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Logging

Logging

The clog package wires log/slog into Salusa. It registers the logger as a DI dependency, attaches request-scoped attributes to a context, and picks a handler: human-friendly colored output on a terminal, text output otherwise, or any custom slog.Handler.

Registering a logger

RegisterDefault(ctx) is called during kernel bootstrap in the generated template. It builds a lazy singleton RootLogger at Info level and registers *slog.Logger as a dependency:

kernel.Register(func(ctx context.Context, c *config.Config) {
	clog.RegisterDefault(ctx)
})

For a custom setup, Register(ctx, cfg) builds the handler from a Config (a type with Handler() (slog.Handler, error)), and RegisterWith(ctx, h) takes a slog.Handler directly. DefaultConfig wraps a slog.Level so the level can come from the environment:

clog.Register(ctx, clog.NewDefaultConfig(level))

DefaultHandler(level) returns a tint handler with error-aware rendering when stderr is a TTY and a plain text handler otherwise.

Using the logger

The *slog.Logger is an ordinary injectable dependency, and clog.Use(ctx) resolves it (falling back to slog.Default if none is registered):

type Handler struct {
	Logger *slog.Logger `inject:""`
}

func (h *Handler) Handle(ctx context.Context) {
	h.Logger.Info("processing order", slog.Int("id", 42))
}

The kernel resolves the logger through clog.Use for its own startup and shutdown messages, and clog.RegisterDefault also sets it as the global log/slog default so anything using slog.Default() participates.

Context attributes

clog.With(ctx, attrs...) returns a context that adds the attributes to any logger resolved from it. Attributes accumulate:

ctx := clog.With(r.Context(), slog.String("request_id", reqID))
ctx = clog.With(ctx, slog.String("user_id", userID))

logger := clog.Use(ctx)  // carries both attributes

The kernel uses this to tag service logs (logger.With("service", name) via StartServices), so every log line from a background service identifies its source.

Sending logs to Loki

The clog/loki subpackage provides a Config that sends logs to a Grafana Loki endpoint through slogloki:

clog.Register(ctx, &loki.Config{
	Level:    slog.LevelInfo,
	URL:      "http://loki:3100/loki/api/v1/push",
	TenantID: "my-app",
})
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