How to Add Context to Log Messages in Go

Add context to log messages in Go by using context.WithValue to attach data to the context and passing that context to your logging function.

ctx := context.WithValue(context.Background(), "user_id", 123)
log.Printf("%s: processing request", ctx.Value("user_id"))

Alternatively, use a structured logger like log/slog which natively supports key-value pairs for context:

ctx := context.Background()
logger := log.Default()
logger.InfoContext(ctx, "processing request", "user_id", 123)

For more complex scenarios, create a custom context key type to avoid collisions:

type contextKey string
const userKey contextKey = "user"

ctx := context.WithValue(context.Background(), userKey, 123)
log.Printf("%s: processing request", ctx.Value(userKey))

Adding context to log messages means including extra information like user IDs or request IDs alongside your standard log text. This helps you trace specific actions through your system, much like adding a case number to a police report. You do this by storing the data in a special container called a 'context' and passing it along with your log call.