Anytime a connection to a long running process is made there is the risk that the connection will be terminated prior to the process completing. A Cancellation Token is used to notify a process that a request should be cancelled.

As part of the connection to an ASP.NET Core Controller there is the HttpContext. This provdes access the a CancellationToken.

To test if the process should cancelled you check the status of the RequestAborted.IsCancellationRequested property within the HttpContext.

[HttpGet]
public async Task<IActionResult> Get(IHttpContextAccessor httpContext)
{
  if(httpContext?.HttpContext?.RequestAborted.IsCancellationRequested)
  {
    // Stop processing
  }
}

The RequestAborted is an instance of a CancellationToken.

Another way to access the CancellationToken is to pass it as part of the Dependency Injection (DI) into the Controller.

[HttpGet]
public async Task<IActionResult> Get(CancellationToken cancellationToken)
{
  // ...
  if(cancellationToken.IsCancellationRequested)
  {
    // Stop processing
  }
  // ...
}

Once the cancellation request has been detected the process should gracefully stop the processing and return.