Dynamic C# API Client Proxies

ABP can dynamically create C# API client proxies to call your remote HTTP services (REST APIs). In this way, you don't need to deal with HttpClient and other low level details to call remote services and get results.

Dynamic C# proxies automatically handle the following stuff for you;

• Maps C# method calls to remote server HTTP calls by considering the HTTP method, route, query string parameters, request payload and other details.
• Authenticates the HTTP Client by adding access token to the HTTP header.
• Serializes to and deserialize from JSON.
• Handles HTTP API versioning.
• Add correlation id, current tenant id and the current culture to the request.
• Properly handles the error messages sent by the server and throws proper exceptions.

This system can be used by any type of .NET client to consume your HTTP APIs.

Service Interface

Your service/controller should implement an interface that is shared between the server and the client. So, first define a service interface in a shared library project, typically in the Application.Contracts project if you've created your solution using the startup templates.

Example:

public interface IBookAppService : IApplicationService
{
    Task<List<BookDto>> GetListAsync();
}

Your interface should implement the IRemoteService interface to be automatically discovered. Since the IApplicationService inherits the IRemoteService interface, the IBookAppService above satisfies this condition.

Implement this class in your service application. You can use auto API controller system to expose the service as a REST API endpoint.

Client Proxy Generation

First, add Volo.Abp.Http.Client nuget package to your client project:

Install-Package Volo.Abp.Http.Client

Then add AbpHttpClientModule dependency to your module:

[DependsOn(typeof(AbpHttpClientModule))] //add the dependency
public class MyClientAppModule : AbpModule
{
}

Now, it's ready to create the client proxies. Example:

[DependsOn(
    typeof(AbpHttpClientModule), //used to create client proxies
    typeof(BookStoreApplicationContractsModule) //contains the application service interfaces
    )]
public class MyClientAppModule : AbpModule
{
    public override void ConfigureServices(ServiceConfigurationContext context)
    {
        //Create dynamic client proxies
        context.Services.AddHttpClientProxies(
            typeof(BookStoreApplicationContractsModule).Assembly
        );
    }
}

AddHttpClientProxies method gets an assembly, finds all service interfaces in the given assembly, creates and registers proxy classes.

The startup templates already comes pre-configured for the client proxy generation, in the HttpApi.Client project.

Endpoint Configuration

RemoteServices section in the appsettings.json file is used to get remote service address by default. Simplest configuration is shown below:

{
  "RemoteServices": {
    "Default": {
      "BaseUrl": "http://localhost:53929/"
    } 
  } 
}

See the "AbpRemoteServiceOptions" section below for more detailed configuration.

Usage

It's straightforward to use. Just inject the service interface in the client application code:

public class MyService : ITransientDependency
{
    private readonly IBookAppService _bookService;

    public MyService(IBookAppService bookService)
    {
        _bookService = bookService;
    }

    public async Task DoIt()
    {
        var books = await _bookService.GetListAsync();
        foreach (var book in books)
        {
            Console.WriteLine($"[BOOK {book.Id}] Name={book.Name}");
        }
    }
}

This sample injects the IBookAppService service interface defined above. The dynamic client proxy implementation makes an HTTP call whenever a service method is called by the client.

IHttpClientProxy Interface

While you can inject IBookAppService like above to use the client proxy, you could inject IHttpClientProxy<IBookAppService> for a more explicit usage. In this case you will use the Service property of the IHttpClientProxy<T> interface.

Configuration

AbpRemoteServiceOptions

AbpRemoteServiceOptions is automatically set from the appsettings.json by default. Alternatively, you can configure it in the ConfigureServices method of your module to set or override it. Example:

public override void ConfigureServices(ServiceConfigurationContext context)
{
    context.Services.Configure<AbpRemoteServiceOptions>(options =>
    {
        options.RemoteServices.Default =
            new RemoteServiceConfiguration("http://localhost:53929/");
    });
    
    //...
}

Multiple Remote Service Endpoints

The examples above have configured the "Default" remote service endpoint. You may have different endpoints for different services (as like in a microservice approach where each microservice has different endpoints). In this case, you can add other endpoints to your configuration file:

{
  "RemoteServices": {
    "Default": {
      "BaseUrl": "http://localhost:53929/"
    },
    "BookStore": {
      "BaseUrl": "http://localhost:48392/"
    } 
  } 
}

AddHttpClientProxies method can get an additional parameter for the remote service name. Example:

context.Services.AddHttpClientProxies(
    typeof(BookStoreApplicationContractsModule).Assembly,
    remoteServiceConfigurationName: "BookStore"
);

remoteServiceConfigurationName parameter matches the service endpoint configured via AbpRemoteServiceOptions. If the BookStore endpoint is not defined then it fallbacks to the Default endpoint.

As Default Services

When you create a service proxy for IBookAppService, you can directly inject the IBookAppService to use the proxy client (as shown in the usage section). You can pass asDefaultServices: false to the AddHttpClientProxies method to disable this feature.

context.Services.AddHttpClientProxies(
    typeof(BookStoreApplicationContractsModule).Assembly,
    asDefaultServices: false
);

Using asDefaultServices: false may only be needed if your application has already an implementation of the service and you do not want to override/replace the other implementation by your client proxy.

If you disable asDefaultServices, you can only use IHttpClientProxy<T> interface to use the client proxies. See the IHttpClientProxy Interface section above.

Retry/Failure Logic & Polly Integration

If you want to add retry logic for the failing remote HTTP calls for the client proxies, you can configure the AbpHttpClientBuilderOptions in the PreConfigureServices method of your module class.

Example: Use the Polly library to re-try 3 times on a failure

public override void PreConfigureServices(ServiceConfigurationContext context)
{
    PreConfigure<AbpHttpClientBuilderOptions>(options =>
    {
        options.ProxyClientBuildActions.Add((remoteServiceName, clientBuilder) =>
        {
            clientBuilder.AddTransientHttpErrorPolicy(policyBuilder =>
                policyBuilder.WaitAndRetryAsync(
                    3,
                    i => TimeSpan.FromSeconds(Math.Pow(2, i))
                )
            );
        });
    });
}

This example uses the Microsoft.Extensions.Http.Polly package. You also need to import the Polly namespace (using Polly;) to be able to use the WaitAndRetryAsync method.