Windows Azure

Caching data is usually a good idea, especially when creating APIs for mobile clients and the user may pay for each transferred byte. There are many approaches to caching data (I recommend reading this article), in my last NancyFX project I used ETag.

ETag

ETag is a HTTP header that acts as a hash of the data. When the server returns a response, it computes a hash of the data and sends it to the client. When the client requests the data again, it includes the ETag in its request. The server compares the ETag with the hash of the current data and if they match (the data did not change), it returns an empty responses with a HTTP 304 status code.

NancyFX is a great .NET framework well suited for creating REST APIs. There are many ways how to approach authentication, the simplest one is the good old Forms Authentication. The idea of Forms Authentication is that the user logs in with a username and password and gets a cookie, the protected endpoints then check the cookie. NancyFX supports Forms Authentication with the Nancy.Authentication.Forms package. The documentation describes how to use it on a web page, but to use it with a REST API a few changes are needed.

Forms Authentication differences for REST API

There are things you want to do differently in a REST API than on a web page. If a user tries to access a protected endpoint, the Forms Authentication on a normal web page redirects him to the login page. In REST API, you typically want the endpoint just to return HTTP 401, no redirects. Also, when a user successfully logs in, you just typically want to return HTTP 200, no redirects.

In a recent larger ASP.NET MVC project running in Windows Azure I needed to provide the advanced users a way to execute custom scripts directly through the webbrowser. A kind of a scripting console where users can create and execute their own scripts, that interact with the project and automate some tasks (that would otherwise require too many clicks).

Choosing the language

I thought about using Lua but I needed an easy way to integrate with some classes (repositories) used in the project. I decided to use C# as the scripting language and implement the scripting console using Roslyn. Roslyn is a really neat project and I recommend taking a look at it, if you do not already know it.

I extracted the main idea of my implementation of the scripting console and posted it on GitHub. I will walk you through it in this blog post.

In my current project, I have chosen NancyFx to implement a REST API. NancyFx is a .NET framework known for its “super-duper-happy-path“.

In one use case I generate a ZIP file in the temp folder and I want the API to return this ZIP file. NancyFx contains a Responses.AsFile helper but it works only with paths relative to the application.

If you have an absolute path of a file, you cannot use it. You need to create a StreamResponse and return the file as an attachment

var file = new FileStream(zipPath, FileMode.Open);
string fileName = //set a filename

var response = new StreamResponse(() => file, MimeTypes.GetMimeType(fileName));
                  
return response.AsAttachment(fileName);

I am currently working on a fairly large Windows Azure projects that among other things conatins a Web Role where I use Ninject as a dependency container. As the business logic library grew larger I found myself writing a lot of repeating boiler plate code to log exceptions in many important methods. I wantet to remove all the boiler plate code and create a custom attribute, say LogErrorAttribute with one simple goal: each method decorated woth this attribute should log info about any occuring exception.

IL weaving?

I have been using Fody for some time to implement the INotifyPropertyChanged calls for me in Windows Phone and Windows 8 projects so it was my first choice.

There is a Fody.MethodDecorator extensions that allows you to execute your code on a methods start, exit and exception. Writing the exception to Console is trivial, but I wanted to use a implementation of my custom ILogFactory.

Windows Azure finally has a good F# support. Creating F# Worker Roles is supported right from the wizzard in Visual Studio and you can create a F# Web Role using the F# C# MVC template. I decided to try it out and the first thing I needed to implement was logging. I decided for logging to Azure Table Storage.

I assume you have a basic idea of how Azure Table Storage works. If not, there is a good guide on the Windows Azure website.

The first thing you need to do is to define your log entry class. You need to create a class, Azure Table Storage does not work with F# records. In my case I want to store a timestamp, message and severity.

[<DataServiceKey("PartitionKey", "RowKey")>]
type LogEntity() =    
    let mutable partitionKey = ""
    let mutable rowKey = ""
    let mutable message = ""
    let mutable timestamp = DateTime.Now
    let mutable severity = ""  
    member x.PartitionKey with get() = partitionKey and set v = partitionKey <- v
    member x.RowKey with get() = rowKey and set v = rowKey <- v
    member x.Message with get() = message and set v = message <- v
    member x.Timestamp with get() = timestamp and set v = timestamp <- v
    member x.Severity with get() = severity and set v = severity <- v

If you want to host your .NET project on the Internet, there are classic ASP.NET webhosting on one side of the spectrum and Microsoft Azure on the other. If you are looking for something in the middle, take a look at AppHarbor.

AppHarbor

AppHarbor is a flexible and scalable .NET Platform-as-a-Service, that you can even use for free, limited to one web or worker role. It runs in AWS and is quite similiar to Heroku.

Variety of Add-ons

There are many add-on supported by AppHarbor, including SQL Server, MongoDB, RavenDB, MySQL, Memcacher … Many of these ad-ons offer free versions so you can test them for free. If you run into some problems, you can use the support forums or StackOverflow.

From time to time I need to develop or maintain a small Windows Azure project. This time I wanted to create the whole project in F#. The first thing I needed to to was to update the Azure toolkit 1.8 (October 2012) but updating Azure Toolkit is always a pain. I started Web Platform Installer, selected Azure Toolkit 1.8, Azure Tools 1.8 for VS2012 and installed everything including the dependencies. What was the result? The whole Azure integration in Visual Studio 2012 stopped working. The Azure templates completely disappeared and Visual Studio only offered me downloading the Azure Toolkit, which failed, because it was already installed. I ended up completely uninstalling everything with the name containing Azure and installing the Azure toolkit again using the link from Visual Studio.

Why cannot this work better?