in structured logging!

Some logs, like startup and shutdown messages, don't need additional metadata... but some do. Common examples in web services are:

• HTTP response code
• Size of request/response bodies
• Duration of the request
• User ID making the request
• Stack traces in the event of failure

All we need to do is add key-value pairs to the log line, for example:

logger.Info("Someone is loose in the server room",
	slog.String("name", "Boots"),
	slog.Int("id", 80045),
)
// 2024-01-15T10:30:45.123Z INFO msg="Someone is loose in the server room" name=Boots id=80045

You can use type-specific helpers like slog.String and slog.Int, or log values directly. Both are valid, but helpers can be more consistent, and sometimes more performant:

logger.Info("Someone is loose in the server room",
	"name", "Boots",
	"id", 80045,
)
// 2024-01-15T10:30:45.123Z INFO msg="Someone is loose in the server room" name=Boots id=80045

Structured logs are more readable, but more importantly, their interface can output JSON (or another format) just by changing the handler. Imagine if your app had thousands of log lines and you had to rewrite all of them just to ship a new format or destination... no fun.

LINKO_LOG_FILE=linko.access.log go run .