Lære Processing Collections Asynchronously | Async Loops and Processing Collections

When you process large collections using asynchronous operations, it is tempting to launch an async task for every item at once. However, running too many concurrent operations can quickly overwhelm system resources such as CPU, memory, network bandwidth, or external services. This is especially true when interacting with databases, APIs, or file systems that have their own limits. To avoid exhausting resources or hitting external rate limits, you need to control how many operations run at a time. Two common strategies for this are batching and throttling.

Definition

Batching means grouping items together and processing them in fixed-size chunks.
Throttling means limiting the number of concurrent operations, often using synchronization primitives like SemaphoreSlim, to avoid overwhelming resources.

Program.cs


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using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            List<int> numbers = Enumerable.Range(1, 20).ToList();
            int batchSize = 5;

            Console.WriteLine("Starting batch processing...");
            foreach (var batch in Batch(numbers, batchSize))
            {
                var tasks = batch.Select(ProcessNumberAsync).ToList();
                await Task.WhenAll(tasks);
            }
            Console.WriteLine("All batches complete.");
        }

        public static IEnumerable<List<T>> Batch<T>(List<T> items, int size)
        {
            for (int i = 0; i < items.Count; i += size)
            {
                yield return items.GetRange(i, Math.Min(size, items.Count - i));
            }
        }

        public static async Task ProcessNumberAsync(int number)
        {
            Console.WriteLine($"Processing {number} on Task {Task.CurrentId}");
            await Task.Delay(500); // Simulate async work
        }
    }
}

This batching approach works by splitting the collection into smaller groups, or batches, and processing each batch in parallel. The Batch method divides the input list into sublists of the specified size. For each batch, you create a list of async tasks and use Task.WhenAll to await their completion before moving to the next batch. This limits the number of concurrent operations to the batch size, which helps control the degree of parallelism and prevents resource exhaustion.

To further control the number of concurrent async operations, especially when you do not want fixed-size batches, you can use a SemaphoreSlim. This allows you to throttle the number of operations running simultaneously, regardless of how many items are in your collection.

ThrottlingExample.cs


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using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

// This is a non-runnable illustrative example.

public class ThrottlingExample
{
    public async Task ProcessWithThrottlingAsync(List<int> items, int maxConcurrency)
    {
        using (SemaphoreSlim semaphore = new SemaphoreSlim(maxConcurrency))
        {
            List<Task> tasks = new List<Task>();
            foreach (var item in items)
            {
                await semaphore.WaitAsync();
                tasks.Add(Task.Run(async () =>
                {
                    try
                    {
                        await ProcessItemAsync(item);
                    }
                    finally
                    {
                        semaphore.Release();
                    }
                }));
            }
            await Task.WhenAll(tasks);
        }
    }

    private async Task ProcessItemAsync(int item)
    {
        // Simulate async work
        await Task.Delay(500);
        Console.WriteLine($"Processed {item}");
    }
}

Batching and throttling are essential techniques in many real-world scenarios. For instance, when calling a third-party API that enforces rate limits, batching your requests ensures you do not exceed the allowed number of concurrent calls. Throttling is similarly important when processing files, sending emails, or performing database operations, where too many simultaneous actions could degrade system performance or cause failures. By applying these strategies, you can build robust, scalable async applications that respect both system and external constraints.

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