slogx

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Installation

go get ella.to/slogx

FilterHandler

FilterHandler wraps any slog.Handler and adds the ability to set different log levels for different packages. It inspects the caller's package path at log time and applies the matching rule.

inner := slog.NewJSONHandler(os.Stdout, nil)

handler := slogx.NewFilterHandler(inner,
    slogx.WithDefaultLevel(slog.LevelInfo),
    slogx.WithLogLevel("myapp/db", slog.LevelDebug),
    slogx.WithLogLevel("myapp/http", slog.LevelWarn),
)

slog.SetDefault(slog.New(handler))

In this setup, most of your app logs at Info and above, but the db package logs everything down to Debug, and the http package only logs Warn and above.

Exclusive Filtering

When you only want to see logs from specific packages and silence everything else:

handler := slogx.NewFilterHandler(inner,
    slogx.WithLogLevel("myapp/auth", slog.LevelDebug),
    slogx.WithExclusiveFiltering(),
)

Only logs from myapp/auth will appear. Everything else is dropped regardless of level.

Runtime Configuration

All settings can be changed while the application is running:

handler.SetDefaultLevel(slog.LevelDebug)
handler.SetLogLevel("myapp/payments", slog.LevelDebug)
handler.RemoveLogLevel("myapp/payments")
handler.SetEnabled(false)  // disable all logging
handler.SetEnabled(true)   // re-enable
handler.SetExclusiveFiltering(true)

Trace ID Filtering

Filter logs to only show entries that carry a specific trace ID. Useful when debugging a particular request in a noisy production environment.

type traceKey struct{}

handler.SetTraceIdKey(traceKey{})
handler.SetFilterTraceId("request-abc-123")

// Only logs where the context contains traceKey{} = "request-abc-123"
// or where an attribute value matches "request-abc-123" will pass through.

Clear the filter to go back to normal:

handler.SetFilterTraceId("")

FileHandler

FileHandler writes structured JSON logs to files with automatic rotation. Each run gets its own timestamped directory, and files rotate when they hit a size or record count limit.

fileHandler, err := slogx.NewFileHandler("/var/log/myapp",
    slogx.WithMaxBytes(50 * 1024 * 1024),  // rotate at 50 MB
    slogx.WithMaxRecords(100_000),           // or at 100k records
)
defer fileHandler.Close()

slog.SetDefault(slog.New(fileHandler))

Directory structure:

/var/log/myapp/
  2025-03-25_14-30-00/
    00001.log
    00002.log
    00003.log
  2025-03-24_09-15-00/
    00001.log

Each log line is a JSON object written by slog.JSONHandler, so it works with any log aggregation tool.

Combining with FilterHandler

The common pattern is to wrap FileHandler with FilterHandler:

fileHandler, _ := slogx.NewFileHandler("/var/log/myapp")
filterHandler := slogx.NewFilterHandler(fileHandler,
    slogx.WithDefaultLevel(slog.LevelInfo),
    slogx.WithLogLevel("myapp/db", slog.LevelDebug),
)

slog.SetDefault(slog.New(filterHandler))

Reading Logs Back

List available sessions, then read logs with pagination:

sessions, err := slogx.ListSessions("/var/log/myapp")
// sessions[0].Name      = "2025-03-25_14-30-00"
// sessions[0].FileCount = 3

reader, err := slogx.NewLogReader(sessions[0].Path, 100) // 100 entries per page

page, err := reader.ReadPage(1, 0) // file 1, line offset 0
for _, entry := range page.Entries {
    fmt.Printf("[%s] %s: %s\n", entry.Level, entry.Time.Format(time.RFC3339), entry.Message)
}

// Or read everything
entries, err := reader.ReadAll()

// Or stream with context cancellation
entryCh, errCh := reader.StreamLogs(ctx)
for entry := range entryCh {
    fmt.Println(entry.Message)
}

HTTP API

Expose FilterHandler controls over HTTP for runtime log management. Mount it as a sub-handler on your existing mux:

mux.Handle("/debug/log/", http.StripPrefix("/debug/log", handler.HttpHandler()))

Endpoints

GET /level — Returns current config as JSON:

{
  "disabled": false,
  "default": "INFO",
  "rules": {"myapp/db": "DEBUG"},
  "exclusive_filtering": false,
  "filter_trace_id": ""
}

POST /level?path=myapp/db&level=DEBUG — Set log level for a path (use path=default for the default level)

DELETE /level?path=myapp/db — Remove a path-specific rule

POST /enable?enable=true — Enable or disable all logging

POST /trace-id?trace-id=abc-123 — Filter logs by trace ID

DELETE /trace-id — Clear trace ID filter

GET /trace-id — Get current trace ID filter

License

MIT — see LICENSE for details.