Wednesday, 30 January 2013

Don't assume anything about the lifetime of ActionFilters in MVC3

In previous versions of ASP.NET MVC, action filters were created per request except in a few cases. This behavior was never a guaranteed behavior but merely an implementation detail and the contract for filters was to consider them stateless. In ASP.NET MVC 3, filters are cached more aggressively. Therefore, any custom action filters which improperly store instance state might be broken.

Taken from the 'breaking changes' section of the release notes for MVC3.

The statement is clear enough to someone feeling the sting still from a recent live issue, but I've outlined a bad and good example of custom action filters below. Basically, if you find yourself needing to store state for the lifetime of a request, use the filterContext.HttpContext.Items collection.

Bad custom action filter

public class BadActionFilter : ActionFilterAttribute
    private string _userState;

    public override void OnActionExecuting(ActionExecutingContext filterContext)
        _userState = someSortOfDataUniqueToUser();

    public override void OnActionExecuted(ActionExecutedContext filterContext)
        // using _userState here is a bad idea!

Good custom action filter

public class GoodActionFilter : ActionFilterAttribute
    public override void OnActionExecuting(ActionExecutingContext filterContext)
        var userState = someSortOfDataUniqueToUser();
        filterContext.HttpContext.Items["userState"] = userState;

    public override void OnActionExecuted(ActionExecutedContext filterContext)
        var userState = filterContext.HttpContext.Items["userState"];
        // using userState here is perfectly fine!

Monday, 28 January 2013

Kanban Land is wonderful


UI and UX

I do not think much of a man who is not wiser today than he was yesterday.

A little older and a little wiser, at least I'd like to think so. I find myself considering design and usability more of late and I'm feeling positive about the results on the projects I'm working on. But I don't think I'm alone on this, thankfully I'm seeing this on a large scale across the web. Sites failing to properly consider design and user experience are beginning to stand out like a sore thumb. I have less time and patience for these sites (perhaps it's my age again?) and I'm quick to find an alternative if needed (perhaps I should drop Ling an email and tell her where she's going wrong?).

Here's some links to some sites I'm finding particularly useful at the moment:

  • Smashing Magazine - An online magazine for designers and developers with some great articles
  • UX Myths - A site dedicated to listing common UX misconceptions and why they don't hold true
  • UX Stack Exchange - A collaboratively edited question and answer site for user experience
  • User-centered Design - (article) type of UI design and a process in which the needs, wants, and limitations of end users of a product are given extensive attention at EACH STAGE of the design process

I'm also reading Rock Surgery Made Easy (by Steve Krugg) at the moment, which is perfect for those wanting to do some DIY usability testing.

Sunday, 13 January 2013

Clearing SQL Server cache for performance testing

You can restart windows or the windows service, but the more elegant way is to use DBCC DROPCLEANBUFFERS.

To drop clean buffers from the buffer pool, first use CHECKPOINT to produce a cold buffer cache. This forces all dirty pages for the current database to be written to disk and cleans the buffers. After you do this, you can issue DBCC DROPCLEANBUFFERS command to remove all buffers from the buffer pool.

USE MyDatabaseName;



PaaS offerings and AppHarbor

This may sound a little obvious, but writing the code for web apps is often the easy part for developers - because that's what developers are good at. I believe developers should be free to focus on writing the code and adding business value. In reality, the developers often get involved with the headaches that come from deploying, load balancing, scaling and monitoring the apps. But with more and more Platform as a Service (PaaS) offerings popping up, it seems like our lives as developers are beginning to get a little easier.

Our technical director at work (Adam Bird) made me aware of around 12 months ago. Adam likes Ruby and had been using Heroku, which was a big hit with Ruby developers because it was an easy-to-use PaaS offering. AppHarbor has been called the 'Heroku for .Net'. It runs on Amazon's AWS infrastructure and is designed to take the pain out of quickly and safely deploying, scaling and managing any .NET web application. Appharbour also offers an impressive add-on feature catalog, allowing developers to easily integrate and configure a wide array of database, queuing, monitoring, indexing and email services with their applications.

Integration with AppHarbor can be achieved using various repository hosting services (e.g. BitBucket, GitHub and Codeplex). Developers can use Git, Mercurial or TFS when deploying code to AppHarbor; integrating deployment in the developer's workflow by letting developers use the tools they love.

My experiences

I've been really impressed with what AppHarbor are offering and was surprised at how easy (and low cost) it was to get an application I had running on an virtual machine elsewhere ported over. One of the biggest selling points for me is integration with GitHub and BitBucket. If you are migrating apps over you need to bear in mind certain limitations too (see below). AppHarbor offer a free tier for you to play around with, and a lot of the add-ons do too. I'd strongly recommend you give it a try.

I've listed what I liked most below, followed by some potential issues/warnings I came across.


  • Low start-up costs
  • Your first deploy can take just a manner of minutes
  • Automatically deploy code from GitHub (or any of the other supported repository hosting services)
  • Support for compiling and running project tests; if the tests fail the app isn't deployed
  • Reverting to a previous app version is easily achieved through the control panel
  • Ability to integrate popular application monitoring add-ons (like New Relic) very easily

Potential issues/warnings...

  • If you are sharing forms auth cookies across apps hosted elsewhere you'll need to implement your own version of form auth and not rely on the .Net libraries (see AppHarbor blog)
  • File system access should be avoided due to the way Appharbor scales your app - put files in a shared data store
  • AppHarbor building and running tests is not going to work for larger projects with complicated tests
  • During my time playing around I've had a handful of intermittent yellow screens of death appear whilst using their control panel (not my own app) - which has made me question their stability