simplelog

Summary

simplelog is an opinionated logging package designed to facilitate easy and structured logging in Go applications with an absolute minimum of boilerplate.

The idea is that you can add a single import line which replaces the stdlib log/slog default handler, and solve the 90% case for logging.

Current Status

Released v1.0.0 2024-06-14. Works as intended. No known bugs.

Features

• if output is a tty, outputs pretty color logs
• if output is not a tty, outputs json
• supports delivering each log message via a webhook
• emits every slog attribute: those passed to a log call, those accumulated with WithAttrs, and those qualified by WithGroup. slog.Group values nest, and slog.LogValuer values are resolved. In json output attributes are object fields (groups become nested objects); in console output they are appended as key=value pairs, with grouped keys written as group.key=value. See Attribute output for the details worth knowing

Planned Features

• supports delivering logs via tcp RELP (e.g. to remote rsyslog using imrelp)

Installation

To use simplelog, first ensure your project is set up with Go modules:

go mod init your_project_name

Then, add SimpleLog to your project:

go get sneak.berlin/go/simplelog

Usage

Below is an example of how to use SimpleLog in a Go application. This example is provided in the form of a main.go file, which demonstrates logging at various levels using structured logging syntax.

package main

import (
	"log/slog"
	_ "sneak.berlin/go/simplelog"
)

func main() {

    // log structured data with slog as usual:
	slog.Info("User login attempt", slog.String("user", "JohnDoe"), slog.Int("attempt", 3))
	slog.Warn("Configuration mismatch", slog.String("expected", "config.json"), slog.String("found", "config.dev.json"))
	slog.Error("Failed to save data", slog.String("reason", "permission denied"))
}

Attribute output

Attributes reach every handler: the ones passed to the log call, the ones accumulated with WithAttrs, and the ones qualified by the groups open at the time they were attached. slog.Group values nest, and slog.LogValuer values are resolved to the value they stand for. A few behaviours are worth knowing before you rely on them.

Your values are never modified. Whatever you log is read and rendered, never written to. A map or a slice you pass to slog.Any comes back from the logger exactly as you handed it over, even when a group later uses the same key.

Durations are nanoseconds in json, and readable on the console. The json and webhook payloads emit a slog.Duration as a number of nanoseconds, matching slog.NewJSONHandler, so a consumer can compare and aggregate the field without parsing it. The console line emits the same duration as 3s, matching slog.NewTextHandler, because a person reads it.

The json payload is an object, with the consequences an object has. The record's own fields are named Time, Level, Message and PC, and they own those names: an attribute keyed after one of them is dropped from the json and webhook output. A key logged more than once keeps its last value there for the same reason, with one exception: when both are slog.Group values sharing a key, the two groups are merged into one object holding the members of both, rather than the second replacing the first. Neither the collision nor the merge applies to the console output, which is a line of text: every pair appears, in order. If you need a field called message, pick a key that does not collide - the collision is silent.

Empty things follow the slog.Handler contract. An empty Attr is ignored, an empty group is elided along with its key, a group with an empty key is inlined into its parent, and WithGroup("") returns the handler unchanged.

License

WTFPL