如何使用Servie discovery

先看client端，如何使用service discovery，範例是Samples\Discovery\src\AspDotNetCore\Fortune-Teller-UI , 先在專案安裝Steeltoe.Discovery.Client或Pivotal.Discovery.Client套件，在專案底下appsettings.json加入給Eureka Servcer的設定

  "eureka": {
    "client": {
      "serviceUrl": "http://localhost:8761/eureka/",
      "shouldRegisterWithEureka": false
    }
  }

"serviceUrl" 代表Eureka Server的位址，
"shouldRegisterWithEureka": false 代表著這個服務不會註冊到Eureka只會使用servicer discovery，

接著在AspDotNetCore\Fortune-Teller-UI\StartUp.cs的ConfigureServices方法裡調用相關的方法

public void ConfigureServices(IServiceCollection services)
{
    services.AddLogging();

    services.AddDiscoveryClient(Configuration);

    services.AddSingleton<IFortuneService, FortuneService>();

    // Add framework services.
    services.AddMvc();
}

services.AddDiscoveryClient(Configuration): 會將Configuration裡面相關的資料註冊到Eureka
services.AddSingleton<IFortuneService, FortuneService>(): 並非是Service Discovery相關的方法，而是.Net core本身提供的DI。FortuneService.cs類似遠端服務在本地端的proxy， 當遇到IFortuneService會自動注入唯一的FortuneService實例

來看一下FortuneService.cs

using Pivotal.Discovery.Client;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;

namespace Fortune_Teller_UI.Services
{
    public class FortuneService : IFortuneService
    {
        DiscoveryHttpClientHandler _handler;
        ILogger<FortuneService> _logger;
        private const string RANDOM_FORTUNE_URL = "http://fortuneService/api/fortunes/random";

        public FortuneService(IDiscoveryClient client, ILoggerFactory logFactory) 
        {
            _handler =  new DiscoveryHttpClientHandler(client, logFactory.CreateLogger<DiscoveryHttpClientHandler>());
            _logger =  logFactory.CreateLogger<FortuneService>();
        }

        public async Task<string> RandomFortuneAsync()
        {
            var client = GetClient();
            var result = await client.GetStringAsync(RANDOM_FORTUNE_URL);
            _logger.LogInformation("RandomFortuneAsync: {0}", result);
            return result;
        }

        private HttpClient GetClient()
        {
            var client = new HttpClient(_handler, false);
            return client;
        }
    }
}

注意一下這個RANDOM_FORTUNE_URL = "http://fortuneService/api/fortunes/random";
中間的"fortunesService" 是在Eureka註冊服務的名字， 對應到的是在Fortune-Teller-Service\appsettings.json裡面的

  "spring": {
    "application": {
      "name":  "fortuneService"
    }

我猜測是藉由DiscoveryHttpClientHandler解析字串來尋找eureka上相對地的服務，細節要找source code看了

如何使用Service registry

範例是Samples\Discovery\src\AspDotNetCore\Fortune-Teller-Service
一樣先在appsettings.json進行設定

  "spring": {
    "application": {
      "name":  "fortuneService"
    }
  },
  "eureka": {
    "client": {
      "serviceUrl": "http://localhost:8761/eureka/",
      "shouldFetchRegistry": false
    },
    "instance": {
      "port": 5000
    }
  }

跟使用service discovery有所不同

"name": "fortuneService" 是會註冊在eureka裡面的服務名，
"shouldFetchRegistry": false 不會從Eureka Server取得已註冊的服務資訊，因為這個服務只做服務註冊

另外需要加上port的資訊
"instance": {
"port": 5000
}
這邊是給Eureka的註冊資訊，所以啟用這個服務的時候指定port也要是跟註冊資訊一樣，否則當要調用這個服務的時候是找不到(這是寫死的，應該有機會自動取得port...)

然後在Startup.cs的ConfigureService方法調用services.AddDiscovery(Configuration)就完成了

Steeltoe在appsettings.json裡面可以對service discovery和registry設定許多選項，有興趣的朋友可以看一下
http://steeltoe.io/docs/steeltoe-discovery/#1-1-3-run-fortune-teller-service