SNekLog/README.md

# slog

Small structured logger for Go with text and JSON output, multiple writers, and optional traceback metadata.

Russian version: [README_ru.md](README_ru.md)

## Features

- Fan out the same record to multiple destinations.
- Text and JSON writers.
- `stdout`, files, and arbitrary external `io.Writer` values.
- Optional timestamps for text output.
- Compact traceback metadata for text writers and full traceback slices for JSON.
- Message replacement for masking secrets or normalizing output.
- Explicit ownership rules for writer closing.

## Installation

```bash
go get git.scuroneko.dev/scuroneko/slog
```

## Quick start

```go
package main

import (
	"log"

	"git.scuroneko.dev/scuroneko/slog"
)

func main() {
	logger := slog.CreateLogger().
		Prefix("API").
		Level(slog.DEBUG).
		PrintTraceback(true).
		JsonPretty(true).
		AddReplacer("SOME_SECRET", "<redacted>")

	text := logger.CreateTextStdoutWriter()
	jsonFile, err := logger.CreateJsonFileWriter("logs/app.json")
	if err != nil {
		log.Fatal(err)
	}

	logger.AddWriters(text, jsonFile)

	logger.Infoln("service started")
	logger.Warnln("cache miss")
	logger.Errorln("request failed")
	logger.Debugln("debug details")
	logger.Infoln("token", "SOME_SECRET")

	if err := logger.Close(); err != nil {
		log.Fatal(err)
	}
}
```

## Defaults

`CreateLogger()` starts with:

- `Prefix("LOG")`
- `Level(slog.FATAL)`
- `PrintTime(true)`
- `PrintTraceback(false)`
- `JsonPretty(false)`

Important: with the current level ordering, `Level(slog.FATAL)` allows `INFO`, `WARN`, `ERROR`, and `FATAL`, but not `DEBUG`. Use `Level(slog.DEBUG)` to enable every level.

## Writers and ownership

`Logger.Close()` only closes writers created by the logger itself:

- `CreateTextFileWriter(...)`
- `CreateJsonFileWriter(...)`

The following writers remain owned by the caller and are not closed by `Logger.Close()`:

- `CreateTextWriter(existingWriter)`
- `CreateJsonWriter(existingWriter)`
- `CreateTextStdoutWriter()`
- `CreateJsonStdoutWriter()`

This makes it safe to plug in `bytes.Buffer`, network writers, and other externally managed resources.

## Output formats

Text writers render records like:

```text
[API] [INFO] [main.go:main:27] [17.03.26 14:05:09] service started
```

The traceback field is included only when `PrintTraceback(true)` is enabled.

JSON writers emit objects with this shape:

```json
{
  "time": "2026-03-17T14:05:09.123456789+03:00",
  "level": "info",
  "prefix": "API",
  "message": "service started",
  "traceback": [
    {
      "method": "main",
      "filename": "main.go",
      "line": 27,
      "signature": "main.main",
      "fullPath": "/path/to/main.go"
    }
  ]
}
```

When `JsonPretty(true)` is enabled, JSON is indented.

## Message replacement

`AddReplacer(old, new)` replaces matching text in every message before the
record reaches any writer. Replacement rules are applied in the order they are
added.

```go
logger := slog.CreateLogger().
	Level(slog.DEBUG).
	AddReplacer("SOME_SECRET", "<redacted>").
	AddReplacer("user@example.com", "<email>")

logger.Infoln("login token:", "SOME_SECRET")
```

This writes `<redacted>` instead of `SOME_SECRET` in both text and JSON output.
An empty `old` value is ignored.

## API summary

- `Info`, `Warn`, `Error`, `Debug`, and `Fatal` accept a list of values.
- `Infof`, `Warnf`, `Errorf`, `Debugf`, and `Fatalf` use `fmt.Sprintf`.
- The `*ln` methods preserve newline semantics, which is useful for `stdout`, Docker, and line-based collectors.
- `Fatal`, `Fatalf`, and `Fatalln` call `os.Exit(1)` after writing the message.
- `AddReplacer` masks or rewrites message text before records are sent to writers.

## Traceback behavior

- Text writers use the nearest user stack frame.
- JSON writers receive the full traceback slice.
- Internal `slog` frames and `runtime` frames are filtered out.

## Repository example

See [examples/main.go](examples/main.go).

## License

This project is licensed under GNU GPLv3. See [LICENSE](LICENSE).