We Keep Coding

How to transfer DI requestscope to an other thread?

Context:
I am using DI in my Web application. (I am using NInject, but hopefully this should not matter)
Some places constructor injection is not possible, for example in my custom log4net database logger (that's not me, who instantiates my custom logger instead the log4net framework). So I am using my DI container there in service locator DP mode, and asking an instance resolve explicitly in the logger code.
Note this is just a sample, in many other cases I had to use NInject as service locator DP instead of constructor injection.
Now the problem:
I have an IAuditContextProvider which serves current request's audit data, like IP etc. The question arises how I configure my DI container to instantiate a concrete provider. So far I've used a request scope (singleton by request) what is supported out of box by NInject.
However recently I faced the fact I had to start a background processing initiated by a request. This is done by
// This is 'outside' it's actually a request serving method running in the request context, btw it is an MVC action method,
// Pseudo code:
var auditProvider = Locator.Resolve<IAuditProvider>()
Task.Run(() =>
{
// I would like to get the very same resolved auditProvider instance here as outside.
// Please note: outer local variables are not solution, because of implicit calls here inside, for example if there is a logging statement here, then the service locator in the custom logger must resolve the very same instance as outside
// Some how I must 'stamp' this thread to be the 'same' as the outside
// request thread in point of view of my custom scope resolver (see below)
}
Note: Configuring the DI container a wide scoped singleton are not solution because of multiple requests are server parallel, and they can not use a common auditProvider.
OK, I thought this is what for custom (resolving) scopes are for. Here is the pseudo code how I am configuring my DI container:
kernel
.Bind(typeof(IAuditContextProvider))
.To(typeof(WebAuditContextProvider)).InScope(dummy =>
{
// Here I have to return a very same object/number/id when in
// 'outside' the request thread, and inside the worker thread.
// This way I hopefully get the very same instance when resolving.
// To be short: I have no idea how?
});

I don't think there is a good answer for your question within the current bounds.
I do have an alternative suggestion - just perform the work synchronously in another process. This would require a form of inter-process communication (IPC) but shouldn't be too difficult.
A simple but effective form of IPC is just writing a record to a database table (acting like a queue) and having a windows service/daemon polling for new records to "process". In this example, you would put a record in the table with the contextual information (user id, etc) and the service would utilize this context to perform the work synchronously, but the workflow would be asynchronous to the Web UI.
This also has a nice side benefit: You can start to build monitoring, retry logic, etc into the service. These things are much harder to do reliably within an ASP.NET model.
You could forgo the database queue completely by using something like message queues/buses/events, but the basic concept is the same.

Update:
Did you try to use closures in C#? Like this:
var auditProvider = Locator.Resolve<IAuditProvider>()
Task.Run(() =>
{
// with closure you'll get that very variable you need:
auditProvider.SomeMethod();
}
You should read whole article about closures by John Skeet and how they can help you together with TPL.
Other useful information:
Such DI is being called as Ambient Context in famous book Dependency Injection by M. Seeman:
A truly universal CROSS-CUTTING CONCERN can potentially pollute a large part of the API for an application if you have to pass an instance around to every collaborator. An alternative is to define a context that’s available to anyone who needs it and that can be ignored by everyone else.
The AMBIENT CONTEXT is available to any consumer via a static property
or method. A consuming class might use it like this:
public string GetMessage() { return SomeContext.Current.SomeValue; }
In this case, the context has a static Current property that a consumer can access. This property may be truly static, or may be associated with the currently executing thread. To be useful in DI scenarios, the context itself must be an ABSTRACTION and it must be possible to modify the context from the outside—in the previous example, this means that the Current property must be writable. The context itself might be implemented as shown in the following listing.
The context is an abstract class, which allows us to replace one context with another implementation at runtime.
public abstract class SomeContext
{
public static SomeContext Current
{
get
{
// Get current context from TLS
var ctx = Thread.GetData(Thread.GetNamedDataSlot("SomeContext")) as SomeContext;
if (ctx == null)
{
ctx = SomeContext.Default;
Thread.SetData(Thread.GetNamedDataSlot("SomeContext"), ctx);
}
return ctx;
}
set
{
Thread.SetData(Thread.GetNamedDataSlot("SomeContext"), value);
}
}
public static SomeContext Default = new DefaultContext();
public abstract string SomeValue { get; }
}
TLS here stands for Thread Local Storage, which can be useful idea for you here.
Also I suggest you to read about OperationContext class, which can be helpful for you if you want to pass some context for your Task, something like this:
// save current context before task start
var operationContext = OperationContext.Current;
Task.Run(() =>
{
// set current operation context inside your Task with closure
OperationContext.Current = operationContext;
// Your work here
}

How to reconfigure SQLTransport based NServicebus in Asp.net Web API?

I am using NServicebus(version 4.6.3) with SQLTransport in my ASP.net web api project. I have different connectionstrings for the queues for different environments (Dev,QA,etc). My configuration looks like below:
public class BusConfigurator
{
public static IStartableBus Bus { get; private set; }
public static void DisposeBus()
{
if (Bus == null)
return;
Bus.Shutdown();
Bus.Dispose();
Bus = null;
}
public static void InitializeServiceBus(string connectionString)
{
var configure = Configure.With()
.DefineEndpointName("MyEndPoint")
.Log4Net(new DebugAppender { Threshold = Level.Warn })
.UseTransport<SqlServer>(connectionString)
.PurgeOnStartup(false)
.SetDefaultTransactionLevel()
.UnicastBus(); // Error is thrown here on second call
configure.MyCustomSQLServerPersistence();
Bus = configure.CreateBus();
}
public static void StartBus()
{
Bus.Start(() => Configure.Instance.ForInstallationOn<NServiceBus.Installation.Environments.Windows>().Install());
}
}
I have a dropdown in the app so that the user can select the environment. Based on the selection, I want to reconfigure the bus. So, I call DisposeBus then pass the connection string to the IntializeServiceBus method followed by the startBus. It works first time but throws error below when it gets called again with different connectionstring:
Unable to set the value for key: NServiceBus.Transport.ConnectionString. The settings has been locked for modifications. Please move any configuration code earlier in the configuration pipeline
Source=NServiceBus.Core
Line=0
BareMessage=Unable to set the value for key: NServiceBus.Transport.ConnectionString. The settings has been locked for modifications. Please move any configuration code earlier in the configuration pipeline
Is NServicebus intended to be used/configured this way? (I am guessing probably not) If not then is there a workaround/different approach for this?

In V4 or below, there is no way to do it by normal human means. There is only one Bus per AppDomain. All of the configuration API is static, so if you try, you get exactly the problems you ran into.
By "human means", I mean that it might be possible to do something crazy with spinning up a new AppDomain within your process, setting up a Bus within that, and then tearing it down when you're finished. It might be possible. I haven't tried it. I wouldn't recommend it.
In V5, the configuration API is completely redesigned, is not static, and so this is possible:
var cfg = new BusConfiguration();
// Set up all the settings with the new V5 Configuration API
using (var justOneBus = NServiceBus.Bus.Create(cfg).Start())
{
// Use justOneBus, then it gets disposed when done.
}
That's right. It's disposable. Then you can do it again. In your case you wouldn't want to put it in a using block - you would want to set it up somewhere, and when the dropdown gets switched, call Dispose on the current instance and rebuild it with the new parameters.
Keep in mind, however, that the Bus is still pretty expensive to create. It's definitely still something you want to treat as an application-wide singleton (or singleton-like) instance. You definitely wouldn't want to spin up a separate one per web request.

How to cache data from repository call

I am working in a multi-layered web application that has ASP.NET MVC as its front-end client. A particular page of this web application is taking a very long time to load. Around 30 seconds.
I downloaded dotTrace and ran it on my application (following this tutorial). I found out that the reason my application is slow.
It turns out it is because one particular method that I have does a load of work (takes time), and that same method gets called a total of 4 times.
Here is a screenshot from dotTrace showing the above:
The method in question is GetTasks(). So in order to improve the speed of the web application I want to cache the data returned from GetTasks() for each request.
If my thinking is correct, this would really improve on the speed issues I am having.
My question is, how can I achieve this? I have never done such a thing before. For each new request, how can I cache the data returned from GetTasks(), and use that for all subsequent calls to GetTasks().

Have you considered the Cache Aside pattern?
You can implement it easily using LazyCache
//probably in my constructor (or use dependency injection)
this.cache = new CachingService()
public List<MyTasks> GetTasks()
{
return cache.GetOrAdd<List<MyTasks>>("get-tasks", () = > {
//go and get the tasks here.
});
}
For more information see https://alastaircrabtree.com/the-easy-way-to-add-caching-to-net-application-and-make-it-faster-is-called-lazycache/

One of the most popsular solution is to cache results. I can shouw you my solution.
First of all install Nuget package: LazyCache
Then you can use wrapper that I've created wrapper: code. You can extract and interface or whatever.
Then you can use it like this:
private readonly CacheManager cacheManager = new CacheManager();
// or injected via ctor
public IEnumerable<Task> GetTasks()
{
return this.cacheManager.Get("Tasks", ctx => this.taskRepository.GetAll());
}
public void AddTask(Task task)
{
this.taskRepository.Create(task);
/// other code
// we need to tell the cache that it should get fresh collectiion
this.cacheManager.Signal("Tasks");
}

SignalR multiple hubs on different paths in same application?

I have an application which contains multiple hubs all on unique paths, so when calling the default :
routes.MapHubs("path", new HubConfiguration(...));
It blows up saying that the signalr.hubs is already defined (as mentioned here MapHubs not needed in SignalR 1.01?).
Now I can understand that it should only be called once, but then you will only get 1 path, so is there any way to handle a path per hub scenario? like how with MVC you specify the controller and action? so something like:
routes.MapHub<SomeHub>("path", new HubConfiguration(...));
== Edit for more info ==
It is mentioned often that you should never need to call this map hubs more than once, and in most scenarios I can agree, however I would not say that this is going to be the case for all applications.
In this scenario it is a website which at runtime loads any plugins which are available, each plugin is exposed the dependency injection framework to include its dependencies and the route table to include its routes. The hubs may have nothing to do with each other (other than the fact that they are both hub objects). So the hubs are not all known up front and are only known after the plugins are loaded, and yes I could wait until after this and try binding the hubs there, however then how do I have custom routes for each one then?
This seems to be a case of SignalR trying to abstract a little too much, as I dont see it being a bad idea to have custom routes rather than the default "/signalr", and as the routes all have different responsibilities it seems bad to have one entry route for them all.
So anyway I think the question still stands, as I dont see this as being a bad use case or bad design it just seems to be that I want to be able to have a route with a hub applied to it, much like in mvc you apply a controller and action to a route.

You shouldn't need more than the signalr.hubs route. If you point your browser to that route, you will see it automatically finds all public types assignable to IHub and creates a JavaScript proxy for them. You can interact with different hubs by name from JavaScript, i.e. if you have the following Hub:
public class GameHub : Hub
You can connect to that specific hub by doing:
var gameHubProxy = $.connection.gameHub;
You can also explicitly specify a name for your hub by adding the HubNameAttribute to the class:
[HubName("AwesomeHub")]
public class GameHub : Hub
You can then retrieve the specific proxy by doing
var awesomeHubProxy = $.connection.awesomeHub;
UPDATE:
I'm not sure whether SignalR will be able to run on multiple paths in the same application. It could potentially mess things up and the default assembly locator won't be able to pick up hubs loaded at runtime anyway.
However, there is a solution where you can implement your own IAssemblyLocator that will pick up hubs from your plugin assemblies:
public class PluginAssemblyLocator : DefaultAssemblyLocator
{
private readonly IEnumerable<Assembly> _pluginAssemblies;
public PluginAssemblyLocator(IEnumerable<Assembly> pluginAssemblies)
{
_pluginAssemblies = pluginAssemblies;
}
public override IList<Assembly> GetAssemblies()
{
return base.GetAssemblies().Union(_pluginAssemblies).ToList();
}
}
After you've loaded your plugins, you should call MapHubs and register an override of SignalRs IAssemblyLocator service:
protected void Application_Start(object sender, EventArgs e)
{
// Load plugins and let them specify their own routes (but not for hubs).
var pluginAssemblies = LoadPlugins(RouteTable.Routes);
RouteTable.Routes.MapHubs();
GlobalHost.DependencyResolver.Register(typeof(IAssemblyLocator), () => new PluginAssemblyLocator(pluginAssemblies));
}
NOTE: Register the IAssemblyLocator AFTER you've called MapHubs because it will also override it.
Now, there are issues with this approach. If you're using the static JavaScript proxy, it won't be re-generated every time it's accessed. This means that if your /signalr/hubs proxy is accessed before all plugins/hubs has been loaded, they won't be picked up. You can get around this by either making sure that all hubs are loaded by the time you map the route or by not using the static proxy at all.
This solution still requires you to get a reference to your plugin assemblies, I hope that's feasible...

Does Resolve<T>() return objects per-session?

In Microsoft Unity IoC, if I call Resolve<SomeType>(), can I guarantee that the object returned is the one that was created during the current session?
For example, three users sign on, and let's say that the object of SomeType that gets created in the container has different values for each user. Will a call to Resolve return the object that was created for the current user? Or would it do something stupid like return the last one that was created?
I'm having troubles testing this myself due to some environment problems and I need to check something in soon, so if someone could answer this it would be very helpful!
Edit
Forgive me for I am very new to Unity, but based on what I read here, it seems like I should be able to register objects in the container with a unique name and retrieve them by that name. So, wouldn't I be able to use a session ID or some other value that persists within a session to retrieve my object?

Oh wow, lifetime management using Unity in am MVC app. Where do I start?
First of all, session singletons are not really possible as there is no ASP.NET system that will guarantee that the same instance will be used between requests in the same session. The session can mimic the same object persisted within the session by serializing and deserializing it between requests.
Transient instances - i.e. simple registrations without lifetime management specification are sufficient 99% of the time. This implies that an instance of registered type will be created every time it is needed.
It is very rarely that you need instances to live throughout the lifetime of the request. However when you need those, you really need those. A connection to a DB is a perfect candidate for this. Request singletons, on the other hand are much easier to create and manage.
The most elegant solution is to use Unity's child container feature. A child container can be created at the beginning of the request, disposed at the end of the request (as an added bonus it will dispose all ContainerControlledLifetimeManager instances).
When creating a child container, all registrations are still available from the parent container, so you need to register request specific stuff with the child container.
Here is pseudo-code to get this working:
private void Application_Start() {
_parentContainer = new UnityContainer();
//creates a transient registration, available at any point in the app.
_parentContainer.RegisterType<IParentIntf, ParentIntfImpl>();
ControllerBuilder.Current.SetControllerFactory(new ServiceLocatorControllerFactory());
}
private void Application_BeginRequest() {
var childContainer = _parentContainer.CreateChildContainer();
//registers a request "singleton"
//This registration is a type registration, an instance of RequestInterfaceImpl
//will be created when needed and then kept in the container for later use.
childContainer.RegisterType<IRequestInterface,RequestInterfaceImpl>(new ContainerControlledLifetimeManager());
//save the child container in the context, so we can use it later
HttpContext.Items["childContainer"] = childContainer;
}
private void Application_EndRequest() {
//dispose the child container
((IUnityContainer)HttpContext.Items["childContainer"]).Dispose();
}
One other thing that needs to be done is to override the Controller Factory to use the child container to create controllers. Controller are the first point of entry into the application and they could simply take a dependency on other components in their constructor.
public class UnityControllerFactory : DefaultControllerFactory {
#region IControllerFactory Members
public override IController CreateController(System.Web.Routing.RequestContext requestContext, string controllerName) {
IController controller;
controllerName = controllerName.ToLower();
var container = ((IUnityContainer)HttpContext.Items["childContainer"])
if(container.IsRegistered<IController>(controllerName))
controller = container.Resolve<IController>(controllerName);
else
controller = base.CreateController(requestContext, controllerName) ;
return controller;
}
}

The default behaviour will be to return a new instance for each resolve call, this isn't what you want.
It would be possible to create and resolve the same instance within a session, but there is no built in support as far as I know. You would have to write your own lifetime manager, and then use this when registering your type.
There is a lifetime manager that can do per thread instances, but this isn't useful for sessions as threads will get re-used, and resolve would need to also work across multiple requests to be truly session-scoped.
It's entirely possible that someone has written a lifetime manager for this.