ASK.HAL

According to Roy Fielding, you may call your API a REST API only if you make use of hypertext

The ASK.HAL project contains components to ease the creation of REST API following the best practices described in the Hypertext Application Language specification and in books like "Rest In Practice" and "REST API Design Cookbook".

The project is composed of 2 components

• ASK.HAL project contains base classes and interfaces, including Json serialization
• ASK.HAL.Mvc project contains component to ease integration of ASK.HAL into ASPNET projects.

Quick example to create a Resource :

using ASK.HAL;
using ASK.HAL.Serialization.Json;

var factory = new ResourceFactory(options);

var author = factory.Create(new Uri("http://example.com/api/authors/12345"))
     .Add(new
     {
         Name = "Marcel Proust",
         BirthDate = new DateTime(1871, 7, 10)
     });

var result = factory.Create(new Uri("http://example.com/api/books/in-search-of-lost-time"))
     .Add(new {Title = "In Search of Lost Time"})
     .AddEmbedded("author",author);

var json = await ResourceJsonSerializer.Serialize(result);

Will return the following Json

{
   "_links": {
     "self": {
       "href": "http://example.com/api/book/in-search-of-lost-time"
     }
   },
   "_embedded": {
      "author": {
         "_links": {
           "self": {
             "href": "http://example.com/api/users/12345"
           }
         },
         "name" : "Marcel Proust",
         "birthdate": "1871-07-10T00:00:00.00"
      }
   },
   "title": "In Search of Lost Time"
}

How to integrate into ASP.NET project

First add the reference to your project

Install-Package ASK.HAL
Install-Package ASK.HAL.Mvc

Update your Program.cs file

var builder = WebApplication.CreateBuilder(args);

// Add Hypertext Application Language services in Dependency Injection
builder.Services.AddHypertextApplicationLanguage();

// Add services to the container.
builder.Services
       .AddControllers(x =>
       {
           // Add Hypertext Formatter to support application/hal+json Content type
           x.AddHypertextApplicationLanguageFormatters();
       })
       .AddJsonOptions(x =>
       {
           x.AddHypertextApplicationLanguageJsonConverter();
           x.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
       });

var app = builder.Build();

...
// Adding HeaderPropagation is required by the IResourceClient
app.UseHeaderPropagation();

...
}

Example of Controller

class  MyController : Controller
{
    private IResourceFactory _factory;
    
    // Inject Resource Factory 
    public MyController(IResourceFactory resourceFactory){
        _factory = resourceFactory;
    }
    
    [HttpGet("/{id}",Name = "getbyid")]
    public ActionResult<IResource> Get(int id, [FromQuery] ResourceRequest request)
    {
        var domainEntity = GetDomainEntity();
        
        var result = _resourceFactory.Create(Url.LinkUri("getbyid", new {id = sampleValue.Id}))
            .AddLink("somelink", new Link(Url.LinkUri("getallvalues")))
            .Add(sampleValue);
         
        return Ok(result);
    }
}

For more information, look at the sample project

IResourceClient

If you need to retrieve a remote resource in your code, you can resolve the IResourceClient from Dependency Injection.

public interface IResourceClient{
    Task<Resource?> GetResource(Uri uri,CancellationToken cancellationToken = new CancellationToken());
}

Important

IResourceClient will propagate automatically the current request Cookie and Authentication Http Headers.

HyperText Cache Pattern

The Hypertext Cache Pattern allows the server to automatically fetch resource links and add the result as an embedded resource.

Example:

From this request retrieving a book

GET http://server/api/books/the-way-of-zen
Accept: application/hal+json

{
   "_links": {
      "self": { "href": "http://server/api/books/the-way-of-zen" },
      "author": { "href": "http://server/api/people/alan-watts" }
   }
   "title": "The way of Zen"
}

We can ask the server to fetch the Author link as an embedded resource using the "expand" parameter.

GET http://server/api/books/the-way-of-zen?expand=author
Accept: application/hal+json

{
   "_links": {
      "self": { "href": "http://server/api/books/the-way-of-zen" },
      "author": { "href": "http://server/api/people/alan-watts" }
   }
   "_embedded": {
     "author": {
       "_links": { "self": { "href": "http://server/api/people/alan-watts" } },
       "name": "Alan Watts",
       "born": "January 6, 1915",
       "died": "November 16, 1973"
     }
   }
   "title": "The way of Zen"
}

The returned resource will automatically contains the embedded resource without changing anything in the Controller.

Enable AutoExpand

builder.Services
       .AddControllers(x =>
       {
           // Add Hypertext Formatter to support application/hal+json Content type
           x.AddHypertextApplicationLanguageFormatters();
           
           // Add AutoExpand Filter
           x.AddHypertextAutoExpand();
       })

Tip

As the AutoExpand ActionFilter retrieve the resources using HTTP GET calls, it may be more efficient to let the controller retrieve the resource if it can be retrieve locally (in the same controller or in another Controller). Therefore, if the controller process the "expand" parameter and add the corresponding embedded resource, the ActionFilter will not process it again.

Delimited values parameters

ASP.NET Core MVC does not support delimited values for query string parameters because it is not standard.

For example, the uri http://someuri/api?param=val1,val2,val3 cannot be mapped as a string[] in your request object. (Official workaround: repeat the attribute name "?param=val1&param=val2...")

To support the delimited values parameters, you can register the following ProviderFactory

builder.Services
       .AddControllers(x =>
       {
           // Add Support for comma separated values mapped as array 
           x.ValueProviderFactories.AddDelimitedValueProviderFactory(',');
       }