rotatingFileOutput function

OutputFunction rotatingFileOutput(String filePath, { int maxSizeBytes = 10 * 1024 * 1024, int maxBackups = 5, })

Returns an OutputFunction like fileOutput, but rotates the file once it reaches maxSizeBytes: before a write that would exceed the limit, the current file is renamed to <filePath>.0, any existing .0 becomes .1, and so on up to .{maxBackups - 1} — the oldest backup beyond that is deleted. A fresh file at filePath is then created for the new entry. Writing (and rotation) is synchronous; for a non-blocking version see AsyncRotatingFileOutput.

The size is read from the file once, when the output is created, and counted in memory from then on (in bytes, as written): the file system is not asked on every write. A file shortened or removed by someone else in the meantime is not noticed until the next rotation — which then comes early or late by that much, and loses nothing.

StructlogConfiguration.configure(
  output: rotatingFileOutput(
    'logs/app.log',
    maxSizeBytes: 1024 * 1024, // 1 MB per file
    maxBackups: 5, // keep app.log.0 .. app.log.4
  ),
);

final log = getLogger();
for (var i = 0; i < 100000; i++) {
  log.info('iteration', context: {'i': i});
}
// logs/app.log holds the newest entries; logs/app.log.0, .1, ...
// hold progressively older ones, oldest discarded past maxBackups.

Implementation

OutputFunction rotatingFileOutput(
  String filePath, {
  int maxSizeBytes = 10 * 1024 * 1024, // 10MB default
  int maxBackups = 5,
}) {
  final file = File(filePath);
  final dir = file.parent;
  if (!dir.existsSync()) {
    dir.createSync(recursive: true);
  }

  void rotate() {
    // Delete oldest backup
    final oldest = File('$filePath.${maxBackups - 1}');
    if (oldest.existsSync()) {
      oldest.deleteSync();
    }
    // Shift backups
    for (var i = maxBackups - 2; i >= 0; i--) {
      final src = File('$filePath.$i');
      final dst = File('$filePath.${i + 1}');
      if (src.existsSync()) {
        src.renameSync(dst.path);
      }
    }
    // Move current to .0
    if (file.existsSync()) {
      file.renameSync('$filePath.0');
    }
  }

  var size = file.existsSync() ? file.lengthSync() : 0;

  return (Map<String, dynamic> entry, LogLevel level) {
    if (size >= maxSizeBytes) {
      rotate();
      size = 0;
    }
    final bytes = utf8.encode('${encodeLogEntry(entry)}\n');
    file.writeAsBytesSync(bytes, mode: FileMode.append);
    size += bytes.length;
  };
}