Serilog Rolling File Sink (roll-on-file-size alternative)

This is a rolling file sink that allows you to specify roll over behaviour based on file size.

Getting started

Install the Serilog.Sinks.RollingFileAlternate package from NuGet:

Install-Package Serilog.Sinks.RollingFileAlternate

To configure the sink in C# code, call WriteTo.RollingFileAlternate() during logger configuration:

var log = new LoggerConfiguration()
    .WriteTo.RollingFileAlternate(".\\logs")
    .CreateLogger();
    
log.Information("This will be written to the rolling file set");

The sink is configured with the path of a folder for the log file set:

logs\20160631-00001.txt
logs\20160701-00001.txt
logs\20160701-00002.txt

Important: Only one process may write to a log file at a given time. For multi-process scenarios, either use separate files or one of the non-file-based sinks.

File size limit

The file size limit, beyond which a new log file will be created, is specified with the fileSizeLimitBytes parameter.

    .WriteTo.RollingFileAlternate(".\\logs", fileSizeLimitBytes: 1024 * 1024)

The default if no limit is specified is to roll every two megabytes.

XML <appSettings> configuration

To use the alternate rolling file sink with the Serilog.Settings.AppSettings package, first install that package if you haven't already done so:

Install-Package Serilog.Settings.AppSettings

Instead of configuring the logger in code, call ReadFrom.AppSettings():

var log = new LoggerConfiguration()
    .ReadFrom.AppSettings()
    .CreateLogger();

In your application's App.config or Web.config file, specify the rolling file sink assembly and sink configuration under the <appSettings> node:

<configuration>
  <appSettings>
    <add key="serilog:using:RollingFileAlternate" value="Serilog.Sinks.RollingFileAlternate" />
    <add key="serilog:write-to:RollingFileAlternate.logsDirectory" value=".\logs" />
    <add key="serilog:write-to:RollingFileAlternate.fileSizeLimitBytes" value="1048576" />

The parameters that can be set through the serilog:write-to:RollingFileAlternate keys are the method parameters accepted by the WriteTo.RollingFileAlternate() configuration method.

In XML and JSON configuration formats, environment variables can be used in setting values. This means, for instance, that the log folder path can be based on TMP or APPDATA:

    <add key="serilog:write-to:RollingFileALternate.logsFolder" value="%APPDATA%\MyApp\logs" />

JSON appsettings.json configuration

To use the alternate rolling file sink with Microsoft.Extensions.Configuration, for example with ASP.NET Core or .NET Core, use the Serilog.Settings.Configuration package. First install that package if you have not already done so:

Install-Package Serilog.Settings.Configuration

Instead of configuring the alternate rolling file directly in code, call ReadFrom.Configuration():

var configuration = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .Build();

var logger = new LoggerConfiguration()
    .ReadFrom.Configuration(configuration)
    .CreateLogger();

In your appsettings.json file, under the Serilog node, :

{
  "Serilog": {
    "WriteTo": [{
      "Name": "RollingFileAlternate",
      "Args": {
        "logDirectory": ".\\logs",
        "fileSizeLimitBytes": 1048576
      }
    }]
  }
}

Controlling event formatting

The alternate rolling file sink creates events in a fixed text format by default:

2016-07-06 09:02:17.148 +10:00 [Information] HTTP "GET" "/" responded 200 in 1994 ms

The format is controlled using an output template, which the alternate rolling file configuration method accepts as an outputTemplate parameter.

The default format above corresponds to an output template like:

    .WriteTo.RollingFileAlternate(".\\logs",
        outputTemplate: "{Timestamp:yyyy-MM-dd HH:mm:ss.fff zzz} [{Level}] {Message}{NewLine}{Exception}")