使用 HttpContext.Current.Application 存储简单数据

Question

I want to store a small list of a simple object (containing three strings) in my ASP.NET MVC application. The list is loaded from the database and it is updated rarely by editing some values in the site's admin area.

I'm thinking of using HttpContext.Current.Application to store it. This way I can load it in the Global.asax:

protected void Application_Start()
{
    RegisterRoutes(RouteTable.Routes);

    HttpContext.Current.Application["myObject"] = loadDataFromSql(); // returns my object
}

And then can easily reference it from any controllers or views as needed. Then in the event, the admin area calls the updateMyObject controller action, I can just update the DB and load it in again and replace HttpContext.Current.Application["myObject"] .

Are there any cons to doing this? It seems like it will work fine for what I am trying to achieve, however does anyone know of a better way to do this, assuming there is some major disadvantage to the method I've laid out?

Answer 1

What you actually do is Caching , and it's great, since you reduce calls to an external storage (a database or a file, whatever). The trade-off is memory usage, of course. Now, almost any modern web framework, including ASP.NET, includes some kind of a caching mechanism. Either you use it, or you use some kind of a global variable.

Storing data in ASP.NET's built-in Cache object has some significant advantages, since this mechanism actually checks the memory usage and removes the cached data according to some rules.

However, if the data you want to cache is intensively used across the application, and its size is not too large (say, smaller than 1 MB), you may want to store it in as a global variable.

In ASP.NET, global variables are achieved by either using the Application object, like you described in your question, or by writing public static properties/fields in an internal/public class.

Here's my solution to static properties. Note that I use a locking object, to protect the inner data from corruption. It looks like this:

public class WhateverClass
{
  private static object theLocker = new object();
  private static YourDataType theData;
  public static YourDataType TheData
  {
    get
    {
      lock (theLocker)
      {
        return theData;
      }
    }
    set
    {
      lock (theLocker)
      {
        theData = value;
      }
    }
  }
}

The usage is very simple:

First time, in Application_Start:

protected void Application_Start()
{
    RegisterRoutes(RouteTable.Routes);

    WhateverClass.TheData = loadDataFromSql();
}

In any controller:

var myData = WhateverClass.TheData;

This approach is better because you have type safety, since this public static property can be explicitly declared with the exact type. In addition, this kind of storage is more testable since it doesn't depend on the web context.

HTH!

Answer 2

HttpContext.Current.Application is essentially a hangover that is needed for backwards compatibility with classic ASP. It's essentially a static Hashtable with classic ASP locking semantics (Application.Lock / Application.UnLock).

As a weakly-typed Hashtable, you will need to cast objects you retrieve:

MyObject myObject = (MyObject) HttpContext.Current.Application["myObject"];

In an ASP.NET application that is not a migration from classic ASP, I would prefer using other standard .NET stuff, such as:

• A static field, using .NET locking semantics if you need locking (eg the C# lock keyword, or a ReaderWriterLockSlim instance, depending on your requirements):

  static MyObject myObject = LoadFromSql();

• The ASP.NET Cache - which has rich functionality for managing expiration, dependencies, ...

Answer 3

Yes, using HttpContext.Current.Application will work fine for what you are doing. No problems.

HttpContext.Current.Application is simply a reference to the static global HttpApplicationState object in .NET for your Web Application, of which there should be one global instance per web application. By storing data there, you provide fast, thread-safe access to your global variables. Be sure to lock them when updating values, as in this example:

System.Web.HttpContext.Current.Application.Lock();
System.Web.HttpContext.Current.Application["WebApplicationPath"] = MyWebApplicationPath;
System.Web.HttpContext.Current.Application.UnLock();

As others have mentioned, you can also create a series of static classes in your App_Code or other folder and there store global static values as well as your HttpContext.Current.Application values, where they can be safely checked for values or updated from the database, or update and check each other, working in tandem. I usually create a static global class to assist in the management and retrieval of the Application variables I store. In this way you have both the state dictionary of the HttpApplicationState class and the web application static objects working together to share and maintain global values. (Keep in mind each static class is assigned per worker process and there may be as many as 10 WP on average by default on many IIS web servers/web applications. So keep data in static types to a minimum.)

Keep in mind as some mentioned server farms do not share Application state. There are many ways to manage this. I'm not a fan of cache because of the ways in which it can expire, fail, become stale, or get corrupted. A simpler solution is to simply use use the database and URL querystrings to communicate across servers and maintain state. Good luck!

Answer 4

If you're deploying to a single web server the approach would work. Consider the Cache object for this as it provides more options for expiration if you need such functionality. (See a comparison, albeit an aged one, here .)

If you're ever going to deploy to a web server farm or equivalent you should use memcached or another web farm friendly caching mechanism. Both the Application and Cache objects only typically exist in a single server context; if your user could be hitting multiple web servers during their session (and the cache needs to be identical) you'll need a shared cache that can be seen from each of the potential web servers.

Regardless of which path you take you will need to invalidate/reload your cache whenever the underlying data changes, which is custom code that varies by app.

This approach works well and can speed things considerably but it's a bit more work than you may realize at first glance...

Answer 5

Application_Start really only gets fired on App Pool Recylce's, IIS Resets or reboots. If your updating these values that infrequently, why not store them in your web.config and access them that way?

That being said, I don't think there is anything wrong with your approach. Though more typically I've seen people using config files for rarely changed values.