I have an application that use a static class too large and complex for this reason can not use the standard Asp.net Session. More telho problems with the stability of my application because when the pool closed by an error in a estarna dlls, all static variables are discharged.
I wonder if there is a setting for each "client" open a pool. So if a User does not fall knocks others.
If you have a static class there is only one of that class for the application pool. If this class has something different for each user it shouldn't be static. If the class only contains general information not pertaining to a specific session and you don't want to make it an instance class then make sure there are no exceptions being thrown in the static class' constructor.
In addition to YetAnotherSoftwareDeveloper's answer, application pooling is used to provide a mechanism that can be used to isolate applications for stability and security reasons, not to isolate individual client sessions.
If you have an application that is unstable, you can keep it from having a detrimental effect on other applications by isolating it in its own application pool. This will not provide any stabilizing effect on the application having problems, but will keep it from crashing other applications on the same server.
Related
In a web application that fires of many ajax requests from different users to carry out actions. One of these requests fires off some database updates. If this is currently in progress I want to make sure other session requests for this action is just ignored. Is it safe to implement a static variable that I can lock so the action can be ignored by other requests if one is already in progress or would this just be a bad design?
Update
After digging more I came across Optimistic Concurrency. I'm using EF6 so to avoid this it sounds like all I need to do is with Concurrency Mode to fixed?
A solution based on static variables may look attractive, because it is easy to implement. However, it quickly becomes a maintenance liability, particularly in a web application environment.
The problem with static variables in web environments, such as IIS, is that they are not shared globally across your entire application: if you configure your app pool to have several worker processes, each process would have its own copy of all static variables. Moreover, if you configure your infrastructure for load balancing, each process on each server would have its own copy, with no control on the part of your application. In your situation this would mean a possibility of multiple updates happening at the same time.
That is why I would avoid using a static variable in situations when it is absolutely critical that at most a single request be in progress at any given time.
In your situation, the persistence layer should be in charge of not corrupting the data no matter how many updates are firing at the same time. Persistence layer needs to decide which requests to execute, and which to throw away. One approach to solving this problem is optimistic locking. See this Q&A for general information on how it could be implemented.
I have a COM+ application written in C# (ServicedComponent.) The application pool size > 1 in all cases. I am using SharedPropertyGroups to retain and share data. From my testing it is not clear if all the running instances of the application is sharing the same values.
Are the properties stored in SharedPropertyGroup are shared across all the instances of the same COM+ application?
Each application pool (DLLHost process) will get it's own shared property manager. From COM+ Shared Property Manager Concepts:
"Shared properties stored in the SPM are available only to objects running in the same process."
So, the shared property manager will let you share transient state inside of one application (pool instance).
If you want to share state between multiple processes then you would probably want to look at an out of process cache approach (e.g. Windows Server AppFabric Caching or a database depending on the requirements).
Also see .NET Enterprise Services and COM+ 1.5 Architecture where they describe some of the issues when using application pooling:
Memory used by the Shared Property Manager (SPM) is process specific.
Application pooling may impact any application that assumes it is
using the only instance of the SPM on that machine. There is no longer
any common highest-level data store (since components can span
processes) for all instances of a COM+ component using application
pooling. Alternatively, you can use a cached middle-tier database to
store common state that will not only span instances in a process but
processes as well. When doing this, you may want to consider using a
pooled component that keeps a persistent connection to a database
specifically for middle-tier serialization operations. In reality,
this is a much better choice even without application pooling, due to
the issues surrounding locking and performance of the SPM.
I have a WCF app on NetTCP Binding based. In client app i have created its proxy class object as static. This client app may run for 4-8 hrs after deployment. Basically at login window I am creating and initializing DataServiceClient proxy class (mainly database insert & updates) and using same object throughout my application until user closes Main Window.
Is there any adverse effect (performance wise) of creating static object of proxy class? If yes then how I can avoid this. Before using static object I was creating individual object at every window (wherever required) but this had increased window loading time.
How I can improve WCF performance. I am satisfied with its performance but it could be my illusion.
Nothing wrong with using the same instance, but make sure your error handling is good. Otherwise the proxy object will go into a faulted state when an error happens and you have to restart the whole application. There are some events you can attach to when the state changes.
After the proxy object goes into the faulted state you have to create a new one, there is no way to recover a faulted proxy object.
I have found that using message headers reduces the amount of methods I actually need to expose, but that really depends on what your service does.
Otherwise I would recommend to use streaming when possible. Keep your data as small as possible. Use the binary formatter.
Looks like you client is a Windows Forms application - a static service proxy should be ok for you as long as you don't do any multi-threading or callbacks on your proxy etc. Essentially, in such case, you need to synchronize the access to static variables.
Talking in general terms, WCF performance can be improved
Designing the service contract carefully - its should be chunky interface and not chatty so that number of service calls gets reduced
Choosing appropriate binding - TCP Binding would be faster than HTTP Binding but it would be .NET propriety and may not work over internet as other ports would be blocked. If your communicating on same machine then named piped binding would be the fastest mode
With 2 web servers, will a singleton class have 2 instances?
Both the web-servers will have separate instances of their application processes be it .net or java. So Yes, both the servers will have their individual instances of your singleton class.
Regardless of the fact that these two web servers are two different physical machines, even if they are on the same server, they will definitely run entirely on different processes. Each process will load its objects in memory separately from any other process.
specifically in case of asp.net -
Even in the single web server, each site will cause separate instance of the Singleton class. Because each site in asp.net worker process is loaded in separate application domain, no two domains can interfere between each others' objects. So in case of asp.net even the single web server having single asp.net worker process can/will have multiple instances of the singleton class each separate from another.
Yes, you have a Singleton per each JVM and even class loaders.
See this When is a Singleton not a Singleton? article (for Java).
What do you mean by "2 web servers"?
Static field (in singleton) is scoped to Application Domain (in .net).
So if your two web servers run in two separate application domains, then yes, otherwise no.
In fact you may well have 2 instances of a singleton calls in ONE web server, if its part of a webapp that is deployed twice.
In Java, the classloader is part of a class's identity. You can load the same class twice with different classloaders, and all static fields will exist twice. C# has a similar mechanism.
It is possible to create a web server that will multiplex over everything - connections, listening sockets, even interfaces. It will be still one instance of the class, only one thread, and on top of that a pretty small memory footprint. The caveat is, that while from most practical standpoints it will look like two servers, it will still be only one server (and if it crashes, it crashes whole...)
This approach is not nearly as popular as multi-threaded webservers though, because while lighter on hardware, it is harder for developer to handle - you have to explicitly multiplex between everything, and juggle all connections in non-blocking calls. If you spawn some extra threads, the OS takes away a lot of work from you, allowing you to write a more feature-rich server easier.
Of course even in a single-threaded server spawning of a second server in a separate task is still possible, just by the user/admin/whoever executing the binary again, with different config. It takes some pretty fancy programming to prevent that from happening.
This is why JSP/Servlets provide the idea of "session" and "application" data. These should be shared across servers in a multi-server environment.
As an extension of this question, specifically for .NET web apps, you also need to pay attention to SessionState handling. Assuming that sessions aren't "sticky" (user stays on one web server once session is established), you'll need to change SessionState to out-of-process. This can either be the ASP.NET session state server, or SQL Server, but the key point to remember is that SessionState isn't automatically shared across servers, unless you make it shared by going out-of-process. Also, anything you put in SessionState needs to be serializable; add the [Serializable] attribute to any classes you use in SessionState.
I want to have an application that works as a Host to many other small applications. Each one of those applications should work as kind of plugin to this main application. I call them plugins not in the sense they add something to the main application, but because they can only work with this Host application as they depend on some of its services.
My idea was to have each of those plugins run in a different app domain. The problem seems to be that my host application should have a set of services that my plugins will want to use and from what is my understanding making data flow in and out from different app domains is not that great of a thing.
On one hand I'd like them to behave as stand-alone applications(although, as I said, they need to use lots of times the host application services), but on the other hand I'd like that if any of them crashes, my main application wouldn't suffer from it.
What is the best (.NET) approach to this kind of situation? Make them all run on the same AppDomain but each one in a different Thread? Use different AppDomains? One for each "plugin"? How would I make them communicate with the Host Application? Any other way of doing this?
Although speed is not an issue here, I wouldn't like for function calls to be that much slower than they are when we're working with just a regular .NET application.
Thanks
EDIT: Maybe I really need to use different AppDomains. From what I've been reading, loading assemblies in different AppDomains is the only way to later be able to unload them from the process.
I've implemented something along these lines using the Managed Addin Framework (MAF) in the System.Addin namespace. With MAF you package your addins as separate DLLs, which your host app can discover and launch in its app domain, in a separate domain for all of the addins, or each addin in its own domain. With shadow copy and separate domains you can even update an addin without shutting down your hostapp.
Your host app and the addins communicate through contracts that you derive from MAF interfaces. You can send objects back and forth between the host and the addins. The cotnracts provide a black-box interface between addins and the host, allowing you to change an addin's implementation unbeknownst to the host.
Addins can even communicate between themselves if the host tells them about each other. In my case a logging addin is shared by the others. This lets me drop in different loggers without touching the other addins or the host.
For my app, the addin use simple supervisor classes that in launch worker classes on their own threads that do all of the processing. Workers catch their own exceptions, which they return to their supervisor through callback methods. Supervisors can restart workers or take other action. The host controls the supervisors through a command contract, which instructs them to start and stop workers and return data.
My host app is a Windows service. The worker threads have thrown exceptions for all the usual reasons (including bugs!), but the host app has never crashed in any of our installations. Since debugging services is inconvenient, addins allow me to build test apps that use the same contracts, with added assurance that I'm testing what I deploy.
Addins can expose UI elements, too. This is very helpful to me as I need to deploy a controller app with the host service, since services do not have UIs. Each plugin includes its own controller interface. The controller app itself is very simple - it loads the addins and displays their UI elements. This allows me to ship an updated addin with an updated interface and not have to ship a new controller.
Even though the controller and the host service use the same addins, they don't step on each other; in fact, they don't even know that another app is using the same addins. The controller and the host talk to each other through a shared database, but you could also use another inter-app mechanism like MSMQ. In the next version the host will be a WCF service with addins on the backend and web services for control.
This is a bit long-winded but I wanted to give you an idea of how versatile MAF is. It's not as complex as it might first look, and you can build rock-solid apps with it.
It depends on how much trust you wish to allow the extensions. I'm working on a similar application and I've chosen to mostly trust the extension code, as this greatly simplifies things. I call into the code from a common thread (in my case, the extensions don't really 'run' in any continuous loop, but rather execute certain tasks that the main application wants to do) and catch exceptions in this thread, so as to provide helpful warnings that loaded extensions are misbehaving.
Currently there's nothing keeping these extensions from launching their own threads that could throw and crash the whole app, but this where I've had to make the trade-off between safety and complexity. My application is not mission-critical (not like a web server or database server), so I consider it an acceptable risk that a buggy extension could bring down my application. I provide safeguards to more politely cover the most common failure cases and leave it to the plugin developers (who will mostly be in-house people for now anyway) to clean up their bugs.
In regards to Unloading, yes, you can only unload the code and metadata for an assembly if you place it in an AppDomain. That said, unless you want to be loading and unloading frequently over the life of your program, the overhead associated with keeping the code in memory is not necessarily an issue. Any actual instances or resources using types from the assembly will still be cleaned up by the GC when you stop 'using' it, so the fact that it's still in memory doesn't imply a memory leak.
If your main use case is a series of plugins that you locate once at startup and then provide an option to instantiate while your app is running, I suggest investigating the real memory footprint associated with loading all of them at start-up and keeping them loaded. If you use AppDomains, there will be additional overhead there as well (for instance, memory for the proxy objects and loaded/JITed code to support AppDomain marshaling). There will also be CPU overhead associated with the marshaling and attendant serialization.
In short, I would only use AppDomains if one of the following were true:
I want to get true isolation for the purposes of code security (i.e. I need to run untrusted code in an isolated way)
My app is mission-critical and I absolutely need to make sure that if a plugin fails, it can't bring down my core app.
I need to load and unload the same plugin repeatedly, in order to support dynamic changes to the DLL. This is mainly if my app can't stop running, but I want to hot-patch plugins while it's still running.
I would not prefer AppDomains for the sole purpose of reducing possible memory footprint by allowing Unload.
This is an interisting question.
My first idea was to simply implement interfaces from your host application in your plugin applications to allow them to communicate through Reflection, but this would only allow communication and would not bring a real "sandbox-like" architecture.
My second thought was to design a service-oriented platform. The host application would be a kind of "plugin broadcaster" which would publish your plugins in a ServiceHost on a different thread. As this need to be really responsive and "no brainer configurated", the host application could communicate with the plugin through named pipes channel (NetNamedPipesBinding for WCF) which means is only communicating with localhost pipes and does not need any network configuration or knowledge at all. I think this could be a good solution to your problem.
Regards.