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How to execute hub methods on database changes

I'm using Entity Framework Core and SQLite as relational database, but also ASP.NET Core to display the data on a website.
Now what I want to do is every time something happens to the database (insert, update, delete), I also want a SignalR method to be executed. This way every client gets noticed about the changes.
Is it possible to observe the SQLite database?
Or should I call the hub methods manually after every statement to the db?
My Hub Class
public class GeneralHub : Hub
{
public async Task ServerSendUpdate()
{
List<int> list = CountTickets();
await Clients.All.SendAsync("ClientReceiveUpdate", list);
}
private List<int> CountTickets()
{
NamedRestClient _namedRestClient = new NamedRestClient();
return _namedRestClient.CountTickets().Result;
}
}
At the moment the amount of objects in my database is displayed on the website by invoking method ServerSendUpdate()

EF core DbContext in a multithreaded API application

tl;dr How can I use Entity Framework in a multithreaded .NET Core API application even though DbContext is not threadsafe?
Context
I am working on a .NET Core API app exposing several RESTful interfaces that access the database and read data from it, while at the same time running several TimedHostedServices as background working threads that poll data regularly from other webservices and store them into the database.
I am aware of the fact that DbContext is not threadsafe. I read a lot of docs, blog Posts and answers here on Stackoverflow, and I could find a lot of (partly contradictory) answers for this but no real "best practice" when also working with DI.
Things I tried
Using the default ServiceLifetime.Scoped via the AddDbContext extension method results in exceptions due to race conditions.
I don't want to work with locks (e.g. Semaphore), as the obvious downsides are:
the code is polluted with locks and try/catch/finally for safely releasing the locks
it doesn't really seem 'robust', i.e. when I forget to lock a region that accesses the DbContext.
it seems redundant and 'unnatural' to artificially syncronize db access in the app when working with a database that also handles concurrent connections and access
Not injecting MyDbContext but DbContextOptions<MyDbContext> instead, building the context only when I need to access the db, using a using statement to immediatelly dispose it after the read/write seems like a lot of resource usage overhead and unnecessarily many connection opening/closings.
Question
I am really puzzled: how can this be achived?
I don't think my usecase is super special - populating the db from a Background worker and querying it from the web API layer - so there should be a meaningful way of doing this with ef core.
Thanks a lot!

You should create a scope whenever your TimedHostedServices triggers.
Inject the service provider in your constructor:
public MyServiceService(IServiceProvider services)
{
_services = services;
}
and then create a scope whenever the task triggers
using (var scope = _services.CreateScope())
{
var anotherService = scope.ServiceProvider.GetRequiredService<AnotherService>();
anotherService.Something();
}
A more complete example is available in the doc

Another approach to create own DbContextFactory and instantiate new instance for every query.
public class DbContextFactory
{
public YourDbContext Create()
{
var options = new DbContextOptionsBuilder<YourDbContext>()
.UseSqlServer(_connectionString)
.Options;
return new YourDbContext(options);
}
}
Usage
public class Service
{
private readonly DbContextFactory _dbContextFactory;
public Service(DbContextFactory dbContextFactory)
=> _dbContextFactory = dbContextFactory;
public void Execute()
{
using (var context = _dbContextFactory.Create())
{
// use context
}
}
}
With factory you don't need to worry about scopes anymore, and make your code free of ASP.NET Core dependencies.
You will be able to execute queries asynchronously, which not possible with scoped DbContext without workarounds.
You always be confident about what data saved when calling .SaveChanges(), where with scoped DbContext there are possibilities that some entity were changed in other class.

Do Redis's GetDatabase() method caches all database tables?

I have cached my database using following code for Redis operations:
public bool InitialiseCache()
{
try
{
_cache = Connection.GetDatabase();
return true;
}
catch (Exception ex)
{
return false;
}
}
I tried to debug and preview value of _cache but it does not display cached data (tables). I wanted to confirm that GetDatabase() method caches all tables. Is there any way to preview all Redis keys, or values?

Short Answer :
No, Redis's GetDatabase() method DOES NOT caches all database tables
Long Answer :
As Per StackExchange.Redis on Github :
Using a redis database
Accessing a redis database is as simple as:
IDatabase db = redis.GetDatabase();
The object returned from GetDatabase is a cheap pass-thru object, and does not need to be stored. Note that redis supports multiple
databases (although this is not supported on "cluster"); this can be
optionally specified in the call to GetDatabase. Additionally, if you
plan to make use of the asynchronous API and you require the
Task.AsyncState to have a value, this can also be specified:
int databaseNumber = ...
object asyncState = ...
IDatabase db = redis.GetDatabase(databaseNumber, asyncState);
Once you have the IDatabase, it is simply a case of using the redis
API. Note that all methods have both synchronous and asynchronous
implementations. In line with Microsoft's naming guidance, the
asynchronous methods all end ...Async(...), and are fully await-able
etc.

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.

stub webserver for integration testing

I have some integration tests where I want to verify certain requires are made against a third-[arty webserver. I was thinking I would replace the third-party server with a stub server that simply logs calls made to it. The calls do not need to succeed, but I do need a record of the requests made (mainly just the path+querystring).
I was considering just using IIS for this. I could 1) set up an empty site, 2) modify the system's host file to redirect requests to that site 3) parse the log file at the end of each test.
This is problematic as for IIS the log files are not written to immediately, and the files are written to continuosly. I'll need to locate the file, read the contents before the test, wait a nondeterministic amount of time after the test, read the update contents, etc.
Can someone think of a simpler way?

You could use the System.Net.HttpListener ( MSDN LINK ).
It works as embedded WebServer, this means you can even check the access on-the-fly without having to parse log files.
A class i used in my Code recently:
class Listener
{
private HttpListener listener = null;
public event EventHandler CommandReceived;
public Listener()
{
this.listener = new HttpListener();
this.listener.Prefixes.Add("http://localhost:12345/");
}
public void ContextReceived(IAsyncResult result)
{
if (!this.listener.IsListening)
{
return;
}
HttpListenerContext context = this.listener.EndGetContext(result);
this.listener.BeginGetContext(this.ContextReceived, this.listener);
if (context != null)
{
EventHandler handler = this.CommandReceived;
handler(context, new EventArgs());
}
}
public void Start()
{
this.listener.Start();
this.listener.BeginGetContext(this.ContextReceived, this.listener);
}
public void Stop()
{
this.listener.Stop();
}
}

Yeah, I don't think you need a whole webserver. You don't need to test HTTP.
What you do need to test is the underlying data structure that you're sending and receiving. So just create tests for that (i.e. make a point at which you can validate your generate dataformat with what is expected, and also with what you intend to receive, etc).
Test the data, not the protocol (unless, obviously, the protocol is custom).

I've done something very similar to this in a number of projects.
You don't want to create stubbed web service. That's just adding a dependency you don't need. What I did was create an interface which mimics the web service's API. I then created a proxy class that will call the web service in the live system. For testing I used RhinoMocks to create mocked classes that return the results I wanted to test for. This was very useful for me, as I could then produce all sorts of 'unexpected' behaviour which wouldn't be possible with the live system.
public interface IServiceFacade {
string Assignments();
}
public class ServiceFacade : IServiceFacade {
private readonly Service _service;
public ServiceFacade(Service service) {
_service = service;
}
public string Assignments() {
return _service.Assignments();
}
}
Then my test code contained stuff like this:
var serviceFacade = MockRepository.GenerateMock<IServiceFacade>();
serviceFacade.Stub(sf => sf.Assignments()).Return("BLAH BLAH BLAH");
or
serviceFacade.Stub(sf => sf.Assignments()).Return(null);
or
serviceFacade.Stub(sf => sf.Assignments()).Throw(new Exception("Some exception"));
I found this very useful.