In my web app on start up I have a number of threads that are currently running and do all sorts of work (such as releasing database locks, monitoring users etc). These threads are started on the application_start() event in my asp.net app.
What concerns me is if the app pool dies for some reason, these threads will not start up again until a request is made to the application (i.e. app domain loaded etc).
I was thinking about scheduling a batch process (via command line and vb-script) every 10 mins to make a web request which will ensure that my application is already loaded and running (threads will be up then).
So far I have not seen any problems with the app pool crashing so it was a precautionary measure.
I noticed my app pool currently has a default setting of 20 mins to recycle.
I was wondering if this approach is correct and I do not introduce any other problems down the road.
We are using IIS 6 so cannot use the Application warm up module which I suppose is not for app pools crashing.
Related
A test WCF webservice that I have hosted using IIS 7.5 is consistently slow to respond to calls made after a period of inactivity (i.e. the first call of each day).
From researching this topic I gather that the problem of "application warmup" is commonly encountered when using IIS (e.g. see here).
I have taken the usual steps that are recommended to try and mitigate this problem:
Installed the Application Initialization Module.
Disabled the application pool Idle Time-out, and the Regular Recycling Time Interval (i.e. set them to '0').
Edited the applicationhost.config file so that autoStart=True and startMode="alwaysRunning" for the necessary app pool, and preloadEnabled="true" for my application.
With these settings, I expect the application pool to immediately spin up a worker process when IIS is started, and spin up a new worker process when the existing one exits. Also, I expect the application to be loaded within the worker process.
However, for the first call of each day, the logs show the difference in time between the client making a call, and the webservice receiving the call, can be as much as 10 seconds. Subsequent calls are typically handled in well under 2 seconds.
Curiously, the long response time is not reproduced by making a call following an iisreset command. I would expect that such a heavy-handed operation would put the webservice in a similarly "cold" situation, however this does not seem to be the case.
I would like to know:
What could still be causing the delay in the application "warming up"?
What is the difference in the state of the webservice following iisreset and a long period of inactivity?
Should I resort to a "heart beat" solution to regularly ping the service to keep it alive?
Thanks in advance for any tips or insight.
I'll try to help with you questions:
What could still be causing the delay in the application "warming up"?
Warm up an application does not mean warm up its resources. For instance, if you configure Auto-start with Application Fabric in your WCF application (https://msdn.microsoft.com/en-us/library/ee677260(v=azure.10).aspx), and this application access database using EF, it will not initiate your DBContext.
If you want these resources initialized after your application warmed up, you need to implement a method to initialize your resources, like cache, DBContext, etc.
What is the difference in the state of the webservice following iisreset and a long period of inactivity?
When the application spend long time of inactivity, probably the application pool goes down and it is restarted when it receives any request, like a recycle does.
This link has interest information about the difference between iisreset and application pool recycle, and it can help to answer your question: https://fullsocrates.wordpress.com/2012/07/25/iisreset-vs-recycling-application-pools/
Should I resort to a "heart beat" solution to regularly ping the service to keep it alive?
If you keep on accessing your service, it will probably keep its resources initialized in memory, so can be a good approach.
Anyway, if your Application Pool is configured to recycle in some interval time, it will be recycled and your resources in memory lost.
If it looks problem to you, just turn off this feature going to IIS -> Application Pool -> Advanced settings and set Regular Time Interval=0
For this issue, it's just some suggestions, you need to make some tests and find out the better solution.
If I create a web application and host it on a Windows Server, then as I understand it, IIS handles the initial request and routes it to the appropriate website or application. I'm under the impression that a w3wp.exe (worker process) instance is created for each application. IIS works with the worker process, which in turn works with the web application.
What happens if the application gets twenty requests per second? Will the worker process create twenty instances of the application to handle each request, or will it queue the requests passing them to a single instance of the application as and when?
I suspect it's the latter. If that is the case then am I right to think that the worker process will keep an application alive whilst it is getting requests?
I'm trying to fully understand what a web application does when it handles many con-current requests. I've tried asking this question before but struggled with the wording, so hopefully this makes sense.
EDIT:
Thanks to Mason I realised that the answer was right in front of me! Web applications use DLLs, which can't run by themselves. It's the w3wp.exe (worker process) which call the DLLs to handle the requests.
The number of worker processes per web site is controlled in the application pool advanced settings (in IIS management console).
The configuration of number of concurrent requests each of the workers can handle depends on the IIS version. In IIS 7 was in the same place, for more recent versions you will have to check your machine.config (looking for maxWorkerThreads)
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 regards 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.
I have several important Quartz events that MUST go off at specific times of the night. Lately I have been noticing that not all the events are run. I have a feeling that overnight our server load is very light (ie. zero users) and that the web server kind of goes to sleep, and hence so does Quartz. Does this seem plausible? I am using Quartz.net within the web server, and not as a separate service.
Yes, that is plausible. In general, it is considered a bad practice to have IIS run scheduled tasks, that is a job better left for a Windows Service, or the built-in Windows Scheduled tasks (which has been much improved for Windows Server 2008).
Your worker process might have been shut down because there are no load. By default, IIS shuts down worker processes after 20 minutes without ingoing requests (you can alter this in the Application Pool settings). Also, worker processes are likely to be restarted after a certain amount of time or requests, or if they consume too much memory.
A quick-fix for your specific problem might be to use the Windows Task Scheduler to request the site periodically to keep it alive - or have it request a URL that triggers your task at the predefined time.
ASP.Net, by default, will shut down AppDomain's after a period of inactivity.
The recommended course of action is to implement timed events either in a Windows Service or as an executable launched from Window's Scheduled Tasks.
It is also possible to change the IIS configuration so that it will not shut down your AppDomain. How exactly this is accomplished varies between versions of IIS, but instructions can easily be found by searching.
Question: When a webapplication gets started, it executes Application_Start in global.asax.
Now, a web application gets started as soon as the first request for a page in that application reaches the server.
But my question is: how long will the application run until the application is stopped.
I mean when after the first page request, there's no traffic on the server.
I need to know because I intend to start a server that listens on a tcp port in global.asax.
And when the application stops, the server ceases to listen to its port.
It depends on your IIS settings. Your application will run in an application pool, which takes a bunch of settings defining the behaviour of this pool.
The thing you're looking for are recycling settings. In IIS 7, you can access these easily from the management console. Go to Application Pools, right click on the application pool your app runs in (if you don't know which one that is, then it's probably the DefaultAppPool) and select recycling.
Here you'll find the options you have to control the recycling behaviour of your app pool, which in turn controls when your app 'resets'.
in a word (well 2) - shared hosting.
on shared hosting beware, (godaddy/webhost4life etc) this timeout could well be less, plus you don't have option to configure that on these hosting environments. i've had cases where the app pool is recycled after 5 mins at certain peek times, so you might have to investigate 'wakeup' routines to poke your app to keep in in the memory. i do this for a few shared hosting apps to great effect using pingalive.com.
hope this helps, even if in an abstract way.
jim