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In this article, let's learn about how to perform background workloads with the help of Channels inside Hosted Service in WebAPI in ASP.NET Core.

Table of Contents

1. Introduction
2. What are Channels?
3. What is Hosted Service?
4. Performing Background Workloads in Hosted Service
5. Summary

Introduction

I got an requirement to do data migration from one of my client and the constraint they have is to do it via an endpoint in web api . Initially I thought it might be simple data migration but then I realized that it is not just a simple data migration but it is a huge data migration. Though we recommended alternate approached the client was not ready to touch production database data outside the application. So we decided to do it via an web api endpoint. But the problem is that the data migration will take more than 30 minutes to complete and the request will timeout. So upon further analysis I came to know about Channels and Hosted Service in ASP.NET Core. Before we start lets understand what are channels and hosted service. This problem provides an example of a potential use case for a hosted service.

Other use cases include:

• Processing uploaded excel in background
• Sending emails in background
• Other long running operations, etc

What are Channels ?

A channel is a synchronisation concept which supports passing data between producers and consumers concurrently. One or many producers can write data into the channel, which are then read by one or many consumers. Logically a channel is effectively an efficient, thread-safe queue. Channels are available under System.Threading.Channels namespace

To know more about Channels is outside the scope of this article. I'll write a separate article on channels in ASP.NET Core. For now lets walkthrough how to use Channels.

In the above code, the DataMigrationChannel class provides a bounded channel for communication between producers and consumers. Producers can add items to the channel using the MigrateData method, and consumers can read all the items from the channel asynchronously using the ReadAllAsync method. The class ensures that only one writer and one reader can access the channel at a time and provides a limit on the number of messages that can be stored in the channel.

What is Hosted Service ?

Hosted services in ASP.NET Core have been available since version 2.1, and they support performing background tasks outside of the main requests flow. The best way to understand when and where hosted services can be applied is to begin using them. In this article, we'll dive straight in and create our first hosted service. Hosted services are based on the abstract concept of a background service. The terms hosted service and background service are often used interchangeably. I'll refer to them by both names throughout this article. Hosted services are available under Microsoft.Extensions.Hosting namespace.

To know more about Hosted Service or Background Service is outside the scope of this article. I'll write a separate article on Background Services in ASP.NET Core. For now lets understand walkthrough how to use Hosted Service.

In the above code, the DataMigrationService class extends BackgroundService and is responsible for executing data migration tasks asynchronously in the background. It reads items from the dataMigrationChannel using an IAsyncEnumerable<bool>, performs the migration logic, and logs information about the start and completion of the migration using the provided logger. The service relies on dependency injection to access the necessary dependencies, such as the DataMigrationChannel, logger, and service provider.

Performing Background Workloads in Hosted Service

So far we learn't what are channels and hosted service. Now lets see how to coordinate request and hosted service to perform background workloads. We'll use System.Threading.Channels to transfer data between requests and hosted services. Our goal will be to offload a long-running processing workload to a hosted service so that we improve the response time for requests received. The user will simply make a request to the API, the API will register a request in Channels and the API will return a response immediately. The hosted service will then process the request in the background.

Before we look at the endpoint code, we need to register the Channel and Hosted Service in the Program class as shown below. Note that DataMigrationChannel needs to be registered as Singleton service.

Note: If you have not done so already, I recommend you read the article on Dependency Injection Lifetimes in .NET.

In the above code, the DataMigration class represents an API controller for handling data migration requests. It exposes a single HTTP PUT action method that triggers the data migration process by invoking the MigrateData method on the injected DataMigrationChannel instance.

Summary

In this article we learn't how to perform background workloads outside request life cycle using channels in hosted services in ASP.NET web api. We started with use case and under stood alternate use cases and then we learn't what are channels and hosted service. Then we learn't how to coordinate request and hosted service to perform background workloads. I hope you enjoyed reading this article. Now it's time to extend the idea like connecting producer and consumer via cloud and start implementing for similar use cases in your projects.