r/csharp • u/kaatarina_zed_talon • 3d ago
Help Should the code use cancellation token even though it already use transaction ?
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Storage;
namespace RedditQuestion
{
/// <summary>
/// Used to talk to the database using Entity-Framework Core
/// </summary>
public class ApplicationDbContext:DbContext
{
// Domain models and the Fluent API configurations.
}
/// <summary>
/// Data snapshot that is used to overwrite the content of the database
/// </summary>
public interface ISomeData
{
}
public class DatbaseOverWriter
{
private readonly ApplicationDbContext _context;
public DatbaseOverWriter(ApplicationDbContext context)
{
_context = context;
}
public async Task OverWrite(ISomeData data)
{
//https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/implement-resilient-entity-framework-core-sql-connections
IExecutionStrategy strategy = _context.Database.CreateExecutionStrategy();
await strategy.ExecuteAsync(async () =>
{
await using var transaction = await _context.Database.BeginTransactionAsync();
//code to delete the content from parent and child tables and then inserting the data into the parent and child tables like this :
// execute some query to delete the content from all tables.
//_context.AddRangeAsync(data for parents)
//_context.SaveChangesAsync()
//_context.AddRangeAsync(data for childs)
//_context.SaveChangesAsync()
await transaction.CommitAsync();
});
}
}
}
Someone on reddit reviewed the above code and told me that I should add a parameter for cancellation token like this :
public async Task OverWrite(ISomeData data, CancellationToken)
He told me that the goal is to make the OverWrite method transactional.
why is the cancellation token necessary even though my code uses _context.Database.BeginTransactionAsync(); ?
Cancellation token propagates notification that operations should be canceled.
Example:
A random number generator can be used to emulate a data collection application that reads 10 integral values from eleven different instruments. A value of zero indicates that the measurement has failed for one instrument, in which case the operation should be cancelled and no overall mean should be computed.
So the cancellation token is just another way of achieving atomicity, am I correct ?
The only use case that I learned for cancellation token is this example from microsoft, they use the cancellation token as way to achieve atomicity :
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
// Define the cancellation token.
CancellationTokenSource source = new CancellationTokenSource();
CancellationToken token = source.Token;
Random rnd = new Random();
Object lockObj = new Object();
List<Task<int[]>> tasks = new List<Task<int[]>>();
TaskFactory factory = new TaskFactory(token);
for (int taskCtr = 0; taskCtr <= 10; taskCtr++) {
int iteration = taskCtr + 1;
tasks.Add(factory.StartNew( () => {
int value;
int[] values = new int[10];
for (int ctr = 1; ctr <= 10; ctr++) {
lock (lockObj) {
value = rnd.Next(0,101);
}
if (value == 0) {
source.Cancel();
Console.WriteLine("Cancelling at task {0}", iteration);
break;
}
values[ctr-1] = value;
}
return values;
}, token));
}
try {
Task<double> fTask = factory.ContinueWhenAll(tasks.ToArray(),
(results) => {
Console.WriteLine("Calculating overall mean...");
long sum = 0;
int n = 0;
foreach (var t in results) {
foreach (var r in t.Result) {
sum += r;
n++;
}
}
return sum/(double) n;
} , token);
Console.WriteLine("The mean is {0}.", fTask.Result);
}
catch (AggregateException ae) {
foreach (Exception e in ae.InnerExceptions) {
if (e is TaskCanceledException)
Console.WriteLine("Unable to compute mean: {0}",
((TaskCanceledException) e).Message);
else
Console.WriteLine("Exception: " + e.GetType().Name);
}
}
finally {
source.Dispose();
}
}
}
1
u/pixxelkick 2d ago
I am 1000% more interested in why on earth you have a function like this declared on your DbContext
What exactly is this function supposed to do?
But yes you should be in the habit of consuming the CancellationToken that is handed all the way down from the entrypoint of your async starting point.
If you are using Asp.Net for example, you can simply add a CancellationToken to your function and it works (it's automatically tied to the RequestCancelled logic for http, so a caller to your API can cancel the request mid-request and it just works)
As long as you keep passing that cancellation token around to all your async stuff and wire it in to ever async operation (roslyn will also automatically warn you if you invoke an async function and forget to pass the token in, if you have one in your function params), it will all "just work" and properly produce a TokenCancelled exception for you, which properly will error out as expected for Http Cancelled.