引言

有时我们会在IIS中启用一些定时服务，但是你必须清楚IIS会定期回收Asp.net的应用程序的。首先来看IIS啥时候回收APPDomain.

APPDomain 回收时机

There are several things that can cause ASP.NET to tear down your AppDomain.

1. When you modify web.config, ASP.NET will recycle the AppDomain, though the w3wp.exe process (the IIS web server process) stays alive.
   
2. IIS will itself recycle the entire w3wp.exe process every 29 hours. It’ll just outright put a cap in the w3wp.exe process and bring down all of the app domains with it.
   
3. In a shared hosting environment, many web servers are configured to tear down the application pools after some period of inactivity. For example, if there are no requests to the application within a 20 minute period, it may take down the app domain.

If any of these happen in the middle of your code execution, your application/data could be left in a pretty bad state as it’s shut down without warning.

So why isn’t this a problem for your typical per request ASP.NET code?

When ASP.NET tears down the AppDomain, it will attempt to flush the existing requests and give them time to complete before it takes down the App Domain. ASP.NET and IIS are considerate to code that they know is running,such as code that runs as part of a request.

Problem is, ASP.NET doesn’t know about work done on a background thread spawned using a timer or similar mechanism. It only knows about work associated with a request.

有哪些风险

There are three main risks, one of which I’ll focus on in this blog post.

1. An unhandled exception in a thread not associated with a request will take down the process. This occurs even if you have a handler setup via the Application_Error method. I’ll try and explain why in a follow-up blog post, but this is easy to deal with.
2. If you run your site in a Web Farm, you could end up with multiple instances of your app that all attempt to run the same task at the same time. A little more challenging to deal with than the first item, but still not too hard. One typical approach is to use a resource common to all the servers, such as the database, as a synchronization mechanism to coordinate tasks.
3. The AppDomain your site runs in can go down for a number of reasons and take down your background task with it. This could corrupt data if it happens in the middle of your code execution.

告诉ASP.NET ，我还在运行

The good news is there’s an easy way to tell ASP.NET about the work you’re doing! In the System.Web.Hosting namespace, there’s an important class,HostingEnvironment. According to the MSDN docs, this class…

Provides application-management functions and application services to a managed application within its application domain

This class has an important static method, RegisterObject. The MSDN description here isn’t super helpful.

Places an object in the list of registered objects for the application.

For us, what this means is that the RegisterObject method tells ASP.NET that, “Hey! Pay attention to this code here!” Important! This method requires full trust!

This method takes in a single object that implements the IRegisteredObjectinterface. That interface has a single method:

public interface IRegisteredObject{    void Stop(bool immediate);}

When ASP.NET tears down the AppDomain, it will first attempt to call Stopmethod on all registered objects.

In most cases, it’ll call this method twice, once with immediate set to false. This gives your code a bit of time to finish what it is doing. ASP.NET gives all instances of IRegisteredObject a total of 30 seconds to complete their work, not 30 seconds each. After that time span, if there are any registered objects left, it will call them again with immediate set to true. This lets you know it means business and you really need to finish up pronto! I modeled my parenting technique after this method when trying to get my kids ready for school.

When ASP.NET calls into this method, your code needs to prevent this method from returning until your work is done. Levi showed me one easy way to do this by simply using a lock. Once the work is done, the code needs to unregister the object.

For example, here’s a simple generic implementation of IRegisteredObject. In this implementation, I simply ignored the immediate flag and try to prevent the method from returning until the work is done. The intent here is I won’t pass in any work that’ll take too long. Hopefully.

public class JobHost : IRegisteredObject{    private readonly object _lock = new object();    private bool _shuttingDown;    public JobHost()    {        HostingEnvironment.RegisterObject(this);    }    public void Stop(bool immediate)    {        lock (_lock)        {            _shuttingDown = true;        }        HostingEnvironment.UnregisterObject(this);     }    public void DoWork(Action work)    {        lock (_lock)        {            if (_shuttingDown)            {                return;            }            work();        }    }}

I wanted to get the simplest thing possible working. Note, that when ASP.NET is about to shut down the AppDomain, it will attempt to call the Stop method. That method will try to acquire a lock on the _lock instance. The DoWork method also acquires that same lock. That way, when the DoWork method is doing the work you give it (passed in as a lambda) the Stop method has to wait until the work is done before it can acquire the lock. Nifty.

Later on, I plan to make this more sophisticated by taking advantage of using aTask to represent the work rather than an Action. This would allow me to take advantage of task cancellation instead of the brute force approach with locks.

With this class in place, you can create a timer on Application_Start (I generally use WebActivator to register code that runs on app start) and when it elapses, you call into the DoWork method here. Remember, the timer must be referenced or it could be garbage collected.

Here’s a small example of this:

using System;using System.Threading;using WebBackgrounder;[assembly: WebActivator.PreApplicationStartMethod(  typeof(SampleAspNetTimer), "Start")]public static class SampleAspNetTimer{    private static readonly Timer _timer = new Timer(OnTimerElapsed);    private static readonly JobHost _jobHost = new JobHost();    public static void Start()    {        _timer.Change(TimeSpan.Zero, TimeSpan.FromMilliseconds(1000));    }    private static void OnTimerElapsed(object sender)    {        _jobHost.DoWork(() => { /* What is it that you do around here */ });    }}

建议

This technique can make your background tasks within ASP.NET much more robust. There’s still a chance of problems occurring though. Sometimes, the AppDomain goes down in a more abrupt manner. For example, you might have a blue screen, someone might trip on the plug, or a hard-drive might fail. These catastrophic failures can take down your app in such a way that leaves data in a bad state. But hopefully, these situations occur much less frequently than an AppDomain shutdown.

Many of you might be scratching your head thinking it seems weird to use a web server to perform recurring background tasks. That’s not really what a web server is for. You’re absolutely right. My recommendation is to do one of the following instead:

• Write a simple console app and schedule it using Windows task schedule.
• Write a Windows Service to manage your recurring tasks.
• Use an Azure worker or something similar.

Given that those are my recommendations, why am I still working on a system for scheduling recurring tasks within ASP.NET that handles web farms and AppDomain shutdowns I call WebBackgrounder (NuGet package coming later)?

I mean, besides the fact that I’m thick-headed? Well, for two reasons.

The first is to make development easier. When you get latest from our source code, I just want everything to work. I don’t want you to have to set up a scheduled task, or an Azure worker, or a Windows server on your development box. A development environment can tolerate the issues I described.

The second reason is for simplicity. If you’re ok with the limitations I mentioned, this approach has one less moving part to worry about when setting up a website. There’s no need to configure an external recurring task. It just works.

But mostly, it’s because I like to live life on the edge.

参考

The Dangers of Implementing Recurring Background Tasks In ASP.NET

https://github.com/NuGet/WebBackgrounder

定时Job在IIS中潜在危险-IIS 定期回收