File log

Location: file module.
var fileLog = new FileLog(new FileLogSettings());
var dynamicallyConfiguredLog = new FileLog(() => ObtainSettings());


  • Customizable output templates.
  • Customizable rolling strategies:
    • By size (roll over to a new file when current one reaches given size)
    • By time (roll over to a new file every second/minute/hour/day)
    • Hybrid (both by size and time)
    • Automatic cleanup of stale files
  • Multiplexing: multiple FileLog instances inside a single process may write to the same file without performance penalties.
  • Log method is nonblocking: log events are placed into an internal queue, buffered and efficiently written to files in the background.


FileLog instances are configured with FileLogSettings, exposing following options:
Default value
Path to the log file. If rolling is enabled, this path serves as a base path and gets combined with various suffixes determined by the rolling strategy.
Position of the rolling suffix can be customized by inserting a special {RollingSuffix} placeholder somewhere in the file name.
This path is interpreted as relative to current working directory, though absolute paths are supported too.
File path is not allowed to point to a directory.
Output template to render log events with.
An optional format provider used when applying custom format specifiers.
Specifies the way to treat an existing log file: append or rewrite.
Type of rolling strategy. Possible values are None, ByTime, BySize and Hybrid.
Period of switching to the next file when using ByTime or Hybrid rolling strategies. Possible values are Day, Hour, Minute and Second.
Size threshold for switching to the next file when using BySize or Hybrid rolling strategies, specified in bytes.
The number of most recent files to keep when using a rolling strategy: older files are automatically removed. Specify zero value to avoid removing old files.
To avoid bringing down app with runaway disk space only the most recent 5 files are stored by default.
Produced file's encoding.
File stream buffer size, specified in bytes.
Debug, Info, Warn, Error, Fatal
A list of enabled log levels.
Capacity of the internal log events queue.
1 second
Cooldown for enforcing file-related settings (file path, rolling strategy, etc). This means that when conditions are met to switch to the next part of log file or reopen the file with another name/options due to change in settings, the switching may be delayed for up to value of this option.
All of these parameters are optional.
Most of listed options support dynamic reconfiguration when a constructor with Func<FileLogSettings> is used and provided delegate returns different instances of FileLogSettings. FileLog will react to these changes by either reopening the file, switching to a new file or simply using the new values where applicable.
The only notable exception to this rule is the EventsQueueCapacity option: it has a per-file scope and can't be changed dynamically.

Usage tips

  • Keep in mind that log messages are written to files in the background. Use Flush or FlushAsync methods of FileLog to ensure that all logged events have been actually written at any given moment.
  • Remember to dispose of FileLog instances when they are no longer needed in order to close file handles. Dispose method also flushes all remaining messages before closing the file.
  • Internal queue with limited capacity implies that log messages may be lost in the event of overflow (see guarantees section for more on this topic). Use EventsLost property of a single FileLog instance or static TotalEventsLost property to check if any events have been discarded.
  • If any log events are lost due to internal queue overflow, FileLog emits a special diagnostic message that looks like this:
    • [FileLog] Buffer overflow. 100 log events were lost (events queue capacity = 50000).
  • When encountering any other severe internal error (such as inability to open the file), FileLog reports it to console.
  • FileLog can be configured from any external source (such as a local settings file) with Vostok.Configuration: just construct the log with a delegate that obtains FileLogSettings from a ConfigurationProvider.