ASP.NET Core Logging Solution

.NET Core Logging

This package makes it a one-liner - loggerFactory.AddFile() - to configure top-quality file logging for ASP.NET Core apps.

• Text or JSON file output
• Files roll over on date; capped file size
• Request ids and event ids included with each message
• Writes are performed on a background thread
• Files are periodically flushed to disk (required for Azure App Service log collection)
• Fast, stable, battle-proven logging code courtesy of Serilog

You can get started quickly with this package, and later migrate to the full Serilog API if you need more sophisticated log file configuration.

Getting started

1. Add the NuGet package as a dependency of your project either with the package manager or directly to the CSPROJ file:

2. In your Program class, configure logging on the web host builder, and call AddFile() on the provided loggingBuilder.

 Host.CreateDefaultBuilder(args)
        .ConfigureLogging((hostingContext, loggingBuilder) =>
        {
            // loggingBuilder.AddFile("Logs/Logs_{Date}.txt");
            loggingBuilder.AddFile(hostingContext.Configuration.GetSection("Logging"));
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup();
        });

3. Add a custom excetion filter as a global exception handler:

    public class GlobalExceptionFilter : IExceptionFilter
    {
        private readonly ILogger _logger;

        public GlobalExceptionFilter(ILogger logger)
        {
            _logger = logger;
        }

        public void OnException(ExceptionContext context)
        {
            var loggingBuilder = new StringBuilder();

            if (context.HttpContext.Request.GetDisplayUrl() != null)
                loggingBuilder.AppendLine($"\tUrl: {context.HttpContext.Request.GetDisplayUrl()}");

            loggingBuilder.AppendLine($"\tIp: {context.HttpContext.Connection.RemoteIpAddress}");

#if DEBUG
            foreach (var key in context.HttpContext.Request.Headers.Keys)
            {
                loggingBuilder.AppendLine($"\t{key}: {context.HttpContext.Request.Headers[key]}");
            }
#endif

            loggingBuilder.AppendLine($"\tError Message: {context.Exception.Message}");
            if (context.Exception.InnerException != null)
            {
                PrintInnerException(context.Exception.InnerException, loggingBuilder);
            }

            loggingBuilder.AppendLine($"\tError HelpLink: {context.Exception.HelpLink}");
            loggingBuilder.AppendLine($"\tError StackTrace: {context.Exception.StackTrace}");

            _logger.LogError(loggingBuilder.ToString());
        }

        public void PrintInnerException(Exception ex, StringBuilder loggingBuilder)
        {
            loggingBuilder.AppendLine($"\tError InnerMessage: {ex.Message}");
            if (ex.InnerException != null)
            {
                PrintInnerException(ex.InnerException, loggingBuilder);
            }
        }
    }

4. In your Startup class, configure the global exception handler on the ConfigureServices method, so we can catch all unhandled exceptions:

public void ConfigureServices(IServiceCollection services)
{
    ...

    services.AddControllersWithViews(options =>
    {
        options.Filters.Add();
    });
  
    ...
}

public IActionResult Privacy()
{
    throw new Exception("Unhandled exception");
    return View();
}

5. In your Startup class, add the log directory as static on the Configure method, so we can view the log directory:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    ...

    app.UseStaticFiles();

    //add the log directory as static, so we can view the log directory
    app.UseFileServer(new FileServerOptions()
    {
        FileProvider = new PhysicalFileProvider(Path.Combine(Directory.GetCurrentDirectory(), @"Logs")),
        RequestPath = new PathString("/Log"),
        EnableDirectoryBrowsing = true
    });

    app.UseRouting();

    ...
}

Done! The framework will inject ILogger instances into controllers and other classes:

class HomeController : Controller
{
    readonly ILogger _log;

    public HomeController(ILogger log)
    {
        _log = log;
    }

    public IActionResult Index()
    {
        _logger.LogInformation("Hello, world!");
        _logger.LogError(new Exception("Custom exception"), "Custom exception");
    }
}

The events will appear in the log file:

2016-10-18T11:14:11.0881912+10:00 0HKVMUG8EMJO9 [INF] Hello, world! (f83bcf75)

File format

By default, the file will be written in plain text. The fields in the log file are:

Field	Description	Format	Example
Timestamp	The time the event occurred.	ISO-8601 with offset	2016-10-18T11:14:11.0881912+10:00
Request id	Uniquely identifies all messages raised during a single web request.	Alphanumeric	0HKVMUG8EMJO9
Level	The log level assigned to the event.	Three-character code in brackets	[INF]
Message	The log message associated with the event.	Free text	Hello, world!
Event id	Identifies messages generated from the same format string/message template.	32-bit hexadecimal, in parentheses	(f83bcf75)
Exception	Exception associated with the event.	Exception.ToString() format (not shown)	System.DivideByZeroException: Attempt to divide by zero\r\n\ at...

To record events in newline-separated JSON instead, specify isJson: true when configuring the logger:

loggingBuilder.AddFile("Logs/myapp-{Date}.txt", isJson: true);

This will produce a log file with lines like:

{"@t":"2016-06-07T03:44:57.8532799Z","@m":"Hello, world!","@i":"f83bcf75","RequestId":"0HKVMUG8EMJO9"}

The JSON document includes all properties associated with the event, not just those present in the message. This makes JSON formatted logs a better choice for offline analysis in many cases.

Rolling

The filename provided to AddFile() should include the {Date} placeholder, which will be replaced with the date of the events contained in the file. Filenames use the yyyyMMdd date format so that files can be ordered using a lexicographic sort:

log-20160631.txt
log-20160701.txt
log-20160702.txt

To prevent outages due to disk space exhaustion, each file is capped to 1 GB in size. If the file size is exceeded, events will be dropped until the next roll point.

Message templates and event ids

The provider supports the templated log messages used by Microsoft.Extensions.Logging. By writing events with format strings or message templates, the provider can infer which messages came from the same logging statement.

This means that although the text of two messages may be different, their event id fields will match, as shown by the two "view" logging statements below:

2016-10-18T11:14:26.2544709+10:00 0HKVMUG8EMJO9 [INF] Running view at "/Views/Home/About.cshtml". (9707eebe)
2016-10-18T11:14:11.0881912+10:00 0HKVMUG8EMJO9 [INF] Hello, world! (f83bcf75)
2016-10-18T11:14:26.2544709+10:00 0HKVMUG8EMJO9 [INF] Running view at "/Views/Home/Index.cshtml". (9707eebe)

Each log message describing view rendering is tagged with (9707eebe), while the "hello" log message is given (f83bcf75). This makes it easy to search the log for messages describing the same kind of event.

Additional configuration

The AddFile() method exposes some basic options for controlling the connection and log volume.

Parameter	Description	Example value
pathFormat	Filename to write. The filename may include {Date} to specify how the date portion of the filename is calculated. May include environment variables.	Logs/log-{Date}.txt
minimumLevel	The level below which events will be suppressed (the default is LogLevel.Information).	LogLevel.Debug
levelOverrides	A dictionary mapping logger name prefixes to minimum logging levels.
isJson	If true, the log file will be written in JSON format.	true
fileSizeLimitBytes	The maximum size, in bytes, to which any single log file will be allowed to grow. For unrestricted growth, passnull. The default is 1 GiB.	1024 * 1024 * 1024
retainedFileCountLimit	The maximum number of log files that will be retained, including the current log file. For unlimited retention, pass null. The default is 31.	31
outputTemplate	The template used for formatting plain text log output. The default is {Timestamp:o} {RequestId,13} [{Level:u3}] {Message} ({EventId:x8}){NewLine}{Exception}	{Timestamp:o} {RequestId,13} [{Level:u3}] {Message} {Properties:j} ({EventId:x8}){NewLine}{Exception}

appsettings.json configuration

The file path and other settings can be read from JSON configuration if desired.

In appsettings.json add a "Logging" property:

{
  "Logging": {
    "PathFormat": "Logs/log-{Date}.txt",
    "LogLevel": {
      "Default": "Debug",
      "Microsoft": "Information"
    }
  }
}

And then pass the configuration section to the AddFile() method:

loggingBuilder.AddFile(Configuration.GetSection("Logging"));

In addition to the properties shown above, the "Logging" configuration supports:

Property	Description	Example
Json	If true, the log file will be written in JSON format.	true
FileSizeLimitBytes	The maximum size, in bytes, to which any single log file will be allowed to grow. For unrestricted growth, passnull. The default is 1 GiB.	1024 * 1024 * 1024
RetainedFileCountLimit	The maximum number of log files that will be retained, including the current log file. For unlimited retention, pass null. The default is 31.	31
OutputTemplate	The template used for formatting plain text log output. The default is {Timestamp:o} {RequestId,13} [{Level:u3}] {Message} ({EventId:x8}){NewLine}{Exception}	{Timestamp:o} {RequestId,13} [{Level:u3}] {Message} {Properties:j} ({EventId:x8}){NewLine}{Exception}

Using the full Serilog API

This package is opinionated, providing the most common/recommended options supported by Serilog. For more sophisticated configuration, using Serilog directly is recommened. See the instructions in Serilog.AspNetCore to get started.

The following packages are used to provide AddFile():

• Serilog - the core logging pipeline
• Serilog.Sinks.RollingFile - rolling file output
• Serilog.Formatting.Compact - JSON event formatting
• Serilog.Extensions.Logging - ASP.NET Core integration
• Serilog.Sinks.Async - async wrapper to perform log writes on a background thread