Can anyone explain the differences, in IIS, between application pools, worker processes and app domains? Also, how do they work together? I've read a couple of articles but it's still a little confusing.
Does each website created in IIS becomes an application?
Is each application associated with one worker process?
Where do app domains come into the picture?
I try to say them with other words.
In a server you can have many asp.net sites that runs together. Each one site is an app domain.
You must assign to each of them one application pool. Many application domains (sites) can have the same application pool, and because they have the same application pool they run under the same processes, and under the same account - and they have the same settings of the pool. If this pool restarts, then all sites under that pools restarts.
Now each pool can have one or more worker process. Each worker process is a different program that's run your site, have their alone static variables, they different start stop calls etc. Different worker process are not communicate together, and the only way to exchange data is from common files or a common database. If you have more than one worker process and one of them make long time calculations, then the other can take care to handle the internet calls and show content.
When you assign many worker process to a single pool then you make the called web garden and your site is like to be run from more than one computer if a computer is one processing machine.
Each worker process can have many threads.
How the more worker process affect you:
When you have one worker process everything is more simple, among your application all static variables are the same, and you use the lock to synchronize them.
When you assign more than one worker process then you still continue to use the lock for static variables, static variables are not different among the many runs of your site, and if you have some common resource (e.g. the creation of a thumbnail on the disk) then you need to synchronize your worker process with Mutex.
One more note. Its sounds that when you make more worker process then you may have more smooth asynchronous page loads. There is a small issue with the session handler of asp.net that is lock the entire process for a page load - that is good and not good depend if you know it and handle it - or change it.
So let talk about one site only with many worker process. Here you face the issue that you need to synchronize your common resource change with Mutex. But the pages/handlers that use session they are not asynchronous because the session locks them. This is good for start because you avoid to make this synchronization of many points your self.
Some questions on this topic:
Web app blocked while processing another web app on sharing same session
jQuery Ajax calls to web service seem to be synchronous
ASP.NET Server does not process pages asynchronously
Replacing ASP.Net's session entirely
Now this session lock is not affect different sites.
Among different sites the more worked process can help to not the one site block the other with long running process.
Also among different sites the more pools also can help, because each pool have at least one worked process, but remember and see by your self using the process explorer, each working process takes more memory of your computer, and one big server with 16G memory and one SQL server can not have too many different worked process - for example on a server with 100 shared sites, you can not have 100 different pools.
One IIS server may have multiple application pools.
One web application binds to one application pool.
One application pool may have more than one worker process (when Web Garden is enable).
One worker process can have multiple app domains. One app domain lives only in one worker process.
One app domain may have multiple threads. One thread can be shared by different app domains in different time.
The meaning to ASP.NET developers: to make your web site scalable, don't use in-proc session and don't use static class variable lock for synchronization.
Yes, though not every application is a website. You can have an application that is nested under a website.
Yes, every application has to have one worker process (application pool), though one application pool can server several applications. A single web application can be distributed (web garden/farm) meaning that it will run in multiple processes.
Each process will run in its own app domain (every application pool is a separate app domain).
From MSDN.
Create a Web Application:
An application is a grouping of content at the root level of a Web site or a grouping of content in a separate folder under the Web site's root directory.
Application Pools:
An application pool defines a group of one or more worker processes, configured with common settings that serve requests to one or more applications that are assigned to that application pool. Because application pools allow a set of Web applications to share one or more similarly configured worker processes, they provide a convenient way to isolate a set of Web applications from other Web applications on the server computer. Process boundaries separate each worker process; therefore, application problems in one application pool do not affect Web sites or applications in other application pools. Application pools significantly increase both the reliability and manageability of your Web infrastructure.
From the source link:-http://weblogs.asp.net/owscott/archive/2007/09/02/application-vs-appdomain.aspx
An application is an IIS term, but it's one that ASP.NET utilizes.
Essentially it creates a sandbox, or a set of boundaries to separate
different sites, or parts of sites, from the others.
An AppDomain is a .NET term. (In IIS7, AppDomains play a larger role
within IIS, but for the most part it's an ASP.NET term)
The worker process is used to process the request of the web application.
Related
I'm troubleshooting restarts in an ASP.NET application. The application is restarting about 20 times a day. We strongly suspect one part of the application because the restarts began when this particular feature when into production. I've added some logging to those pages using the log4net library, but I'm having trouble interpreting the logs.
When an ASP.NET application is running in an Application Pool, does only a single instance of that application run, or will multiple instances of that application run? I know several worker process threads will be spawned. What is the relationship of the worker process threads to the application(s) running in the App Pool?
I am thinking that I may not be interpreting the results correctly if there are multiple applications logging to the same log. If one is restarted but the other is not, the logs aren't really telling me much about what was happening when the restart occurred.
UPDATE 1
Looking at this page, I find the following information:
An application pool defines a group of one or more worker processes, configured with common settings that serve requests to one or more applications that are assigned to that application pool. Because application pools allow a set of Web applications to share one or more similarly configured worker processes, they provide a convenient way to isolate a set of Web applications from other Web applications on the server computer. Process boundaries separate each worker process; therefore, application problems in one application pool do not affect Web sites or applications in other application pools.
But I am still confused. I know from experience that I can assign two entirely different applications to use the same App Pool. Does that mean that exactly two worker processes will be spawned? Or can there be multiple worker processes spawned for the first app, and multiple worker processes spawned for the second app? If a problem happens in one worker process, can it take down every application running in that App Pool?
UPDATE 2
From this page about using WinDbg, I found this information (emphasis mine):
In IIS 5.x, there is only one Aspnet_wp.exe worker process and one debugger thread. Consequently, only one debugger can be attached to the Aspnet_wp.exe process at a time. This can pose a problem if you're dealing with multiple Web applications on the same machine. In IIS 6.0, you can coerce an AppDomain to run in a separate application pool. (For more information, see "IIS 5.x Process Model" and "IIS 6.0 Process Model" in Chapter 1.) Separate application pools provide multiple W3wp.exe processes. Multiple debugger threads are created in these processes (one in each), allowing you to debug more efficiently.
This sounds to me like each App Pool gets one w3wp.exe process. Am I interpreting that right? And if so, does that still apply in IIS 7.5?
Yes, each application pool is typically a single process1, but can contain multiple threads. You can assign multiple sites to an application pool, and those sites will all run under the same process, however they will run under different "app domains", which are security contexts that separate the code of one site from another, even if they're running on the same app pool.
Two users hitting the site at the same time can run on different threads, meaning they can run concurrently. That means any logging can have values interspersed. You might want to add a session value to your logging so you can sort based on session.
App pool restarts (recycling) are normal, 20 restarts in a day does not seem unusual. They can happen multiple times per day, and IIS controls when app pools are restarted. It does this whenever it feels it needs to clean up the pool.2 Your applications should be written in such a way as to recover gracefully from this (ie, do not keep anything in session that cannot be easily recreated if the app pool restarts).
The app pool can also restart when an unhandled exception occurs in your app. In that case, you want to address the cause of this. Such exceptions are usually logged in the event log.
1 – While you can configure an application pool to have multiple worker processes (this is known as a Web Garden), this is not a typical (nor generally recommended) configuration in my experience.
2 – Note that using IIS Manager you can configure an application to log recycle events to the Windows Event Log. You can also use IIS Manager to set the threshold for when several of the different types of recycle events occur.
Hosted customers in IIS7 can use asp.net and System.Diagnostics to list all the system's process ID. They can also kill the ones that belong to their own application pools. Seems like a big security problems in IIS7 for shared hosting environment. Any suggestions on how to prevent normal users from accessing System.Diagnostics? How to limit it to administrators only?
Unlike with Windows 2003 and IIS6, many shared Windows 2008/IIS7 hosting environments provide their customers with dedicated application pools and Full Trust.
Whilst customers may be able to launch and kill their own processes (including their own worker processes), provided that the identity of the account that the site runs under is locked down then no real harm can be done. Also what would be the benefit to a customer having code that constantly kills their own application pool (other than to force a restart of a worker process to allow Application_Start type events to fire if you need to reload some settings there)?
I work for a shared hoster, we actually provide customers with the ability to start, stop and recycle their dedicated pools via our admin system, all they would be doing in code is pretty much the same thing.
The worst that can happen is that a customer launches a process that consumes large amounts of memory or an excessive amount of CPU (but then even their own ASP.NET code can run away out of control doing the same). We monitor our servers continuously for such anomalous behaviour and can track down the culprit within 2-3 minutes of being alerted. The end user will get a friendly warning and told not to do this again, if they do then their site is instantly shut down.
The only time I'd be worried if the hoster was running shared pools at full trust, but if they're doing that then they have a whole other security headache to overcome, process killing would be the least of their worries.
for IIS7
Does a webapplication run faster when Maximum Worker Processes is more than one?
By increasing the Maximum Worker Processes over 1 you're creating a Web Garden. So the short answer is: likely no... unless:
To quote Chris Adams an ex IIS PM's article I have flowers... should I get a Web Garden?:
Web gardens was designed for one single reason – Offering applications that are not CPU-bound but execute long running requests the ability to scale and not use up all threads available in the worker process.
The examples might be things like -
Applications that make long running database requests (e.g. high computational database transaction)
Applications that have threads occupied by long-running synchronous and network intensive transactions
The question that you must ask yourself -
What is the current CPU usage of the server?
What is the application’s threads executing and what type of requests are they?
Based on the criteria above, you should understand better when to use Web Gardens. Web Gardens, in the metabase, equals the MaxProcesses metabase property if you are not using the User Interface to configure this feature.
cscript adsutil.vbs set w3svc/apppools/defaultapppool/maxprocesses 4
I hope that I get some mileage out of having this blog and more importantly I hope it helps you understand this better…
You may want to look at "What is Web Garden?" from Deploying ASP.NET Websites on IIS 7.0 [codeproject.com] which says:
By default each Application Pool runs with a Single Worker Process (W3Wp.exe). We can assign multiple Worker Processes With a Single Application Pool. An Application Pool with multiple Worker process is called "Web Gardens". Many worker processes with the same Application Pool can sometimes provide better throughput performance and application response time. And each worker process should have their own Thread and Own Memory space.
WebGarden is faster than single worker process in case if application contains locks that prevent its parallelization. For example, GDI+ based image processing.
See this and this questions for more info.
Web farm is used for multiple request and multiple user using switching among them.
Web Garden creates worker processes for every processor individually for one user.
Is it possible to use worker process as a switch and convert it into Web Farm that can allocate multiple processor on bases of algorithm?
From my point of view (and please correct me if I mistake)
the practical diferent betwing web farm and web garden is that the web farm is a plan running on many computers, and web garden is a plan running on a single machine.
You do not need from my point of view to try to convert web garden to web farm, is with out any point. (please correct me if I mistake)
If you have one machine, and you open more than 1 worker process on Web Garden iis settings, then you only need to make your setup running as web farm, have one database connect, use mutex or other lock for be sure that 2 or more worker process are synchronize together.
In my application I have use them both and my only issue is the synchronization of the data.
I have a WCF service installed on IIS7. I noticed that the first call to my service is always very very slow. The subsequent calls are much faster & acceptable.
If there are no calls made to the service for some time, it again goes to sleep mode. After this the next call again takes a long long time.
Any remedies for this problem?
It is because of process management on IIS. When there are no calls for certain period of time IIS release the recourses and stops the process.
This is why you can notice that it is slow for first request and for requests after a long delay. Because while the first request or after long period of silence IIS loads everything from scratch. JIT complier runs and etc...
Also note :
When you are hosting WCF services on IIS, the WCF services enjoy all the features of ASP.NET applications. You have to be aware of these features because they can cause unexpected behavior in the services world. One of the major features is application recycling, including application domain recycling and process recycling. Through the IIS Management Console, you can configure different rules when you want the recycling to happen. You can set certain thresholds on memory, on time, and on the amount of processed requests. When IIS recycles a worker process, all the application domains within the worker process will be recycled as well
If you need automatic starting: The Windows Service Control Manager allows you to set the startup type to automatic, so that as soon as Windows starts, the service will be started, without an interactive logon on the machine. So you can use Windows service as a host.
More details you can check in Hosting and Consuming WCF Services.
There is another approach through which you can make it better. We have some kind of scehduled process which keeps hitting our server like every 5 mins with very light 'fetch' requests to keep all servers "hot" (by loading most of the required dlls etc) so that user experience is far better.
I agree it is not a fool proof way but still is something you can consider apart from increasing the recycling settings in IIS.