Why does IIS stop after a while?_问答_开发者_运维开发者技术经验分享

I am not too clear about the IIS lifecycle, but my general understanding is:

Every couple of hours IIS resets itself. This is apparently done so as to fix up any memory leaks, resource deadlocks etc. etc. ie. It seems to be a cleanup operation.
Every couple more hours (I think I read 23 hours) the server just stops listening to inbound requests and runs Application_End. An external page request will restart the app.

Can I get a bit more reasoning to why these behaviors occur? Especially with rega开发者_如何学Pythonrds to item #2... My server runs internal scheduling behaviors which completely died last night. The reason was that Application_End occurs and no customer requests were happening to start the IIS server again. This seems weird. Why not just clean up memory leaks etc. and then keep IIS running exactly as it was? The only reason I can think of is that it lets the server reclaim memory/cpu used by IIS, but that seems nonsensical and the cause of bugs, such as my scheduler issue!

Each website in IIS sits in an application pool and you have three different sections that influence when an application pool recycles it's worker process; Recycling, Performance and Health. When the process recycles, a new worker process (w3p.exe) is created first to handle any new requests. Any existing requests are completed on the old process before that is then closed. Application_Start and Application_End will run on each process so you can setup and teardown resources appropriately.

The Recycling settings have the most direct impact on when the worker process will recycle and you can choose to restart after a specific number of minutes running, number of requests processed or at a specific time each day. In a web farm using a specific time can ensure that you never have all the severs in the farm recycle at the same time. You can turn all these off so your worker process doesn't recycle but as you stated in your question this leaves the server vulnerable to memory leaks and threads hanging which will stop IIS serving any requests for websites in that application pool.

The Performance settings can shutdown a worker process if it has been idle for an specified number of minutes or if the CPU reaches a specified threshold. You can also increase the number of worker processes for an application pool and create a web garden.

The Health settings monitor worker processes and will shut them down if they fail repeatedly and will check that they start and stop within a specified time.

Technically, IIS is not stopping or resetting. It's the application pool that is being recycled which ensures that the application domain in which your web applications run does not bog down over time due to bugs/inefficiencies in your code, bugs in the framework, etc.

The IIS model is actually very good for the health of a long-running application. Windows Services for example don't get these benefits. If the process crashes, it's done. But because IIS can measure various aspects of your web application like response time, memory consumption, inactivity, etc, it can offer to reset your application under certain circumstances. They're all configurable but you should always strive to develop web applications in such a way that one request does not depend upon a prior request.

You should also not rely on things happening in the web application that are not directly in response to a web request. So if you are starting up a background thread to do some background tasks then I'd recommend moving that out into a separate process (such as a Windows Service or Scheduled Task.) Although if you really don't want to do that, there is an IIS 7 Application Warm-Up Module that will periodically ping your web application in order to start it.

If you are using in-process session state and the resets are causing problems, you may want to consider using a SQL-based session state provider.

In any event, you can read more about configuring the IIS 7 application pool recycling behavior here. http://technet.microsoft.com/en-us/library/cc753179(WS.10).aspx

I think the other posters have already answered your main question sufficiently well however I'd like to address the final part of your question.

Why not just clean up memory leaks etc. and then keep IIS running exactly as it was? The only reason I can think of is that it lets the server reclaim memory/cpu used by IIS, but that seems nonsensical and the cause of bugs, such as my scheduler issue!

and

Why do I need to wait for a web page request to start up my pool, rather then having the server automatically running and gung-ho about receiving client web requests?

Let's think about the following scenario and what would happen if IIS behaved in this way. If we have a machine hosting several thousand websites (i.e. your typical shared hosting environment) each website has it's own application pool (w3p.exe) running. Say that IIS started up a worker pool for each website regardless of if a request to that site had been made, you'd have a few thousand processes starting up at once each idling at say 2MB of RAM. If you've got 2000 websites you've just allocated 4GB of RAM to sit and essentially do nothing and the OS might start eating into the page file without any real need.

Is this desirable? I think you'd agree that the answer is no.

These behaviors can be controlled by changing the app pool recycle settings of your website. Our production website at work recycles its pool every night at 3am, but our QA environment recycles several times a day.