Object reuse with ObjectPool in ASP.NET Core

By Günther Foidl, Steve Gordon, and Samson Amaugo

Microsoft.Extensions.ObjectPool is part of the ASP.NET Core infrastructure that supports keeping a group of objects in memory for reuse rather than allowing the objects to be garbage collected. All the static and instance methods in Microsoft.Extensions.ObjectPool are thread-safe.

Apps might want to use the object pool if the objects that are being managed are:

• Expensive to allocate/initialize.
• Represent a limited resource.
• Used predictably and frequently.

For example, the ASP.NET Core framework uses the object pool in some places to reuse StringBuilder instances. StringBuilder allocates and manages its own buffers to hold character data. ASP.NET Core regularly uses StringBuilder to implement features, and reusing them provides a performance benefit.

Object pooling doesn't always improve performance:

• Unless the initialization cost of an object is high, it's usually slower to get the object from the pool.
• Objects managed by the pool aren't de-allocated until the pool is de-allocated.

Use object pooling only after collecting performance data using realistic scenarios for your app or library.

NOTE: The ObjectPool doesn't place a limit on the number of objects that it allocates, it places a limit on the number of objects it retains.

ObjectPool concepts

When DefaultObjectPoolProvider is used and T implements IDisposable:

• Items that are not returned to the pool will be disposed.
• When the pool gets disposed by DI, all items in the pool are disposed.

NOTE: After the pool is disposed:

• Calling Get throws an ObjectDisposedException.
• Calling Return disposes the given item.

Important ObjectPool types and interfaces:

• ObjectPool<T> : The basic object pool abstraction. Used to get and return objects.
• PooledObjectPolicy<T> : Implement this to customize how an object is created and how it's reset when returned to the pool. This can be passed into an object pool that's constructed directly.
• IResettable : Automatically resets the object when returned to an object pool.

The ObjectPool can be used in an app in multiple ways:

• Instantiating a pool.
• Registering a pool in Dependency injection (DI) as an instance.
• Registering the ObjectPoolProvider<> in DI and using it as a factory.

How to use ObjectPool

Call Get to get an object and Return to return the object. There's no requirement to return every object. If an object isn't returned, it will be garbage collected.

ObjectPool sample

The following code:

• Adds ObjectPoolProvider to the Dependency injection (DI) container.
• Implements the IResettable interface to automatically clear the contents of the buffer when returned to the object pool.

using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.ObjectPool;
using System.Security.Cryptography;

var builder = WebApplication.CreateBuilder(args);

builder.Services.TryAddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();

builder.Services.TryAddSingleton<ObjectPool<ReusableBuffer>>(serviceProvider =>
{
    var provider = serviceProvider.GetRequiredService<ObjectPoolProvider>();
    var policy = new DefaultPooledObjectPolicy<ReusableBuffer>();
    return provider.Create(policy);
});

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

// return the SHA256 hash of a word 
// https://localhost:7214/hash/SamsonAmaugo
app.MapGet("/hash/{name}", (string name, ObjectPool<ReusableBuffer> bufferPool) =>
{

    var buffer = bufferPool.Get();
    try
    {
        // Set the buffer data to the ASCII values of a word
        for (var i = 0; i < name.Length; i++)
        {
            buffer.Data[i] = (byte)name[i];
        }

        Span<byte> hash = stackalloc byte[32];
        SHA256.HashData(buffer.Data.AsSpan(0, name.Length), hash);
        return "Hash: " + Convert.ToHexString(hash);
    }
    finally
    {
        // Data is automatically reset because this type implemented IResettable
        bufferPool.Return(buffer); 
    }
});
app.Run();

public class ReusableBuffer : IResettable
{
    public byte[] Data { get; } = new byte[1024 * 1024]; // 1 MB

    public bool TryReset()
    {
        Array.Clear(Data);
        return true;
    }
}

NOTE: When the pooled type T doesn't implement IResettable, then a custom PooledObjectPolicy<T> can be used to reset the state of the objects before they are returned to the pool.