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Distributing Requests In Parallel

I have like that scenario:
I have an endpoint and this endpoint will save the requests in List or Queue in memory and it will return immediately success response to the consumer. This requirement is critical, the consumer should not wait for the responses, it will get responses from a different endpoint if it needs. So, this endpoint must return as quickly as possible after saving the request message in memory.
Another thread will distribute these requests to other endpoints and save the responses in memory as well.
What I did till now:
I created a controller api to save these requests in the memory. I saved them in a static request List like below:
public static class RequestList
{
public static event EventHandler<RequestEventArgs> RequestReceived;
private static List<DistributionRequest> Requests { get; set; } = new List<DistributionRequest>();
public static int RequestCount { get => RequestList.Requests.Count; }
public static DistributionRequest Add(DistributionRequest request)
{
request.RequestId = Guid.NewGuid().ToString();
RequestList.Requests.Add(request);
OnRequestReceived(new RequestEventArgs { Request = request });
return request;
}
public static bool Remove(DistributionRequest request) => Requests.Remove(request);
private static void OnRequestReceived(RequestEventArgs e)
{
RequestReceived?.Invoke(null, e);
}
}
public class RequestEventArgs : EventArgs
{
public DistributionRequest Request { get; set; }
}
And another class is subscribed to that event that exists in that static class and I am creating a new thread to make some background web requests to be able to achieve 2. item which I stated above.
private void RequestList_RequestReceived(object sender, RequestEventArgs e)
{
_logger.LogInformation($"Request Id: {e.Request.RequestId}, New request received");
Task.Factory.StartNew(() => Distribute(e.Request));
_logger.LogInformation($"Request Id: {e.Request.RequestId}, New task created for the new request");
//await Distribute(e.Request);
}
public async Task<bool> Distribute(DistributionRequest request)
{
//Some logic running here to send post request to different endpoints
//and to save results in memory
}
And here is my controller method:
[HttpPost]
public IActionResult Post([FromForm] DistributionRequest request)
{
var response = RequestList.Add(request);
return Ok(new DistributionResponse { Succeeded = true, RequestId = response.RequestId });
}
I tried that approach but it did not work as I expected, it should return within milliseconds since I am not waiting for responses but it seems to wait for something, and after every single request waiting time is increasing as below:
What am I doing wrong? Or Do you have a better idea? How can I achieve my goal?

Based on you example code I tried to implement it without "eventing". Therefore I get much better request times. I cannot say if this is related to your implementation or the eventing itself for this you have to do profiling.
I did it this way
RequestsController
Just like you had it in your example. Take the request and add it to the requests list.
[Route("requests")]
public class RequestsController : ControllerBase
{
private readonly RequestManager _mgr;
public RequestsController(RequestManager mgr)
{
_mgr = mgr;
}
[HttpPost]
public IActionResult AddRequest([FromBody] DistributionRequest request)
{
var item = _mgr.Add(request);
return Accepted(new { Succeeded = true, RequestId = item.RequestId });
}
}
RequestManager
Manage the request list and forward them to some distribor.
public class RequestManager
{
private readonly ILogger _logger;
private readonly RequestDistributor _distributor;
public IList<DistributionRequest> Requests { get; } = new List<DistributionRequest>();
public RequestManager(RequestDistributor distributor, ILogger<RequestManager> logger)
{
_distributor = distributor;
_logger = logger;
}
public DistributionRequest Add(DistributionRequest request)
{
_logger.LogInformation($"Request Id: {request.RequestId}, New request received");
/// Just add to the list of requests
Requests.Add(request);
/// Create and start a new task to distribute the request
/// forward it to the distributor.
/// Be sure to not add "await" here
Task.Factory.StartNew(() => _distributor.DistributeAsync(request));
_logger.LogInformation($"Request Id: {request.RequestId}, New task created for the new request");
return request;
}
}
RequestDistributor
Distribution logic can be implemented here
public class RequestDistributor
{
public async Task DistributeAsync(DistributionRequest request)
{
/// do your distribution here
/// currently just a mocked time range
await Task.Delay(5);
}
}
Wire up
... add all these things to your dependency injection configuration
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
services.AddSingleton<RequestDistributor>();
services.AddSingleton<RequestManager>();
}
Tests
With the here provided code pieces I received all the requests back in less than 10 ms.
Note
This is just an example try to always add interfaces to your services to make them testable ;).

What happens when we don't await an async method

I have a .NET CORE 2 backend. In one of my controller endpoints, I'm creating invitations to be sent out via email. This seems to be a huge bottleneck on the endpoint and after thinking about it, I don't really need to wait for these invitations. If the email fails to send out, I can't really do anything about it anyway.
If I don't do await sendFn() would it essentially be a fire and forget method? I was reading on another stackoverflow thread that I'd have to do sendFn().ContinueWith(t => throw(t)) to be able to catch the exception since it'll be in another thread.
I have similar mailing functions around the code base. They each do slightly different things, but is there a service fn I can do to wrap these to make them fire and forget? I think some places I can just not use await (if that works), but some things alter the database context so if I don't await them I can potentially run into a case where something is accessing the same db context.
[HttpPost]
public async Task<IActionResult> CreateEvent([FromBody] Event val)
{
_ctx.Event.Add(val);
await _ctx.SaveChangesAsync();
await SendInvitations(val); // fn in question
return Ok();
}
public async Task SendInvitation(Event event)
{
forEach (var person in event.people)
{
await _ctx.Invitation.Add(person); // This shouldn't happen while another iteration or some other async code elsewhere is using the db ctx.
_ctx.SaveChangesAsync();
await _mailService.SendMail(person.email,"you have been invited"); // don't really need to await this.
}
}
I'm posting to my server with data about an event. After I create and save the event to the database, I go and create invitations for each person. These invitations are also database items. I then send out an email. I'm mostly worried that if I drop the await, then when I'm creating invitations, it may conflict with db context elsewhere or the next iteration.

To get your code to compile and run I had to make these changes:
public async Task<IActionResult> CreateEvent(Event val)
{
_ctx.Event.Add(val);
await _ctx.SaveChangesAsync();
await SendInvitation(val);
return Ok();
}
public async Task SendInvitation(Event #event)
{
foreach (var person in #event.people)
{
await _ctx.Invitation.Add(person);
await _ctx.SaveChangesAsync();
await _mailService.SendMail(person.email, "you have been invited");
}
}
I also had to write this harness code:
public OK Ok() => new OK();
public class Event
{
public List<Person> people = new List<Person>();
}
public class Person
{
public string email;
}
public interface IActionResult { }
public class OK : IActionResult { }
public class Invitation
{
public Task Add(Person person) => Task.Run(() => { });
}
public static class _ctx
{
public static List<Event> Event = new List<Event>();
public static Invitation Invitation = new Invitation();
public static Task SaveChangesAsync() { return Task.Run(() => { }); }
}
public static class _mailService
{
public static Task SendMail(string email, string message) { return Task.Run(() => { }); }
}
Then I updated SendInvitation like this:
public async Task SendInvitation(Event #event)
{
Thread.Sleep(2000);
foreach (var person in #event.people)
{
await _ctx.Invitation.Add(person);
await _ctx.SaveChangesAsync();
await _mailService.SendMail(person.email, "you have been invited");
}
Console.WriteLine("Done `SendInvitation`.");
}
Now, I can run it all like so:
var e = new Event();
e.people.Add(new Person() { email = "foo#bar.com" });
CreateEvent(e).ContinueWith(t => Console.WriteLine("Done `CreateEvent`."));
Console.WriteLine("Done `Main`.");
That outputs:
Done `Main`.
Then 2 seconds later:
Done `SendInvitation`.
Done `CreateEvent`.
If I simply change CreateEvent to this:
public async Task<IActionResult> CreateEvent(Event val)
{
_ctx.Event.Add(val);
await _ctx.SaveChangesAsync();
Task.Run(() => SendInvitation(val));
return Ok();
}
Then I get this output:
Done `Main`.
Done `CreateEvent`.
Then 2 seconds later:
Done `SendInvitation`.
That seems to be what you want.

The short answer is that you have no guarantees that that the execution of that code will complete.
That's why ASP.NET Core has infrastructure for background work: Implementing background tasks in .NET Core 2.x webapps or microservices with IHostedService and the BackgroundService class

Using SignalR Core to send messages from a Controller Method to Angular

I'm trying to use SignalR for Asp Net Core 2.1 in order to send a message from a controller method which call is triggered from a test button in Angular.
The behavior I'd expect is that when I click the button, my service invokes the controller method, which sends the test message. Then, I will simply log the message.
I want to manage this in a service in order to avoid code duplication in all of the components.
I've read some examples like this question about using SignalR in a service (I've used the second solution) and this article and the official docs but even with applying these concepts it don't seems to work. (So, or I'm absolutely applying them in a wrong way or there's still something missing but I can't find out what...)
The client connects to the Message Hub successfully and if I click the button, the method is getting hit but I don't get any message and instead I get this warning in the Chrome console:
Warning: No client method with the name 'SendAsync' found.
Sending messages works fine, the issue is just with receiving them...
The question is: what am I doing wrong? Is the error on the back-end side or in the Angular side?
I share with you all of my code (the button and the service to call the controller method are not relevant since the call to the service goes fine):
> Startup.cs
public void ConfigureServices(IServiceCollection services)
{
//...
services.AddSignalR();
}
//...
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
//...
app.UseSignalR(routes =>
{
//...
routes.MapHub<MessageHub>("/messagehub");
//...
});
}
> MessageHub.cs
public class MessageHub : Hub<ITypedHubClient>
{
public async Task SendMessage(string user, string message)
{
await Clients.All.SendAsync("ReceiveMessage", user, message);
}
}
public interface ITypedHubClient
{
Task SendAsync(string title, string name, string message);
}
> MessageController.cs
IHubContext<MessageHub, ITypedHubClient> _messageHubContext;
public MessageController(IHubContext<MessageHub, ITypedHubClient> messageHubContext)
{
_messageHubContext = messageHubContext;
}
[HttpPost("Test")]
public async Task<IActionResult> Test()
{
try
{
await _messageHubContext.Clients.All.SendAsync("ReceiveMessage","test", "test");
return Ok(true);
}
catch (Exception e)
{
return BadRequest(e);
}
}
> communication.service.ts
#Injectable()
export class CommunicationService {
private _hubConnection: HubConnection | undefined;
public async: any;
message = '';
messages: string[] = [];
private newmessage = new Subject<string>();
message$ = this.newmessage.asObservable();
constructor() {
this._hubConnection = new signalR.HubConnectionBuilder()
.withUrl('/messagehub')
//.configureLogging(signalR.LogLevel.Information)
.configureLogging(signalR.LogLevel.Debug)
.build();
this._hubConnection.start().catch(err => console.error(err.toString()));
this._hubConnection.on('SendMessage', (user: any, message:any) => {
const received = `Received: ${message}`;
//this.messages.push(received);
this.newmessage.next(received);
console.log("got something new...", received);
});
}
clear() {
this.newmessage.next("");
}
public sendMessage(): void {
const data = `Sent: ${this.message}`;
if (this._hubConnection) {
this._hubConnection.invoke('SendMessage', 'AAA' ,data);
}
this.messages.push(data);
}
}

In signalr core 2.1 you can use strongly typed hubs to declare in an interface what actions can be called on the clients :
public class MessageHub : Hub<ITypedHubClient>
{
public async Task SendMessage(string title, string user, string message)
{
await Clients.All.SendMessageToClient(title, user, message);
}
}
public interface ITypedHubClient
{
Task SendMessageToClient(string title, string name, string message);
}
in the controller :
IHubContext<MessageHub, ITypedHubClient> _messageHubContext;
public async Task<IActionResult> Test()
{
await _messageHubContext.Clients.All.SendMessageToClient("test", "test", "test");
return Ok("ok");
}
in the client :
_hubConnection.on('SendMessageToClient', (title, user, message) => {
const received = `title: ${title}, name: ${user}, message: ${message}`;
console.log(received);
});
If you don't use strongly typed hub, then to call the same method in the client it becomes :
public class MessageHub : Hub
{
public async Task SendMessage(string title, string user, string message)
{
await Clients.All.SendAsync("SendMessageToClient", title, user, message);
}
}
In that case you can use the SendAsync method on the client proxy, it's first parameter is the name of the method you want to call.
Update :
When we define a strongly typed hub with an interface, all interface methods must return a Task. With custom methods, signalr generates methods that call SendCoreAsync. That allow us to call these methods asynchronously.
If the return type of the interface methods is not a Task we get the error : All client proxy methods must return 'System.Threading.Tasks.Task'

In-memory database doesn't save data

I have a simple web-app with angular on client-side and asp.net core web-api on server-side. I use InMemoryDatabase
services.AddDbContext<ItemsContext>(options => options.UseInMemoryDatabase("ItemsDB"));
to store data for the simplisity of the development. But I've encountered an issue with that. I have one controller on web-api to response for users' requests:
[Route("api/[controller]")]
public class ItemsController : Controller
{
private readonly IApiService apiService;
public ItemsController(IApiService apiService)//using DI from Startup.cs
{
this.apiService = apiService;
}
[HttpPost, Route("addItem")]
public async Task<Response> Add([FromBody]Item item)
{
return await apiService.Add(item);
}
[HttpDelete("{id}")]
public async Task<Response> Delete(int id)
{
return await apiService.Delete(id);
}
[HttpPut]
public async Task<Response> Put([FromBody]Item item)
{
return await apiService.Put(item);
}
}
and the following Startup.cs configurations:
public void ConfigureServices(IServiceCollection services)
{
services.AddMvc();
services.AddDbContext<ItemsContext>(options => options.UseInMemoryDatabase("ItemsDB"));
services.AddSingleton<IUnitOfWork, UnitOfWork>(provider => {
var context = services.BuildServiceProvider().GetService<ItemsContext>();
return new UnitOfWork(context);
});
services.AddSingleton<IApiService, ApiService>(provider => {
return new ApiService(services);
});
}
The problem is, that when I add new item, everything goes just fine...but then I post another request to delete this item it may show there there is no such an item at all or sometimes it may delete it...so in other words, the database exists and then disappears and I'm not sure when. Here is some additional code refering to the above
public class ApiService: IApiService
{
private readonly IUnitOfWork database;
private readonly IServiceProvider provider;
public ApiService(IServiceCollection serviceCollection)
{
provider = serviceCollection.BuildServiceProvider();
}
public IUnitOfWork Database
{
get
{
return provider.GetService<IUnitOfWork>();
}
}
public async Task<Response> Add(Item item)
{
Database.Items.Add(item);
await Database.SaveAsync();
var id = Database.Items.LastItem().Id;
return new Response() { Result = true, ItemId = id };
}
public async Task<Response> Delete(int id)
{
var item = await db.Items.Find(id);
Database.Items.Remove(item);
await Database.SaveAsync();
return new Response() { Result = true };
}
public async Task<Response> Put(Item item)
{
Database.Items.Update(item);
await Database.SaveAsync();
return new Response() { Result = true };
}
}
Update:
UnitOfWork Implementation:
public class UnitOfWork: IUnitOfWork
{
private readonly DbContext context;
private IRepository<Item> itemsRepository;
public UnitOfWork(DbContext dbContext)
{
context = dbContext;
}
public IRepository<Item> Items
{
get
{
return itemsRepository ?? (itemsRepository = new Repository<Item>(context));
}
}
public void Dispose()
{
context.Dispose();
}
public void Save()
{
context.SaveChanges();
}
public async Task SaveAsync()
{
await context.SaveChangesAsync();
}
}

Your code has multiple serious problems, let's go through them.
services.AddDbContext adds a Scoped service, meaning that instances will be created and disposed on each request. services.AddSingleton adds a Singleton service, so only a single instance will ever be created. You cannot add a scoped service to a singleton one, because the reference the singleton service uses will be disposed and you will end up with a disposed context.
This code:
return provider.GetService<IUnitOfWork>();
represents the service locator anti-pattern. As you can guess, an anti-pattern is something you want to avoid. I also don't know why you would want a service to build the entire DI container nor why you would want a service to have the responsibility of getting the dependencies it needs itself.
This part here is where your question actually comes from:
Database.SaveAsync();
You are calling an asynchronous function and not awaiting for it to finish. The task may finish or not, it may throw an error or not, you will never know what happened.
The best thing is that all of these could be avoided if people stopped attempting to create a Unit of Work + Repository pattern over yet another Unit of Work and Repository. Entity Framework Core already implements these:
DbContext => Unit of Work
DbSet => Repository (generic)
Why do you want yet another abstraction? Will you really ever throw away EF Core from the project to justify the maintenance cost of your code?
The entire question code could have just been this:
[Route("api/[controller]")]
public class ItemsController : Controller
{
private readonly YourContext _context;
public ItemsController(YourContext context)
{
_context = context;
}
[HttpPost]
public async Task<IActionResult> Add([FromBody]Item item)
{
context.Items.Add(item);
await context.SaveChangesAsync();
return Ok(item.Id);
}
[HttpDelete("{id}")]
public async Task<IActionResult> Delete(int id)
{
var item = await context.Items.FindAsync(id);
context.Items.Remove(item);
await context.SaveChangesAsync();
return Ok();
}
[HttpPut]
public async Task<IActionResult> Put([FromBody]Item item)
{
context.Items.Update(item);
await context.SaveChangesAsync();
return Ok();
}
}

Sending messages from Server to clients in SignalR 2.0

I am learning SignalR and it pretty simple. However, I am having a problem sending Messages from Server to client.
My Hub class is as follows:
public class UpdateHub : Hub
{
private readonly UpdateBroadcast _broadcaster;
public UpdateHub() : this(UpdateBroadcast.Instance) { }
public UpdateHub(UpdateBroadcast broadCaster)
{
_broadcaster = broadCaster;
}
}
And I am calling this hub in my broadcaster class like this:
public class UpdateBroadcast
{
private readonly static Lazy<UpdateBroadcast> _instance = new Lazy<UpdateBroadcast>(
() => new UpdateBroadcast(GlobalHost.ConnectionManager.GetHubContext<UpdateHub>()));
private IHubContext _context;
private UpdateBroadcast(IHubContext context)
{
_context = context;
}
public static UpdateBroadcast Instance
{
get { return _instance.Value; }
}
public void UpdatePost(Post post)
{
_context.Clients.All.updatePost(post);
}
}
In my MVC Controller I am calling the UpdatePost method:
public JsonResult AddPost(Post post)
{
UpdateBroadcast broadcaster = UpdateBroadcast.Instance;
Post result = dbFunctions.AddPost(post);
broadcaster.UpdatePost(post);
return Json(new { success = result != null }, JsonRequestBehavior.DenyGet);
}
When I debug the code, I can see that UpdatePost is executed but there is no activity on the client side. My client-side function is like this:
$(function () {
var update = $.connection.updateHub;
update.client.updatePost = function (data) {
alert("called update post");
};
});
I don't seem to understand what is causing this.

Please check below 2 links. I got really helpful with successfully implementation of signalR. Hopefully, this links help you.
https://github.com/vgheri/ChatR
http://www.codeproject.com/Articles/524066/SignalR-Simple-Chat-Application-in-Csharp