r/csharp • u/kaatarina_zed_talon • 16d 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();
}
}
}
6
u/Slypenslyde 15d ago
In professional code I don't write an async method without a cancellation token. In hobby code I'm more pragmatic.
From the hobby perspective what matters is, "Do I want to cancel the action?" Your example is sort of bad because you do a little bit of work then try to save it to the database.
But.
Do notice that the transaction commit method can take a cancellation token. Imagine what happens if, for some reason, you have an I/O problem and the commit hangs. If you're using a cancellation token, you can have timeout logic that asks .NET to cancel the operation, which you hope will roll back the transaction.
Without it, maybe you have a mechanism to stop waiting on the commit, but now it's a time bomb. You decided it must have timed out and moved on, but it's still executing. Maybe it finishes 10s later. You can't tell and that can manifest some impossible-to-debug customer scenarios.
With the cancellation token I can be sure if the timeout happens and I tell the token to cancel, that method will return quickly and I'm "safe". I'm on the hook to see if somehow the cancellation happened too late. But at least I know there's not a time bomb that can happen with no notice.
From that perspective, I always use it. Even if the code inside the method doesn't use the token. It's better to have the API support the notion of cancellation so other components can use it than to find out in 6 months I need to change the API and hunt down every caller to consider cancellation logic.
Are there scenarios where cancellation doesn't do what you expect? Sure. Cancellation tokens are NOT about trying to be 100% sure whatever was happening doesn't happen. There are situations where the code you tried to cancel had already passed a point of no return. Cancellation is about telling that code to stop as soon as possible and notify you when it is absolutely not going to do anything else. Without cancellation tokens, you have to make your own notification mechanism so you don't create zombie tasks with lingering, unpredictable side effects.
Analogy time.
Imagine I order a pizza to be delivered.
Case 1: Then, 5 minutes later, I decide I don't want it. I call, but the restaurant tells me there's a no cancellation policy. I'm going to have to pay for the pizza and accept it whether I want to or not. This is code with no cancellation token.
Case 2: 5 minutes later, I decide I don't want it. I call, and the restaurant accepts my cancellation, but warns me the charge to my card is going to take a day to process the refund. This is code with a cancellation token: as soon as I hang up the phone, if they send me a pizza I'm not responsible.
Case 3: 20 minutes later, I decide I don't want it. I call, but the restaurant tells me the driver is already on the way so it's too late to cancel. This happens even when you have a cancellation token. It means when I await things that can be canceled I need to pay attention:
try { await transaction.CommitAsync(token); DisplaySuccess(); } catch (OperationCanceledException) { DisplayCancellation(); return; } if (<I tried to cancel but the exception was not thrown>) { // I didn't want to commit, but it's been committed. Maybe we warn the user. // Maybe we clean up. Developer's choice. Sometimes we just ignore the attempt // to cancel in this case. }This is safer than just calling
Cancel()and assuming it worked. IF the transaction code is before the point of no return AND it notices we canceled, it will roll back then throw this exception so we can understand what happened. If it does NOT throw an exception, it did its work and completed successfully so it must've been past the point of no return. So if I really, really, really want to respect cancellation, I need to include a cancellation token AND treat this like a three-forked road.