In a previous post, I discussed how we can use Configuration API in .net core console application to retrieve custom configuration settings from a
Json file, utilising a convenient feature of the Microsoft.Extensions.Configuration namespace.
In this post, I will provide an example of how to develop .NET Core console applications using
IHost, generic interface to develop headless services.
One of the most common uses for a Console Application is to build what is known as headless services, this is particularly common when working with Docker containers and Kubernetes (K8S).
Using headless services, it is possible to create a service grouping that does not allocate an IP address or forward traffic, enabling you to definitively control what specific pods you connect and communicate with.
What is .net core generic host
A common scenario or use case for headless services is to run in a Fire And Forget asynchronous pattern, which facilitate background processing tasks, perhaps handling messages on a queue for example, common in a cloud native, container based architectures.
The goal of the Generic Host is to decouple the HTTP pipeline from the Web Host API to enable a wider array of host scenarios. The Generic Host is new in ASP.NET Core 2.1 and isn’t suitable for web hosting, but is ideally used for Console Applications which are typically used to develop headless services.
The Generic Host library is available in
Microsoft.Extensions.Hosting namespace and provided by
To add the namespace using the .net core CLI
Introducing IHost and the HostBuilder
In .NET Core 2.1 Generic Host enables developers easily set up cross-cutting concerns for non-web based applications including:
- Dependency Injection (DI)
IHost interface offers a number of benefits and features:
- Graceful shut down
- Dependency Injection
These features are particularly important when developing complex data processing tasks in Docker containers. Especially graceful shutdown, as it helps to keep application state consistent.
Implement IHost in .net core console applications
IHost interface and the
HostBuilder class provide similar experience to the respective
WebHostBuilder .net core web developers are familiar with when using ASP.net core.
You need to bear in mind that applications which will implement the
IHost interface will typically be required to run asynchronously. This will usually require marking the
Main method as
In order to continue developing our application we will need to add several more package references
HostBuilder does not provide an extension method enabling the use of a
The HostingHostBuilder provides extension methods to configure host services. The following extension available:
ConfigureAppConfiguration– Application Configuration
ConfigureContainer– Configure the instantiated dependency injection container
ConfigureLogging– Configure Logging
ConfigureServices– Adds services to the Dependency Injection container
RunConsoleAsync– Enables console support
UseConsoleLifetime– Listens for shutdown signals
UseContentRoot– Specify the content root directory
UseEnvironment– Specify the environment
RunConsoleAsync will start services and wait on an exit signal in the application.
Building a Host
Using the Main method create a HostBuilder and use extension methods to register services with DI, read configuration and configure the logging for your application.
To configure the Host we will use
ConfigureHostConfiguration to initialize the
IHostingEnvironment for use later in the build process. Environment variable configuration isn’t added by default, so we have to call
AddEnvironmentVariables on the host builder to configure the host from environment variables.
AddEnvironmentVariables accepts an optional user-defined prefix, by convention use your application name in our example we’ve declared a constant string value with
_prefix set with the application name.
ConfigureAppConfiguration configuration providers to construct the final representation of configuration values for our application.
In the example below we read the configuration values from an appsettings.json file first, followed by environment variables and finally from any arguments passed into the application.
ConfigureServices is where we configure the services we want our application to run, registering services with the ServiceCollection. Registration is performed using extension methods on the ServiceCollection and once complete, enabling DI in our application.
The actual implementation of the service is defined using
IHostedService interface. When an application is started, it will call
StopAsync is called and the service cleans up a little before the application is killed.
IHostedService implementation defined and registered with DI container using
Usually in Linux daemons, your service implementation will be a
Singleton class – only one instance of the service for the application lifetime.
It also adds the configuration POCO (Plain Old C# Object) to the the services for dependency injection. This implements the
IOption interface in .NET Core, which can take a type that it will attempt to bind CLI parameters and environment variables to based on the field name.
Console application with one or more IHostedServices are ideal for developing Headless Services for data processing running in Docker containers. When the container is shut down, the processing can stop gracefully.
Dependency Injection gives you many options to test your code, enabling separation of concerns.
There are a number of concepts introduced with Generic Host, which are essential to simplify the development of microservices . A single common pattern for web applications as well as services, with easy access to things like DI, logging and configuration is extremely welcome.
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