I'm using a singleton pattern for the datacontext in my web application so that I dont have to instantiate it every time, however I'm not sure how web applications work, does IIS open a thread for every user connected? if so, what would happend if my singleton is not thread safe? Also, is it OK to use a singleton pattern for the datacontext? Thanks.
I'm using a singleton pattern for the datacontext in my web application
"Singleton" can mean many different things in this context. Is it single-instance per request? Per session? Per thread? Per AppDomain (static instance)? The implications of all of these are drastically different.
A "singleton" per request (stored in the HttpContext) is fine. A singleton per session is discouraged, but can be made to work. A singleton per thread may appear to work but is likely to result in unexpected and difficult-to-debug behaviour. A singleton per Application or AppDomain is a disaster waiting to happen.
so that I dont have to instantiate it every time
Creating a DataContext is very, very cheap. The metadata is globally cached, and connections aren't created until you actually execute a query. There is no reason to try to optimize away the construction of a DataContext instance.
however I'm not sure how web applications work, does IIS open a thread for every user connected?
IIS uses a different thread for every request, but a single request may use multiple threads, and the threads are taken from the Thread Pool, which means that ultimately the same user will have requests on many different threads, and conversely, different users will share the same thread over multiple requests and an extended period of time. That is why I mention above that you cannot rely on a Thread-Local Singleton.
if so, what would happend if my singleton is not thread safe?
Very bad things. Anything that you cache globally in an ASP.NET application either needs to be made thread safe or needs to be locked while it is in use.
Also, is it OK to use a singleton pattern for the datacontext? Thanks.
A DataContext is not thread-safe, and in this case, even if you lock the DataContext while it is in use (which is already a poor idea), you can still run into cross-thread/cross-request race conditions. Don't do this.
DataContext instances should be confined to the scope of a single method when possible, using the using clause. The next best thing is to store them in the HttpContext. If you must, you can store one in the Session, but there are many things you need to be aware of (see this question I answered recently on the ObjectContext - almost all of the same principles apply to a DataContext).
But above all, do not create "global" singleton instances of a DataContext in an ASP.NET application. You will deeply regret it later.
Many people keep the DataContext around for the duration of the request by keeping it in the HttpContext.Current.Items Thereby it is also private to the request.
Have a look at this blogpost by Steve Sanderson, and the UnitOfWork pattern.
Static variables are visible to all users on the per app domain, not per session. Once created, the variable will sit in memory for the lifetime of the app domain, even if there are no active references to the object.
So if you have some sort of stateful information in a web app that shouldn't be visible to other users, it should absolutely not be static. Store that sort of information in the users session instead, or convert your static var to something like this:
public static Data SomeData
{
get
{
if (HttpContext.Session["SomeData"] == null)
HttpContext.Session["SomeData"] = new Data();
return (Data)HttpContext.Session["SomeData"];
}
}
It looks like a static variable, but its session specific, so the data gets garbage collected when the session dies and its totally invisible to other users. There safety is not guaranteed.
Additionally, if you have stateful information in a static variable, you need some sort of syncronization to modify it, otherwise you'll have a nightmare of race conditions to untangle.
#ryudice the web server creates a new thread for each request. I think the best approach is to have a datacontext bound to each request, meaning that you should create a new datacontext every time you serve a request. A good way of achieving this is by using a DI tool, such as StructureMap. These kind of tools allow you to setup the lifecycle of the instances you configure, so for example in your case you would configure your XDataContext class to be HttpContext scoped.
Regards.
here are Microsoft's examples on how to do multi-tier with LINQ-To-SQL.
http://code.msdn.microsoft.com/multitierlinqtosql
Related
As you know, the HttpContext.Current returns the current context in the application pipeline.
Also this property is static, so logically any changes on that or on its properties should affect the other pipelines.
A static field identifies exactly one storage location. No matter how
many instances of a class are created, there is only ever one copy of
a static field.
More
How IIS handle this to prevent conflict on the other pipelines and every HttpContext.Current be unique on each pipeline?
For example for two users that already logged into the system, the HttpContext.Current.User.Identity.Name gives the username of the user who sent the request to the server.
ASP.NET Pipeline:
Current is the property, not field, so it's a static method actually.
This method can return different instances for different threads, and it really does.
If you're developing multithread web application, keep in mind a few things.
Don't use ThreadStaticAttribute. It works in Windows and console applications, but it may not work in web applications, since a single request can be handled by different threads, if you use async, await, and Task<T>.
Use HttpContext.Current.Items instead of ThreadStaticAttribute. These Items are "static" in each HttpContext.
Use SynchronizationContext if you need important settings of HttpContext (regional settings, logged user, and your own HttpContext.Items) after asynchronous calls (if you're not using await).
The reason why you should be careful is a thread pool. It's quite possible that your asynchronous method starts to run in a first thread, continues in a second, and ends in a third. Since each thread has its own copy of the thread static field, you can get unpredictable different values of the field in different locations of your method. SynchronizationContext allows you to return to the initial thread with correct values of regional settings, HttpContext.Items, etc. The await operator does it work for you, so you shouldn't care about context, if you're using the await (thanks to #StephenCleary for the correction).
Now for the thread-static fields. When ASP.NET gets a HTTP request, it creates the new instance of HttpContext with empty HttpContext.Items collection. At the same time ThreadStatic fields are initialized already by previous HTTP request. Therefore f.e. a Singleton class, based on a thread-static field may not work properly. It's important both in synchronous and asynchronous methods of a web application.
The answer lies in thread-local storage, implemented with ThreadStatic in .NET. The ambient context design pattern is also relevant here.
I know that the AmazonDynamoDBClient is thread-safe and I can init a singleton when my app starts and keep using it throughout the apps lifetime.
Does the same apply to DynamoDBContext?
Yes, the DynamoDBContext is thread-safe.
It also maintains a number of caches (like table descriptions) which are populated from DynamoDB, so it's best to keep an instance around for as long as possible. This will help avoid making extra service calls to repopulate the caches.
How do I handle a Linq-to_SQL DataContext across multiple threads?
Should I be creating a global static DataContext that all the threads use and commit changes at the very end or should I create a Context per thread and use that instance for everything inside that thread?
DataContext is not thread safe; using it directly from multiple threads would cause #fail; having a global static data-context would cause #fail and would cause uncontrolled memory growth (the data-context includes an identity manager and change tracker for every object fetched; this only grows over time, as more objects are touched)
Data context should ideally be used for a unit of work; spin one up; do something (that is bound in scope - i.e. not the entire app lifetime), and dispose it. So IMO the real answer here is "tie it to that unit of work". Only you can know what that is in your application; it could be a single method, it could a page request on a web page, it could be a timer "tick" in a service. Who knows...
I have a RESTful C# web service (using Open Rasta) that I want to run IronPython scripts that talk to a CouchDB.
One thing I could use some clarification on is: How often do I need a new instance of the python engine and the scope? one each per application? per session? per request?
I currently have a static engine at the application level along with a dictionary of compiled scripts; then, per request, I create a new scope and execute the code within that scope...
Is that correct? thread safe? and as performant as it could be?
EDIT: regarding the bounty Please also answer the question I posed in reply to Jeff: Will a static instance of the engine cause sequential requests from different clients to wait in line to execute? if so I will probably need everything on a per-request basis.
A ScriptRuntime/ScriptEngine per application and a Scope per request is exactly how it should be done. Runtimes/Engine are thread-safe and Scopes are not.
Per request is the way to go unless all of your code is thread safe. You may get better performance using per application (per session implies you have th notion of "sesions" between you client and server), however the implication there is that all of your code in the "application" is thread safe.
So per-request is what you should use unless you know your code to be thread safe.
Note also that per application will be faster only if:
In order to make things thread safe
you've not blocking threads in any
way.
To a certain extent if the
business layer/data layer are
extremely "heavy" (take a lot of
time to instantiate) then some
performance benefit may be gained.
I have a web app that currently uses the current HttpContext to store a LINQ Data Context. The context is persisted for the current request, on a per user basis, per Rick Strahl's blog:
string ocKey = "ocm_" + HttpContext.Current.GetHashCode().ToString("x")
Thread.CurrentContext.ContextID.ToString();
if (!HttpContext.Current.Items.Contains(ocKey))
{
// Get new Data Context and store it in the HTTP Context
}
However, I have some scripts that execute from the global.asax file, that don't have an HttpContext. The HttpContext.Current is NULL, because the server is the one making the "request".
Is there an equivalent object that I can use to store the Data Context? So I don't have to worry about re-creating it, and attaching/detaching objects? I only want to persist the context for the lifetime of my processes.
UPDATED:
I am currently trying to use a static variable in my DAL helper class. on the first call to one of the methods in the class the DataContext is instantiated, and stored in the static variable. At the end of my process, I call another method that calls Dispose on the DataContext, and sets the static variable to NULL.
Can you not just use a static variable specifically for those scripts? That will have the same life-time as the AppDomain. You should probably think carefully about any concurrency concerns, but it sounds like the simplest way to keep a value around.
(I've just checked, and although one instance of HttpApplication can be used to service multiple requests, each one only serves one request at a time - which suggests that multiple instances are created for concurrent request processing. I haven't validated this, but it does sound like it wouldn't be safe to keep it in an instance variable.)
EDIT: Josh's answer suggests that you want this to be per-thread. That sounds slightly odd to me, as unless you've got a lot of these events occurring, you're quite likely to only ever see them execute on different threads, making the whole sharing business pointless. If you really do want that sort of thing, I'd suggest just using an instance variable in the HttpApplication-derived class - for exactly the reason described in the paragraph above :)
Why not use the current HttpContext? The scripts in your global.asax file are all the result of a request coming into the server, so there should be a context associated with that request which you can grab.
I don't understand the need for generating the key based on the hashcode or the thread. There is going to be a separate instance of HttpContext for each request that comes in, and that instance is going to be specific to the thread that is processing the request. Because of that, the key is pretty much worthless when it's based on the instance of HttpContext and the thread.
Also, how do you dispose of the DataContext when you are done? It implements IDisposable for a reason, so I would recommend against a shared instance like this.
UPDATE
In the comments, it indicates that there is a timer that is running that is executing the scripts. Instead of the timer, I would recommend setting up a Scheduled Task which will call a webservice or predetermined page on the site which will perform the task. Then you will always have an HttpContext to work with.
HttpContext.Current is a static method and should be available from anywhere as long as the code is executing within the context of a request.
In your case your not executing within the context of a request, You could look at using Application.Cache but I would caution against holding a DataContext open. I am not very famillar with linq to entities, so I could be wrong, but generally caching data base related items such as connections is bad.
I would also recommend that you consider moving the logic out of your global.asax and to a windows service. This would let you have more control over these tasks, for example you can shut them down seperatley of the web site.
Edit
As JS points out you could use a static variable. You could also define an instance variable marked with ThreadLocal attribute. This will give each thread its own copy of the variable, and can eliminate contention. Since you want each thread to have its own copy anyways.
Is there a reason why these need to be handled the same way as the other DataContexts? It seems to me that if the context is only needed inside the event handling routine, you shouldn't need to keep it around. Especially if it is in Application_Start (as per your comment), I wouldn't bother caching it anywhere -- just use it locally and pass it to the other methods as needed.
Set the DataContext as the state parameter when creating the timer. Based on the info you posted on the comments, it seems to me that your DataContext is more related to the timers than anything else.
Also avoid using the same DataContext for different timers, because you would end up with mixed modifications from the different timers. Also make sure your same timer logic isn't run twice, since it would cause the same i.e. too short period with no control.