# File log
**Location**: [file module](/logging/modules/file.md).
```csharp
var fileLog = new FileLog(new FileLogSettings());
var dynamicallyConfiguredLog = new FileLog(() => ObtainSettings());
```
### Features
* Customizable [output templates](/logging/concepts/formatting/output-templates.md).
* 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.
### Configuration
`FileLog` instances are configured with `FileLogSettings`, exposing following options:
| Option | Default value | Description |
| ---------------------------- | --------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `FilePath` | `logs/log` | 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.
|
| `OutputTemplate` | `OutputTemplate.Default` | [Output template](/logging/concepts/formatting/output-templates.md) to render log events with. |
| `FormatProvider` | `null` | An optional format provider used when applying custom [format specifiers](/logging/concepts/formatting/format-specifiers.md). |
| `FileOpenMode` | `Append` | Specifies the way to treat an existing log file: append or rewrite. |
| `RollingStrategy.Type` | `None` | Type of rolling strategy. Possible values are `None`, `ByTime`, `BySize` and `Hybrid`. |
| `RollingStrategy.Period` | `Day` | Period of switching to the next file when using `ByTime` or `Hybrid` rolling strategies. Possible values are `Day`, `Hour`, `Minute` and `Second`. |
| `RollingStrategy.MaxSize` | `1073741824` | Size threshold for switching to the next file when using `BySize` or `Hybrid` rolling strategies, specified in bytes. |
| `RollingStrategy.MaxFiles` | `5` | 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.
|
| `Encoding` | `UTF8` | Produced file's encoding. |
| `OutputBufferSize` | `65536` | File stream buffer size, specified in bytes. |
| `EnabledLogLevels` | `Debug, Info, Warn, Error, Fatal` | A list of enabled log levels. |
| `EventsQueueCapacity` | `50000` | Capacity of the internal log events queue. |
| `FileSettingsUpdateCooldown` | `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` 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](/logging/guarantees.md) 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](https://vostok.gitbook.io/configuration/): just construct the log with a delegate that obtains `FileLogSettings` from a `ConfigurationProvider`.
---
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