HostInitActions 1.3.1

Host init actions

This is a simple library for defining asynchronous operations to be performed before the application starts. Typically, it is an asynchronous initialization of singleton services registered in an IoC container. This means that there is no need to perform this initialization in a blocking manner before registering to the IoC container. HostInitActions are based on the IHostedService implementation, which means they only work in IHost implementation environments that support the IHostedService work flow. For example, within a regular WebHost in an ASP.NET Core application.

Registration of initialization actions

Registration of initialization actions is done in the ConfigureServices method of the host builder or Startup class.

var builder = WebApplication.CreateBuilder(args);
builder.WebHost.ConfigureServices(services =>
{
    services.AddSingleton<IDatabaseService, DatabaseService>();
	...
    services.AddAsyncServiceInitialization()
        .AddInitAction<IDatabaseService>(async (databaseService, cancellationToken) =>
        {
            await databaseService.CreateIfNotExistsAsync(cancellationToken);
        });
});

First, it is necessary to register the initialization service using the AddAsyncServiceInitialization method which returns a collection for registering specific init actions that will be executed before the application starts.

The AddInitAction method expects the generic parameter of the service type that is required for initialization and also the function that executes the required init action on this service. The initialized service will be retrieved from the IoC Container and passed as a parameter to the initialization function along with the optional CancellationToken.

You can define multiple init actions and they will be executed sequentially in the order of registration.

 services.AddAsyncServiceInitialization()
        .AddInitAction<IDatabaseService>(async (databaseService, cancellationToken) =>
        {
            await databaseService.CreateIfNotExistsAsync(cancellationToken);
        })
        .AddInitAction<IDeviceClient>(async deviceClient =>
        {
            await deviceClient.InitializeAsync();
        });

It is also possible to define more services (max 5) for one init action in case you need to have better control over the execution of individual initializations.

 services.AddAsyncServiceInitialization()
        .AddInitAction<IDatabaseService, IDeviceClient>(async (databaseService, deviceClient) =>
        {
            await deviceClient.PreInitializeAsync();
            await Task.WhenAll(new []
            {
                deviceClient.InitializeAsync(),
                databaseService.CreateIfNotExistsAsync(),
            });
            await deviceClient.ConnectToDeviceAsync(port: 1223);
        });

Initialization executors

Another way to perform an initialization action with multiple dependencies is the custom implementation of the IAsyncInitActionExecutor interface. This requires implementing only the ExecuteAsync method that executes your initialization logic. The dependencies needed to perform the initialization action are defined as class dependencies using the constructor and are injected from the IoC container.

    public class MyInitActionExecutor : IAsyncInitActionExecutor
    {
        private readonly IDependency1 _d1;
        private readonly IDependency2 _d2;
        ...
        private readonly IConfig _config;

        public MyInitActionExecutor(IDependency1 d1, IDependency2 d2, ... , IConfig config) {...}

        public async Task ExecuteAsync(CancellationToken cancellationToken)
        {
            await _d1.SomeAsyncAction(_config.SomeValue);
            await _d2.OtherAsyncAction(_config.OtherValue);
            ...
        }
    }

Registering custom IAsyncInitActionExecutor is done using one of the AddInitActionExecutor method overloads on IInitActionCollection.

    services.AddAsyncServiceInitialization()
        .AddInitActionExecutor<MyInitActionExecutor>()
        .AddInitActionExecutor(otherExecutorInstance)
        .AddInitActionExecutor(serviceProvider => new LastExecutor(...));

You can freely combine registrations using AddInitAction and AddInitActionExecutor. The execution order of initialization actions and initialization executors is still defined by the order of registration.

Use init stages!

Always consider using init stages when registering your initialization actions. Init stage is a set of actions that will be run in parallel during initialization. Creating a stage is done by calling the GetOrAddStage method which accepts a key that allows to return to the stage and register other actions to it again. Different methods can then use the same key to register other init actions to the same stage, which will then be run in parallel to maximize the benefits of asynchronous code.

    var stageKey = "default-stage";

    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stageKey)
            .AddInitAction<IService>(async (service, cancellation) => 
            {
                await service.InitAsync(cancellation);
            })
            .AddInitActionExecutor<MyInitActionExecutor>();

    ...
    
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stageKey)        
            .AddInitActionExecutor(otherExecutorInstance)
            .AddInitActionExecutor(serviceProvider => new LastExecutor(...));

Stage behaves like a normal initialization action in terms of other initialization actions. That is, it will run after all actions/stages that were registered before and will end (all actions it contains) before the actions/stages registered after stage are started.

But, the stages give the possibility to influence in advance the order in which the stages will be executed. This is possible because the stage is already created by calling the GetOrAddStage method. This makes it possible to first pre-create all init stages and then register all initialization actions into them.

    var stage1Key = "stage-1";
    var stage2Key = "stage-2";
    var stage3Key = "stage-3";

    // pre-creation of stages
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stag1eKey)
        .GetOrAddStage(stag2eKey)
        .GetOrAddStage(stag3eKey);

    //registration of init actions with a pre-guaranteed order
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stag2eKey)
            .AddInitAction<IServiceX>(async (serviceX) => await serviceX.InitAsync()); 
    ...
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stag3eKey)
            .AddInitAction<IServiceY>(async (serviceY) => await serviceY.InitAsync()); 
    ...
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stag2eKey)
            .AddInitAction<IServiceXX>(async (serviceXX) => await serviceXX.InitAsync()); 
    ...
    services.AddAsyncServiceInitialization()
        .GetOrAddStage(stag1eKey)
            .AddInitAction<IServiceZ>(async (serviceZ) => await serviceZ.InitAsync()); 

In the above example, ServiceZ will be initialized first, then ServiceX and ServiceXX will be initialized in parallel, and ServiceY will be initialized last.

Invocation of init actions

Init actions are executed before:

• The app's request processing pipeline is configured.
• The server is started and IApplicationLifetime.ApplicationStarted is triggered.
• The Kestrel is started (in WebHost)

Explicit invocation of init actions

For applications or tests that do not run in the Host's environment but only use the SecviceCollection container to provide dependency injection, it is possible to explicitly invoke the execution of registered init actions directly on the ServiceProvider.

    services.AddServicesThatRegisterSomeInitActionsInternally();
    ...
    var serviceProvider = services.BuildServiceProvider();

    // Performs all initialization actions, if any are registered.
    await serviceProvider.ExecuteInitActionsAsync();
    ...

Thanks to the ability to explicitly invoke init actions on the IServiceProvider, you can take advantage of the abstraction offered by the library even if your application uses only ServiceCollection. Also, in tests it is not necessary to run the Host instance to execute all init actions but you can simplify the test by simply using the ServiceCollection and the ExecuteInitActionsAsync method on the built IServiceProvider.