Diagnostics

Nancy has a built in diagnostics dashboard that enables you to inspect the state and behavior of your application at runtime. The dashboard can easily be extended with custom functionality and can be tailored to suit the needs of your application.

The diagnostics dashboard is embedded in the main Nancy assembly. The back-end is a Nancy application that is serving a backbone.js front-end. It also makes use of the SuperSimpleViewEngine to render out views. It is built to be fast, extensible and cross-browser compliant.

You reach the dashboard by pointing your browser to http://<address-of-your-application>/_Nancy/. However before being able to use the dashboard, you first need to configure it.

Configuring access to the dashboard

The dashboard is automatically available to you, in your application. However in order to gain access, you need to configure a password. Without the password you will be presented with a page telling you that the dashboard needs to be configured in order for you to use it, along with instructions on how to perform the configuration.

To configure a password for your diagnostics dashboard, you need to override the DiagnosticsConfiguration property of your Bootstrapper. Once overridden, you should return an instance of the DiagnosticsConfiguration that has had a password assigned to the Password property.

public class CustomBootstrapper : DefaultNancyBootstrapper
{
    protected override DiagnosticsConfiguration DiagnosticsConfiguration
    {
        get { return new DiagnosticsConfiguration { Password = @"A2\6mVtH/XRT\p,B"}; }
    }
}

When this has been done, you will be presented with a login screen instead of a configuration notification, the next time you browse the dashboard. A valid login will produce a login cookie that has a sliding expiration set to 15 minutes.

Each access to the dashboard will extend the lifetime, of the cookie, for another 15 minutes. If the cookie expires, before you revisit the dashboard, you will be redirected back to the login form.

Removing diagnostics

As mentioned earlier, the diagnostics dashboard is automatically wired up for your application. However, should you wish to completely remove the dashboard you can do so by performing a call to DiagnosticsHook.Disable() and it will no longer be accessible.

It is recommended that the call to DiagnosticsHook.Disable() is performed from the ApplicationStartup method in your bootstrapper. This will ensure that the dashboard is disabled as soon as your application starts up.

public class CustomBootstrapper : DefaultNancyBootstrapper
{
    protected override void ApplicationStartup(TinyIoc.TinyIoCContainer container, IPipelines pipelines)
    {
        DiagnosticsHook.Disable(pipelines);
    }
}

So what kind of tools are there?

The dashboard presents you with four options when you access it; Information, Interactive Diagnostics, Request Tracing and Configuration. These are the main sections of the dashboard and your gateway to taking a peek under the hood of your application at runtime.

Information

The information page provides information about the setup of the Nancy environment, such as assembly version, which components are being used by Nancy, what view engines are available, what host is being used and so on.

Configuration

In Nancy the StaticConfiguration type can be used to control the behavior of your application, for example controlling the case-sensitivity, if caching should be used and so on.

The configuration page provides access to the StaticConfiguration through a user interface, so that the behavior of the application can be altered at runtime.

It should be noted that any changes that are applied through the configuration page are not persisted and will be reset, to their configured value, when the application is restarted.

To configure these behaviors permanently you should still alter their value by setting them from the ApplicationStartup method in your bootstrapper.

Request tracing

As you would expect, request tracing, provides a window for inspecting the behavior of individual requests as they are processed by Nancy.

For performance reasons, request tracing is disabled by default and can either be turned on using the Configuration page on the dashboard or by setting the StaticConfiguration.EnableRequestTracing property to true

public class CustomBootstrapper : DefaultNancyBootstrapper
{
    protected override void ApplicationStartup(TinyIoC.TinyIoCContainer container, IPipelines pipelines)
    {
        StaticConfiguration.EnableRequestTracing = true;
    }
}

The trace log is accessible though the NancyContext and it is easy to add your own information, in order to provide an even richer experience for your application.

public class HomeModule : NancyModule
{
    public HomeModule()
    {
        Get["/"] = parameters => {
            this.Context.Trace.TraceLog.WriteLog(s => s.AppendLine("Root path was called"));
            return HttpStatusCode.Ok;
        };
    }
}

The WriteLog method takes a function that will be passed an instance of a StringBuilder at runtime. The reason it is taking a function is that when diagnostics is disabled, Nancy will simply not invoke the function so there will be no performance penalties for leaving the trace code in your application.

Interactive diagnostics

What if you could query your application about pretty much anything during runtime? This is exactly what the interactive diagnostics feature enables you to do. It does not provide a query language for you to write expressions in (even though it would be possible for you to add in your own application if you so wish), but it enables you to implement diagnostics tools as normal C# types and then invoke them from the dashboard.

This enables you to use the full capabilities of the language, platform and Nancy (including dependency injection) to deliver pretty much any diagnostics capability you can think of. As long as your type implements the IDiagnosticsProvider interface, then the Nancy diagnostics will automatically discover it and expose it as an interactive diagnostics tool.

The IDiagnosticsProvider interface

Anything that implements this type lets Nancy know that it is capable of providing interactive diagnostics tools for the dashboard. The interface itself is very basic and provides mostly metadata about the provider

/// <summary>
/// Defines the functionality a diagnostics provider.
/// </summary>
public interface IDiagnosticsProvider
{
    /// <summary>
    /// Gets the name of the provider.
    /// </summary>
    /// <value>A <see cref="string"/> containing the name of the provider.</value>
   string Name { get; }

   /// <summary>
   /// Gets the description of the provider.
   /// </summary>
   /// <value>A <see cref="string"/> containing the description of the provider.</value>
   string Description { get; }

   /// <summary>
   /// Gets the object that contains the interactive diagnostics methods.
   /// </summary>
   /// <value>An instance of the interactive diagnostics object.</value>
   object DiagnosticObject { get; }
}

The most important member, of the IDiagnosticsProvider interface, is the DiagnosticObject property. The object returned by this property is the one that will be wired up on the dashboard.

So what does the diagnostics object look like?

There is nothing special about a diagnostics object. It does not have to be the same type as the one that implements the IDiagnosticsProvider interface, nor does it have to implement any other interfaces. The thing to understand is that any public method on the object are going to be exposed on the interactive diagnostics dashboard.

These methods can have primitive (string, int and so on) parameters and return any object, as long as it is possible to JSON serialize it. This means that method invocation will be strongly-typed and no need to perform cast or type checks.

When you want to invoke one of these methods, from the diagnostics dashboard, Nancy presents you with a form that contains input fields that will be used to provide values to the methods parameter once it is invoked. The input fields contains information about the parameter name and type.

The output of the method will then be rendered using the JSON Report Format script, created by ServiceStack.

Providing a description of your method

It is possible to provide a description for a diagnostics method that will be included in the form that is used to invoke the method.

There are two ways you can provide the description; by using an attribute on the method or by providing a property that will return the description.

To use the attribute you simply add the Nancy.Diagnostics.DescriptionAttribute to the method and provide a description and that is all there is to it. However sometimes you may find yourself requiring a bit more control of the description string, for example if you want to provide localized strings.

When you find yourself in these situations you can use the property approach. Nancy uses a convention whereby it tries to find a property that has the same name as the method, but with an added Description suffix appended to it. An example of this would be a NameOfYourMethodDescription property for the NameOfYourMethod method.

If your method has been decorated with the DescriptionAttribute and you also have a description property declared, then the property will take precedence over the attribute.

Customizing the output with templates

As previously mentioned, the output of your diagnostics method will be rendered using the JSON Report Format and will use the formatting templates that it declares. However, Nancy provides you with the option to use custom output templates.

An output template is a Mustache template and you can specify a unique template for each of your diagnostics method. Just like with method descriptions, you have the possibility, to provide the template, using either an attribute or a property.

For templates, the attribute that is used is the Nancy.Diagnostics.TemplateAttribute and the naming convention for the property is name of the property with a Template suffix.

Creating a simple diagnostics provider

Let's create a simple diagnostics provider with a method that will greet someone by their name. Not really a real scenario but it is a basic enough sample to show the anatomy of a diagnostics provider without getting distorted by the implementation.

public class CustomDiagnosticsProvider : IDiagnosticsProvider
{
    public string Name
    {
        get { return "Custom diagnostics provider"; }
    }

    public string Description
    {
        get { return "Provides custom diagnostics capabilities"; }
    }

    public object DiagnosticObject
    {
        get { return this; }
    }

    [Description("Greets a person using their name")]
    [Template("<h1>{{model.Result}}</h1>")]
    public string Greet(string name)
    {
        return string.Concat("Hi, ", name);
    }
}

The provider exposes a single method, the Greetmethod. The method takes a single string parameter which is the name of the person to greet. In addition the method has been given a description, using the DescriptionAttribute, that will be displayed in the form that will be used to invoke the method from the dashboard.

Instead of using the default output format, the method also provides a custom Mustache template that will be used to format the output.

A proper diagnostics provider would probably take a couple of constructor dependencies in order to be able to access the various pieces of the application.

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