I have some ASP.NET web services which all share a common helper class they only need to instantiate one instance of per server. It's used for simple translation of data, but does spend some time during start-up loading things from the web.config file, etc. The helper class is 100% thread-safe. Think of it as a simple library of utility calls. I'd make all the methods shared on the class, but I want to load the initial configuration from web.config. We've deployed the web services to IIS 6.0 and using an Application Pool, with a Web Garden of 15 workers.
I declared the helper class as a Private Shared variable in Global.asax, and added a lazy load Shared ReadOnly property like this:
Private Shared _helper As MyHelperClass
Public Shared ReadOnly Property Helper() As MyHelperClass
Get
If _helper Is Nothing Then
_helper = New MyHelperClass()
End If
Return _helper
End Get
End Property
I have logging code in the constructor for MyHelperClass(), and it shows the constructor running for each request, even on the same thread. I'm sure I'm just missing some key detail of ASP.NET but MSDN hasn't been very helpful.
I've tried doing similar things using both Application("Helper") and Cache("Helper") and I still saw the constructor run with each request.
You can place your Helper in the Application State. Do this in global.asax:
void Application_Start(object sender, EventArgs e)
{
Application.Add("MyHelper", new MyHelperClass());
}
You can use the Helper that way:
MyHelperClass helper = (MyHelperClass)HttpContext.Current.Application["MyHelper"];
helper.Foo();
This results in a single instance of the MyHelperClass class that is created on application start and lives in application state. Since the instance is created in Application_Start, this happens only once for each HttpApplication instance and not per Request.
It's not wise to use application state unless you absolutely require it, things are much simpler if you stick to using per-request objects. Any addition of state to the helper classes could cause all sorts of subtle errors. Use the HttpContext.Current items collection and intialise it per request. A VB module would do what you want, but you must be sure not to make it stateful.
I 'v done something like this in my own app in the past and it caused all kinds of weird errors.
Every user will have access to everyone else's data in the property. Plus you could end up with one user being in the middle of using it and than getting cut off because its being requested by another user.
No there not isolated.
Related
There are numerous places you can have initialization code be executed in ASP.NET:
web.config is processed
WebActivator PreApplicationStartMethod
WebActivator PostApplicationStartMethod
Global.asax Application_Start
What is the ordering of these occurences? Are there any other additional items that should be to this list?
Edit: Since it was mentioned that statics are relevant to first invocation location, I'm going to break this up for them
Foo class that is used in a WebActivator PreApplicationStartMethod
static constructor
static readonly field
Bar class that is used in a WebActivator PostApplicationStartMethod
static constructor
static readonly field
Baz class that is used in a Global.asax Application_Start
static constructor
static readonly field
For clarity purposes, suppose that in the above examples each of those depends on the Foo/Bar/Baz class being used in the location and that the class contains a static constructor and static readonly field.
Static constructors and static field initialization is determined by the runtime, not ASP.NET. Eric Lippert recently posted a great four-part blog series detailing how they work.
As for the rest of the items you mentioned, methods marked with the System.Web.PreApplicationStartMethodAttribute are executed first. According to the MSDN documentation for this attribute, there is no guarantee of the order in which these methods are called.
According to a blog post by Phil Haack, this attribute gives developers the opportunity to call two other methods during the application's startup: BuildProvider.RegisterBuildProvider and BuildManager.AddReferencedAssembly. The MSDN documentation for BuildManager.AddReferenceAssembly states that this method can only be executed during the Application_PreStartInit stage of the application, which suggests that's when all methods marked by the System.Web.PreApplicationStartMethodAttribute are executed.
WebActivator uses the framework's PreApplicationStartMethodAttribute to hook into the application's startup. Once called, it will search for and execute all methods marked by the WebActivator.PreApplicationStartMethodAttribute before it dynamically registers an HttpModule that will later invoke all methods marked by the PostApplicationStartMethodAttribute - after Application_Start has been called in the HttpApplication class.
So, to summarize, the order is:
Web.config is read into memory
Methods marked with a PreApplicationStartMethodAttribute
HttpApplication.Application_Start
Methods marked with WebActivator.PostApplicationStartMethodAttribute
The application life cycle looks like this:
A request is made for an application resource.
The unified pipeline receives the first request for the application.
Response objects are created for each request.
An HttpApplication object is assigned to the request
The request is processed by the HttpApplication pipeline.
Additionally, here are the events that occur in the request pipeline:
See ASP.NET Application Life Cycle Overview
Static constructors and static readonly fields (instantiated inline) are initialized the first time that type is used by your code. That could happen any point in the application lifetime.
Specifically answering your question
According to the WebActivator project page, this is the order of events:
web.config is processed
WebActivator PreApplicationStartMethod
Global.asax Application_Start
WebActivator PostApplicationStartMethod
As far as static initialization goes, see Eric Lipperts posts that Justin linked in his answer.
Your current list has some things that aren't necessarily related to ASP.NET only (static readonly fields, etc.), but this link describes the ASP.NET lifecycle. There are a ton of things that happen, many of them allow customization where you could inject some of your own code if you had good reason to.
Your question is pretty broad. Is there something you're trying to accomplish here that could hope hone in on what part of the process would be ideal for what you want to do?
I have a simple app where I use global.asax to map a serviceroute to a wcf service through a custom servicehostfactory in Application_Start. The constructor of that service does some initial processing to set up the service which takes a bit of time.
I need this constructor to fire when its serviceroute is added automatically. I tried creating a clientchannel from global.asax and making a dummy call to spin up the service, but discovered the service isn't up yet -- it seems application_start has to return?
So how do I get the constructor of the service to fire when first mapped through global.asax without having to manually hit the service? Unfortunately AppFabric isn't an option for us, so I can't just use it's built-in autostart..
UPDATE
I was asked for a bit more detail;
This is like a routing management service. So I have Service1 -- it gets added as a serviceroute in global.asax. Now I have http://localhost/Service1
Inside Service1 I have a method called 'addServiceRoute'. When called, it also registers a route for Service2. Now I have http://localhost/Service1/Service2.
My initial solution from global.asax was to build a channelfactory to http://localhost/service1 but that wouldn't work. Service1 wasn't up yet and wouldn't come up till Application_Start returned (Still not sure why?). So then I thought I'd cheat and move that initial addserviceroute call to the constructor of service1. Also didn't work.
It was mentioned that this shouldnt be in the constructor -- i agree, this is just testing code.
A singleton was also mentioned, which might be ok, but I intend to have more than one instance of Service1 on a machine (in the same app pool) so I don't think that'll work?
** UPDATE #2 **
I was asked for sample code.. here it is from global.asax (trimmed a bit for brevity).. So http://localhost/Test DOES come up.. But if I have to use appfabric to warm up Test and get its constructor to fire, then don't I need Test.svc or something? How do I get appfabric to even see this service exists?
protected void Application_Start(object sender, EventArgs e)
{
RouteTable.Routes.Ignore("{resource}.axd/{*pathInfo}");
RouteTable.Routes.Add(
new ServiceRoute("Test", new MyServiceHostFactory(ITestService, BindingType.BasicHttpBinding, true), TestService));
}
What you describe requires singleton service (something you should avoid) because normally each call or session gets a new instance = a new call to constructor. In self hosted WCF service you can instantiate singleton service instance and pass it to ServiceHost constructor. In case of IIS hosted service used together with ServiceRoute you can try to create your own class derived from ServiceHostFactory and pass created service instance as a parameter to its constructor. In this factory class implement CreateServiceHost method and pass that existing service instance into ServiceHost constructor. To make this work your service class must still be handled as singleton through service behavior.
Btw. constructor should not do any time consuming operation. Constructor is for constructing object not for initializing infrastructure. Using constructor for such initialization is bad practice in the first place.
AppFabric autostart is what I would recommend - even though you say you cannot use it - this is the problem it was meant to solve (warming up your service).
As an alternative before AppFabric existed, you would have to use a scheduled task (a.k.a cron job) with an executable that calls into the service you want initialized. The way AppFabric autostart works is by using named pipes (net.pipe) to trigger the warm up - but it just does this exact thing when the service is recycled. The difference between the scheduled task approach and the AppFabric autostart is that the scheduled task doesn't know when your application pool has been recycled - you would need to poll your service periodically to keep it warm.
Alternatively you could consider hosting your WCF application outside of IIS via self-hosting. This would avoid the warm-up issue, but you wouldn't achieve many of the benefits of the IIS hosted container. See HttpSelfHostServer in the new MVC Web API or review using a standard ServiceHost.
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()) {}
}
Well - exactly as the question subject states - any ideas on how you might do this?
I've been looking over the objects in System.Web.Hosting but nothing is standing out.
The reason? I'm getting one or two application errors which are typically occuring during a recycle (they happen about 25 hours apart and I've left my app pool recycle time at the default) and so I want to know if they're happening on a thread that's in the pool that's shutting down, or the one that's start(ed/ing) up.
I recently stumbled across this article on Brain.Save() which talks about exactly this issue from the point of view of hosting WCF (he's Steve Maine - A program manager at Redmond on the Connected Servies Division).
They need to be able to do this when a WCF service is hosted inside Asp.Net since they need to be able to shutdown any open listeners so that the WCF engine in the new app domain will be able to open them all up again.
As the article demonstrates, the answer is to implement the IRegisteredObject interface, call ApplicationManager.CreateObject to create an instance of your object and then register it with HostingEnvironment.RegisterObject (all detailed in the MSDN documentation for the interface).
When this object's IRegisteredObject.Stop(bool) implementation is called with false as the parameter, this is notification that the app domain is being shut down and that the object should be unregistered (kind of like a global dispose) with a call to HostingEnvironment.UnregisterObject.
When it's called with true it means you've not unregistered in good time, and that if you don't Unregister immediately, it'll be done for you.
I can certainly use this mechanism to find out, when an exception occurs, if the AppDomain is being killed or not. The nature of the object in question that throws the exception means that if it's not at shutdown, it must be during initial startup.
Equally, however, I may well start looking at this persistence mechanism for some of my other more complicated static information!
The History
The article also explains some of the history, and rationale, of why you would want to use IRegisteredObject rather than Application_Start and Application_End methods in global.asax:
Traditional ASP.NET applications can hook application lifecycle events (application startup/shutdown) by implementing methods like Application_Start and Application_Stop in global.asax. However, global.asax is for application code. Infrastructure pieces (of which the WCF hosting system is one) need a mechanism of hooking AppDomain lifecycle events that do not involve dumping infrastructure code in your global.asax file. That space is reserved for you, the user, and it would be rude of use to pollute that with a bunch of hosting goo we need to make the whole thing work. Instead, the ASP.NET folks did some great work during the Whidbey release to open up the hosting API’s and make it easy for people like WCF to come along and hook these lifecycle events in a way that’s invisible to application code.
You can check the value of System.Web.Hosting.HostingEnvironment.ShutdownReason, when the app pool is not in the process of closing / recycling it will have the ShutdownReason of None.
Adding the actual code to do this:
public class RecycleWatcher : IRegisteredObject
{
public static bool IsRecycling { get; private set; }
public void Register()
{
HostingEnvironment.RegisterObject(this);
}
public void Stop(bool immediate)
{
IsRecycling = true;
}
}
Then enable it by running
new RecycleWatcher().Register();
After that just check that property for IsRecycling to know if you are recyling or not.
if (RecycleWatcher.IsRecycling) DoSomething();
Not sure exactly what you want to do when the appication pool recycles but if you add the below event handler to Global.asax then the code in it will run when the application is shut down.
protected void Application_End(object sender, EventArgs e)
{
}
I got a webapp that stores a config object in ApplicationState.
This object contains the connection string to the database among other things.
Sometimes i start a async thread to do a few longer running tasks, like sending emails and updating the database.
However since this thread don't have a HttpContext i can't get at the config object.
I know this design that everything depends on HttpContext is bad, but thats too late to change now.
Looking at reflector i see that the HttpContext class just uses a static internal class to get the ApplicationState. Is there any other way to get at it?
All those internal classes in .net are really annoying.
Just pass whatever you like to your thread when you start it. Use a ParameterizedThreadStart delegate to start it instead of just a ThreadStart delegate. You could either pass it HttpContext.Current, or else bundle together the information you want your thread to have, and pass it.
If you really need access to Application State (or similar) from async handlers you should modify your HttpApplication subclass (e.g. Global.asax) to store the Application State instance (this.Application) to a static property during Application_Start:
public static HttpApplicationStateWrapper State { get; private set; }
protected void Application_Start()
{
State = new HttpApplicationStateWrapper(this.Application);
}
It would be more appropriate to use a DI framework to register this instance, but if you have one available you could probably avoid the use of Application State altogether for storing config. Further, there is a configuration framework in .NET that directly addresses this need and provides the ability to read configuration from anywhere.