We're writing a class we'll use in our asp.net site. This class will pull down some json using HttpClients and such, and use it to provide information to other clients.
Some of this information will change very infrequently and it doesn't make sense to query for it on each client request.
For that reason I'm thinking of making a static constructor in this new class for the slow-changing information and stashing the results in a few static member variables. That'll save us a few HttpRequests down the line-- I think.
My question is, how long can I expect that information to be there before the class is recycled by ASP.Net and a new one comes into play, with the static constructor called once more? Is what I'm trying to do worth it? Are there better ways in ASP.Net to go about this?
I'm no expert on ASP.Net thread pooling or how it works and what objects get recycled and when.
Typical use of the new class (MyComponent, let's call it) would be as below, if that helps any.
//from mywebpage.aspx.cs:
var myComponent = new MyComponent();
myComponent.doStuff(); //etc etc.
//Method calls like the above may rely on some
//of the data we stored from the static constructor call.
Static fields last as long as the AppDomain. It is a good strategy that you have in mind but consider that the asp runtime may recycle the app pool or someone may restart the web site/server.
As an extension to your idea, save the data locally (via a separate service dedicated to this or simply to the hard drive) and refresh this at specific intervals as required.
You will still use a static field in asp.net for storing the value, but you will aquire it from the above local service or disk ... here I recommend a System.Lazy with instantiation and publication options on thrread safe (see the constructor documentation).
I have an application that uses a cache for certain objects, and that cache is implemented as a static Dictionary. I'm looking at using some of this code for a new ASP.NET project. Since the cache will be stored on the server, instead of built every time the application is run, I will need to refresh the cache from time to time.
My question is this: will application pool recycling ever rerun the static constructor of my class which sets up the cache?
I don't know much about how application pools work, but from my understanding, they"shut down" when the site isn't accessed for a long time, and restart when it is accessed again. During this restart, would static objects be "instantiated" again, or are those still preserved in memory through the "shut down" process?
Yes. Static constructors would re-run.
Yet, it's still wiser to initialize data on Application_Start event rather than relying on static constructors because it behaves in more predictable way.
I'm creating a generic flexible site that I can use over and over again but with different features activated or deactivated. I'm looking for the best method to approach my solution.
Specific areas will need to be closed off in certain circumstances, for instance, the application will contain an event management page, but it will not always be required. The application will pull out the active and deactivated features from a data source.
They're going to be like Application wide settings, that will be required on each page, hiding away those settings that are turned off from the menu and not allowing users to access the deactivated feature pages.
I have thought of a number of ways to achieve this :
Store the feature statuses in the database, then on each time the page / menu is accessed / displayed, call the database requesting whether to hide the page / menu item.
Store the feature statuses in the database and access them on the application startup, store them application wide then they can be accessed as and when.
Put the feature statuses in the web config, this way we don't need to query the database every single time or have globally accessible properties.
I would like you advice on which method would be best, or if a better method is available I would be grateful. I don't want to hit the database too many times, or would a simple check like this not be too performance expensive? I'm also not sure if the web config is secure enough for managing active site features.
Regards,
I think putting configuration in configuration file(web.config) would be better option. Because while displaying page/loading menu going to database every time to see whether it should be de/active, required database trip which is overhead.
And if the configuration is stored in web.config it is easily accessible by admin and also easy to modify without compiling or changing anything.
So, in my view best option is to store configuration in web.config. In addition to that you can also make administration page which will change configuration settings and should be reflected in configuration file.
I suggest 2nd approach - store the settings in the database and then retrieve them at startup and store in application scoped containers.
The advantage of such approach over the web.config is that you can easily take client database and immediately run it in your development environment for testing/debugging. If, on the other hand, some settings are stored outside of the database, cloning the site means that you not only have to clone the database but also all the settings from various other resources (like the web.config).
The answer depends on how often you will be changing the status of the features.
If you are going to only set the statuses once when you clone the site and will never touch it again, go with option #2 (load on application start).
If there is any possibility that you will need to change the status of the features in the future, go with option #1 (get statuses on each page load). A simple datareader read to the db will not affect the speed of your site. The db is your friend, remember that's what it is there for. Also, if you ever need to change the statuses while the site is up and running, this method allows you to do so without restarting the entire application.
Whatever method you finally decide to implement, make sure the "application wide" location you store the settings is multi-threaded ready. Remember, each page request will be run on a separate thread and they will all access the same resource.
My suggestion taken from MS (multi-thread safe Singleton Pattern):
using System;
public sealed class Singleton
{
private static volatile Singleton instance;
private static object syncRoot = new Object();
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
lock (syncRoot)
{
if (instance == null)
instance = new Singleton();
}
}
return instance;
}
}
}
The context:
(Note: in the following I am using "project" to refer to a collection of software deliverables, intended for a single customer or a specific market. I am not referring to "project" as it is used in Visual Studio to refer to a configuration that builds a single EXE or DLL, within a solution.)
We have a sizable system that consists of three layers:
A layer containing code that is shared across projects
A layer containing code that is shared across different applications within a project
A layer containing code that is specific to a particular application or website within a project.
The first two layers are built into DLL assemblies. The top layer is an assortment of EXEs and/or .aspx web applications.
IIRC, we have a number of different projects that use this pattern. All four share layer 1 (though often in slightly different versions, as managed by the VCS). Each of them has its own layer 2. Each of them has its own set of deliverables, which can range from a website, or a website and a background service, to our largest and most complex (and the bread-and-butter of our business) which consists of something like five independent web applications, 20+ console applications/background services, three or four independent web services, half-a-dozen desktop GUI apps, etc.
It's been our intent to push as much code into levels 1 and 2 as possible, to avoid duplicating logic in the top layers. We've pretty much accomplished that.
Each of layers 1 and 2 produce three deliverables, a DLL containing the code that is not web-related, a DLL containing the code that is web-related, and a DLL containing unit tests.
The problem:
The lower levels were written to make extensive use of singletons.
The non-web DLL in layer 1 contains classes to handle INI files, logging, a custom-built obect-relational mapper, which handles database connections, etc. All of these used singletons.
And when we started building things on the web, all of those singletons became a problem. Different users would hit the website, log in, and start doing different things. They'd do something that generated a query, which would result in a call into the singleton ORM to get a new database connection, which would access the singleton configuration object to get the connection string, and then the connection would be asked to perform a query. And in the query the connection would access the singleton logger to log the SQL statement that was generated, and the logger would access the singleton configuration object to get the current username, so as to include it in the log, and if someone else had logged in in the meantime that singleton configuration object would have a different current user. It was a mess.
So what what we did, when we started writing web applications using this code base was to create a singleton factory class, that was itself a singleton. Every one of the other singletons had a public static instance() method that had been calling a private constructor. Instead, the public static instance() method obtained a reference to the singleton factory object, then called a method on that to get a reference to the single instance of the class in question.
In other words, instead of having a dozen classes that each maintained its own private static reference, we now had a single class that maintained a single static reference, and the object that it maintained a reference to contained a dozen references to the other, formerly singleton classes.
Now we had only one singleton to deal with. And in its public static instance() method, we added some web-specific logic. If we had an HTTPContext and that context had an instance of the factory in its session, we'd return the instance from the session. If we had an HTTPContext, and it didn't have a factory in its session, we'd construct a new factory and store it in the session, and then return it. If we had no HTTPContext, we'd just construct a new factory and return it.
The code for this was placed in classes we derived from Page, WebControl, and MasterPage, and then we used our classes in our higher-level code.
This worked fine, for .aspx web applications, where users logged in and maintained session. It worked fine for .asmx web services running within those web applications. But it has real limits.
In particular, it won't work in situations where there is no session. We're feeling pressure to provide websites that serve a larger user base - that might have tens or hundreds of thousands of users hitting them dynamically. Up to now our users have been pretty typical desktop business users. They log into our websites, and stay in them much of the day, using our web apps as an alternative to a desktop app. A given customer might have as many as six users who might use our websites, and while we have a thousand or more customers, combined they don't make for all that heavy a load. But our current architecture will not scale to that.
We're also running into situations where ASP.NET MVC would be a better fit for building the web UI than .aspx web forms. And we're exploring building mobile apps that would be communicating with stand-alone WFC web services. And while in both of these, it looks like it's possible to run them in an environment that has a session, it looks to limit their flexibility and performance fairly severely.
So, we're really looking at ways to eliminate these singletons.
What I'd really like:
I'm trying to envision a series of refactors, that would eventually lead to a better-structured, more flexible architecture. I could easily see the advantages of an IoC framework, in our situation.
But here's the thing - from what I've seen of IoC frameworks, they need their dependencies provided to them externally via constructor parameters. My logger class, for example, needs an instance of my config class, from which to obtain the current user. Currently, it is using the public static instance() method on the config class to obtain it. To use an IoC framework, I'd need to pass it as a constructor.
In other words, from where I sit, the first, and unavoidable task, is to change every class that uses any of these singletons so as to take the singleton factory as a constructor parameter. And that's a huge amount of work.
As an example, I just spent the afternoon doing exactly that, in the level 1 libraries, to see just how much work it is. I ended up changing over 1300 lines of code. The level 2 libraries will be worse.
So, are there any alternatives?
Typically, you should try to wrap the contextual information into its own instance and provide a static accessor method to refer to it. For example, consider HttpContext and its available every where in web application via HttpContext.Current.
You should try to devise something similar so that instead of returning singleton instance, you would return the instance from the current context. That way, you need to not change your consumer code that refers to these static methods (e.g. Logger.Instance()).
I generally roll-up information such as logger, current user, configuration, security permissions into application context (can be more than one class if need arises). The AppContext.Current static method returns the current context. The method implementation goes something like
public interface IContextStorage
{
// Gets the stored context
AppContext Get();
// Stores the context, context can be null
void Set(AppContext context);
}
public class AppContext
{
private static IContextStorage _storageProvider, _defaultStorageProvider;
public static AppContext Current
{
get
{
var value = _storageProvider.Get();
// If context is not available in storage then lookup
// using default provider for worker (threadpool) therads.
if (null == value && _storageProvider != _defaultStorageProvider
&& Thread.CurrentThread.IsThreadPoolThread)
{
value = _defaultStorageProvider.Get();
}
return value;
}
}
...
}
IContextStorage implementations are application specific. The static variables _storageProvider gets injected at the application start-up time while _defaultStorageProvider is a simple implementation that looks into current call context.
App Context creation happens in multiple stages - for example, a global information such as configuration gets read and cached at application start-up while specific information such as user & security gets formed at authentication stage. Once all info is available, the actual instance is created and stored into the app specific storage location. For example, desktop application will use a singleton instance while web application can probably store the instance into the session state. For web application, you may have logic at start of each request to ensure that the context is initialized.
For a scalable web applications, you can have a storage provider that will store the context instance into the cache and if not present in the cache then re-built it.
I'd recommend starting by implementing "Poor Man's DI" pattern. This is where you define two constructors in your classes, one that accepts an instance of the dependencies (IoC), and another default constructor that new's them up (or calls a singleton).
This way you can introduce IoC incrementally, and still have everything else work using the default constructors. Eventually when you have IoC being used in most places you can start to remove the default constructors (and the singletons).
public class Foo {
public Foo(ILogger log, IConfig config) {
_logger = log;
_config = config;
}
public Foo() : this(Logger.Instance(), Config.Instance()) {}
}
I have an ASP.net web application running in the default application pool. A user has reported getting results returned that were meant for a different user. As far as I know there is no way two users on two different machines should ever see each other's results.
Here is very roughly what the internal structure looks like
namespace SomeApplication
{
public partial class _Default : System.Web.UI.Page
{
static ArrayList results = new ArrayList();
protected void Button_Click(object sender, EventArgs e)
{
// Add data to results
// Display in asp:Table element
}
}
}
User1 and User2 were both using the above application at roughly the same moment. User1 got normal results but user2 got a mix of their results and User1's results appear in the asp:Table.
Now I know the results object is in the global scope, but it was my understanding that is this a per-instance of the application object.
Static variables are not user specific, they are global within the ASP.NET worker process currently running. It is entirely possible that this one process served both your users. This means that any code running in the process will see this static variable. Statics are also dangerous because IIS can recycle the worker process at any point it pleases, this will bin the values in the variables.
Instead you may need to use one of the state/caching mechanisms provided in ASP.NET, like Session State:
http://msdn.microsoft.com/en-us/library/ms972429.aspx
As already pointed out by Adam, static variables are not user specific but are per AppDomain. So in ASP.NET scenario where you are running multiple sites in one app-pool, each app-pool process (you can more than one worker process per app pool based on configuration) will have one app domain for each web application. So all your users will share same static variables.
Not only that there is no thread-safty built in for static variables and you can actually see in-consistent data when two different threads are accessing those variables.
For the example that you have sited, you will be better off using a instance variable which corresponds to per request scope (in ASP.NET scenario) but full-fill most of use cases (and you don't have to worry about thread-safty). For data that you want to maintain for the same page over repeated post-back, view-state is the way to go. For data that has more life time, you can choose between Session State, ASP.NET Cache, Application state etc. First two are thread-safe implementations while Application state provides means to lock the slot before accessing them.
Refer these links for various state management options that you have available in ASP.NET:
http://msdn.microsoft.com/en-us/library/z1hkazw7.aspx
http://msdn.microsoft.com/en-us/library/75x4ha6s.aspx