Dynamic HttpHost

In some scenarios, the API host address needs to be determined dynamically at runtime rather than being fixed at compile time.

Using UriAttribute for Absolute Uri

[LoggingFilter]
public interface IUserApi
{
    [HttpGet]
    ITask<User> GetAsync([Uri] string urlString, [PathQuery] string id);
}

Pass the complete URL when calling:

var user = await userApi.GetAsync("http://api.example.com/detail", "id001");

Custom HttpHostBaseAttribute

A more flexible approach is to create a custom HttpHostBaseAttribute to implement dynamic host resolution:

[ServiceNameHost("baidu")]
public interface IUserApi
{
    [HttpGet("api/users/{id}")]
    Task<User> GetAsync(string id);

    [HttpPost("api/users")]
    Task<User> PostAsync([JsonContent] User user);
}

public class ServiceNameHostAttribute : HttpHostBaseAttribute
{
    public string ServiceName { get; }

    public ServiceNameHostAttribute(string serviceName)
    {
        this.ServiceName = serviceName;
    }

    public override Task OnRequestAsync(ApiRequestContext context)
    {
        // HostProvider is a custom service; the data source can be a database, configuration center, etc.
        var hostProvider = context.HttpContext.ServiceProvider.GetRequiredService<HostProvider>();
        string host = hostProvider.ResolveHost(this.ServiceName);

        // Set the RequestUri of the request message
        context.HttpContext.RequestMessage.RequestUri = new Uri(host);
        return Task.CompletedTask;
    }
}

Integration with Service Discovery

In a microservices architecture, you can integrate with service discovery components:

public class ServiceDiscoveryHostAttribute : HttpHostBaseAttribute
{
    private readonly string _serviceName;

    public ServiceDiscoveryHostAttribute(string serviceName)
    {
        _serviceName = serviceName;
    }

    public override async Task OnRequestAsync(ApiRequestContext context)
    {
        var discovery = context.HttpContext.ServiceProvider.GetRequiredService<IServiceDiscovery>();
        var instances = await discovery.GetInstancesAsync(_serviceName);
        
        // Select an instance using load balancing
        var instance = instances.First();
        context.HttpContext.RequestMessage.RequestUri = new Uri(instance.Address);
    }
}

Tenant-Based Host Switching

In multi-tenant scenarios, you can dynamically switch API hosts based on the current tenant:

[TenantHost]
public interface IOrderApi
{
    [HttpGet("orders/{id}")]
    Task<Order> GetAsync(string id);
}

public class TenantHostAttribute : HttpHostBaseAttribute
{
    public override Task OnRequestAsync(ApiRequestContext context)
    {
        var tenantContext = context.HttpContext.ServiceProvider.GetRequiredService<ITenantContext>();
        var host = tenantContext.GetCurrentTenantHost();
        
        context.HttpContext.RequestMessage.RequestUri = new Uri(host);
        return Task.CompletedTask;
    }
}