Sending scheduled emails by Web API using background tasks - asp.net-web-api

Question

I have researched too much about the ways for sending scheduled emails by .NET core Web API using background tasks. I know it's better that I should implement the background tasks in a windows service which runs separately with app domain.
But my requirement is from web client I will have a table with each row is a promotion event for customer, I can choose to active, pause, stop for each of them, then it will make call to API and from here.
I have to implement each background tasks for each of them that can run synchronous. I have to do that by Web API because end users don't have a place to host the service.
Actual solution:
After one day I came up with the solution which is using IHostedService with BlockingCollection to control the background tasks in runtime as below:
Code for background task using IHostedService:
namespace SimCard.API.Worker
{
internal class TimedHostedService : IHostedService, IDisposable
{
private CancellationTokenSource _tokenSource;
private readonly ILogger _logger;
private Timer _timer;
private readonly TasksToRun tasks;
private readonly IEmailService emailService;
public TimedHostedService(ILogger<TimedHostedService> logger, TasksToRun tasks, IEmailService emailService)
{
this.emailService = emailService;
this.tasks = tasks;
_logger = logger;
}
public Task StartAsync(CancellationToken cancellationToken)
{
tasks.Dequeue();
_logger.LogInformation("Timed Background Service is starting.");
_timer = new Timer(DoWork, null, TimeSpan.Zero,
TimeSpan.FromSeconds(5));
return Task.CompletedTask;
}
private void DoWork(object state)
{
emailService.SendEmail("ptkhuong96#gmail.com", "Test", "OK, Done now");
_logger.LogInformation("Mail sent!");
}
public Task StopAsync(CancellationToken cancellationToken)
{
_logger.LogInformation("Timed Background Service is stopping.");
_timer?.Change(Timeout.Infinite, 0);
return Task.CompletedTask;
}
public void Dispose()
{
_timer?.Dispose();
}
}
}
Here is the code for BlockingCollection:
namespace SimCard.API.Worker
{
public class TasksToRun : ITasksToRun
{
private readonly BlockingCollection<int> _tasks;
public TasksToRun() => _tasks = new BlockingCollection<int>();
public void Enqueue(int settings) => _tasks.Add(settings);
public void Dequeue() => _tasks.Take();
}
}
And the code in controller with get called from web client:
[HttpPost("/api/worker/start")]
public IActionResult Run()
{
tasks.Enqueue(15);
return Ok();
}
Code for Startup.cs:
services.AddHostedService<TimedHostedService>();
services.AddSingleton<TasksToRun, TasksToRun>();
Issue:
After click active button for the first event => controller will get called and one instance of this background task will run. How to pause that task and resume it?
If the first issue is solved, how can I create each background task for each event in the table, think about I may could create more and more event in the future, how can one event get actived, stopped, paused, resumed without affect to another one?
I'm really stuck with this requirement and don't know how to proceed further. If you have a different approach that can adapt my case, you could recommend me also.
Thank you very much for your support.