本文介绍Ocelot中的服务发现（Service Discovery），Ocelot允许指定一个服务发现提供器，之后将从中寻找下游服务的host和port来进行请求路由。
关于服务发现的详细介绍请点击。
在Ocelot中使用了Consul作为服务发现的provider。

1、Consul下载安装

从官方下载页选择合适的平台下载，解压出一个二进制文件并保存到相应位置，并将路径存入path中，本文以windows版本为例（其他平台操作类似）。
打开 cmd/powershell 运行 consul agent -dev 输出如下文本表示成功以dev方式启动consul

==> Starting Consul agent...
==> Consul agent running!
           Version: 'v1.0.7'
           Node ID: '3fc1edca-b635-56cc-b767-01a942423f73'
         Node name: 'Weidaicheng-PC'
        Datacenter: 'dc1' (Segment: '<all>')
            Server: true (Bootstrap: false)
       Client Addr: [127.0.0.1] (HTTP: 8500, HTTPS: -1, DNS: 8600)
      Cluster Addr: 127.0.0.1 (LAN: 8301, WAN: 8302)
           Encrypt: Gossip: false, TLS-Outgoing: false, TLS-Incoming: false

==> Log data will now stream in as it occurs:

    2018/04/29 09:22:03 [DEBUG] agent: Using random ID "3fc1edca-b635-56cc-b767-01a942423f73" as node ID
    2018/04/29 09:22:03 [INFO] raft: Initial configuration (index=1): [{Suffrage:Voter ID:3fc1edca-b635-56cc-b767-01a942423f73 Address:127.0.0.1:8300}]
    2018/04/29 09:22:03 [INFO] raft: Node at 127.0.0.1:8300 [Follower] entering Follower state (Leader: "")
    2018/04/29 09:22:03 [INFO] serf: EventMemberJoin: Weidaicheng-PC.dc1 127.0.0.1
    2018/04/29 09:22:03 [INFO] serf: EventMemberJoin: Weidaicheng-PC 127.0.0.1
    2018/04/29 09:22:03 [INFO] consul: Adding LAN server Weidaicheng-PC (Addr: tcp/127.0.0.1:8300) (DC: dc1)
    2018/04/29 09:22:03 [INFO] consul: Handled member-join event for server "Weidaicheng-PC.dc1" in area "wan"
    2018/04/29 09:22:03 [INFO] agent: Started DNS server 127.0.0.1:8600 (udp)
    2018/04/29 09:22:03 [INFO] agent: Started DNS server 127.0.0.1:8600 (tcp)
    2018/04/29 09:22:03 [INFO] agent: Started HTTP server on 127.0.0.1:8500 (tcp)
    2018/04/29 09:22:03 [INFO] agent: started state syncer
    2018/04/29 09:22:03 [WARN] raft: Heartbeat timeout from "" reached, starting election
    2018/04/29 09:22:03 [INFO] raft: Node at 127.0.0.1:8300 [Candidate] entering Candidate state in term 2
    2018/04/29 09:22:03 [DEBUG] raft: Votes needed: 1
    2018/04/29 09:22:03 [DEBUG] raft: Vote granted from 3fc1edca-b635-56cc-b767-01a942423f73 in term 2. Tally: 1
    2018/04/29 09:22:03 [INFO] raft: Election won. Tally: 1
    2018/04/29 09:22:03 [INFO] raft: Node at 127.0.0.1:8300 [Leader] entering Leader state
    2018/04/29 09:22:03 [INFO] consul: cluster leadership acquired
    2018/04/29 09:22:03 [INFO] consul: New leader elected: Weidaicheng-PC
    2018/04/29 09:22:03 [DEBUG] consul: Skipping self join check for "Weidaicheng-PC" since the cluster is too small
    2018/04/29 09:22:03 [INFO] consul: member 'Weidaicheng-PC' joined, marking health alive
    2018/04/29 09:22:03 [DEBUG] agent: Skipping remote check "serfHealth" since it is managed automatically
    2018/04/29 09:22:03 [INFO] agent: Synced node info
    2018/04/29 09:22:03 [DEBUG] agent: Node info in sync
    2018/04/29 09:22:04 [DEBUG] agent: Skipping remote check "serfHealth" since it is managed automatically
    2018/04/29 09:22:04 [DEBUG] agent: Node info in sync

浏览器输入http://localhost:8500可以看到UI后台界面

Consul UI.png

或者打开另一个PowerShell输入 consul members 输出如下

Node            Address         Status  Type    Build  Protocol  DC   Segment
Weidaicheng-PC  127.0.0.1:8301  alive   server  1.0.7  2         dc1  <all>

至此，Consul安装完成

2、代码修改

继续使用上一篇的项目，在WebApiA和WebApiB中分别新建一个api控制器 CounterController 代码如下

//WebApiA
using Microsoft.AspNetCore.Mvc;

namespace WebApiA.Controllers
{
    [Produces("application/json")]
    [Route("api/[controller]/[action]")]
    public class CounterController : Controller
    {
        private static int _count = 0;

        [HttpGet]
        public string Count()
        {
            return $"Count {++_count} from WebapiA";
        }
    }
}

//WebApiB
using Microsoft.AspNetCore.Mvc;

namespace WebApiB.Controllers
{
    [Produces("application/json")]
    [Route("api/[controller]/[action]")]
    public class CounterController : Controller
    {
        private static int _count = 0;

        [HttpGet]
        public string Count()
        {
            return $"Count {++_count} from WebapiB";
        }
    }
}

修改OcelotGetway项目中的配置文件 configuration.json
在 ReRoutes 节点中添加

{
      "DownstreamPathTemplate": "/api/Counter/Count",
      "DownstreamScheme": "http",
      "UpstreamPathTemplate": "/count",
      "UpstreamHttpMethod": [ "Get" ],
      "ServiceName": "Count",
      "LoadBalancer": "RoundRobin",
      "UseServiceDiscovery": true
}

其中，ServiceName指定服务发现的服务名称，在之后Consul中注册服务的时候会用到，LoadBalancer指定使用的负载均衡模式（目前Ocelot仅支持时间片轮询(RoundRobin)和最少连接(LeastConnection)模式），UseServiceDiscovery标志使用服务发现。
注：如果不指定负载均衡，将不使用负载均衡，本文中使用时间片轮询为例。
然后再根节点添加全局配置

"GlobalConfiguration": {
    "ServiceDiscoveryProvider": {
      "Host": "localhost",
      "Port": 8500
    }
  }

指定服务发现提供器的位置。目前Ocelot只支持共用一个provider。

3、注册服务

在consul所在的目录下创建文件夹 consul.d，文件夹位置及文件夹名称可以随意，进入consul.d文件夹中创建两个json文件 Count_1.json 和 Count_2.json，分别添加如下文本

{
    "service": {
        "id": "count1",
        "name": "Count",
        "tags": ["dev"],
        "address": "localhost",
        "port": 5001
    }
}

{
    "service": {
        "id": "count2",
        "name": "Count",
        "tags": ["dev"],
        "address": "localhost",
        "port": 5002
    }
}

以第一个为例，注册一个服务id为count1，服务名称为Count，和之前配置中使用的相同，还有地址和端口。
使用 Ctrl+C 关闭正在运行的consul，然后输入 consul agent -dev -config-dir=C:/consul_1.0.7/consul.d 其中 C:/consul_1.0.7/consul.d 是本机的配置文件路径。
可以看到如下输出

==> Starting Consul agent...
==> Consul agent running!
           Version: 'v1.0.7'
           Node ID: '91af5002-a1d6-409e-e36b-f7a081f84d24'
         Node name: 'Weidaicheng-PC'
        Datacenter: 'dc1' (Segment: '<all>')
            Server: true (Bootstrap: false)
       Client Addr: [127.0.0.1] (HTTP: 8500, HTTPS: -1, DNS: 8600)
      Cluster Addr: 127.0.0.1 (LAN: 8301, WAN: 8302)
           Encrypt: Gossip: false, TLS-Outgoing: false, TLS-Incoming: false

==> Log data will now stream in as it occurs:

    2018/04/29 09:41:38 [DEBUG] agent: Using random ID "91af5002-a1d6-409e-e36b-f7a081f84d24" as node ID
    2018/04/29 09:41:38 [INFO] raft: Initial configuration (index=1): [{Suffrage:Voter ID:91af5002-a1d6-409e-e36b-f7a081f84d24 Address:127.0.0.1:8300}]
    2018/04/29 09:41:38 [INFO] raft: Node at 127.0.0.1:8300 [Follower] entering Follower state (Leader: "")
    2018/04/29 09:41:38 [INFO] serf: EventMemberJoin: Weidaicheng-PC.dc1 127.0.0.1
    2018/04/29 09:41:38 [INFO] serf: EventMemberJoin: Weidaicheng-PC 127.0.0.1
    2018/04/29 09:41:38 [INFO] consul: Adding LAN server Weidaicheng-PC (Addr: tcp/127.0.0.1:8300) (DC: dc1)
    2018/04/29 09:41:38 [INFO] consul: Handled member-join event for server "Weidaicheng-PC.dc1" in area "wan"
    2018/04/29 09:41:38 [INFO] agent: Started DNS server 127.0.0.1:8600 (udp)
    2018/04/29 09:41:38 [INFO] agent: Started DNS server 127.0.0.1:8600 (tcp)
    2018/04/29 09:41:38 [INFO] agent: Started HTTP server on 127.0.0.1:8500 (tcp)
    2018/04/29 09:41:38 [INFO] agent: started state syncer
    2018/04/29 09:41:38 [WARN] raft: Heartbeat timeout from "" reached, starting election
    2018/04/29 09:41:38 [INFO] raft: Node at 127.0.0.1:8300 [Candidate] entering Candidate state in term 2
    2018/04/29 09:41:38 [DEBUG] raft: Votes needed: 1
    2018/04/29 09:41:38 [DEBUG] raft: Vote granted from 91af5002-a1d6-409e-e36b-f7a081f84d24 in term 2. Tally: 1
    2018/04/29 09:41:38 [INFO] raft: Election won. Tally: 1
    2018/04/29 09:41:38 [INFO] raft: Node at 127.0.0.1:8300 [Leader] entering Leader state
    2018/04/29 09:41:38 [INFO] consul: cluster leadership acquired
    2018/04/29 09:41:38 [INFO] consul: New leader elected: Weidaicheng-PC
    2018/04/29 09:41:38 [DEBUG] consul: Skipping self join check for "Weidaicheng-PC" since the cluster is too small
    2018/04/29 09:41:38 [INFO] consul: member 'Weidaicheng-PC' joined, marking health alive
    2018/04/29 09:41:39 [DEBUG] agent: Skipping remote check "serfHealth" since it is managed automatically
    2018/04/29 09:41:39 [INFO] agent: Synced service "count1"
    2018/04/29 09:41:39 [INFO] agent: Synced service "count2"
    2018/04/29 09:41:39 [DEBUG] agent: Node info in sync
    2018/04/29 09:41:39 [DEBUG] agent: Service "count1" in sync
    2018/04/29 09:41:39 [DEBUG] agent: Service "count2" in sync
    2018/04/29 09:41:39 [DEBUG] agent: Node info in sync
    2018/04/29 09:41:39 [DEBUG] agent: Skipping remote check "serfHealth" since it is managed automatically
    2018/04/29 09:41:39 [DEBUG] agent: Service "count1" in sync
    2018/04/29 09:41:39 [DEBUG] agent: Service "count2" in sync
    2018/04/29 09:41:39 [DEBUG] agent: Node info in sync

在倒数第三行和倒数第二行可以看到count1和count2的两个服务注册成功，使用postman/浏览器请求一下http://localhost:8500/v1/catalog/service/count
可以看到注册的两个Count服务的信息

[
    {
        "ID": "91af5002-a1d6-409e-e36b-f7a081f84d24",
        "Node": "Weidaicheng-PC",
        "Address": "127.0.0.1",
        "Datacenter": "dc1",
        "TaggedAddresses": {
            "lan": "127.0.0.1",
            "wan": "127.0.0.1"
        },
        "NodeMeta": {
            "consul-network-segment": ""
        },
        "ServiceID": "count1",
        "ServiceName": "Count",
        "ServiceTags": [
            "dev"
        ],
        "ServiceAddress": "localhost",
        "ServiceMeta": {},
        "ServicePort": 5001,
        "ServiceEnableTagOverride": false,
        "CreateIndex": 6,
        "ModifyIndex": 6
    },
    {
        "ID": "91af5002-a1d6-409e-e36b-f7a081f84d24",
        "Node": "Weidaicheng-PC",
        "Address": "127.0.0.1",
        "Datacenter": "dc1",
        "TaggedAddresses": {
            "lan": "127.0.0.1",
            "wan": "127.0.0.1"
        },
        "NodeMeta": {
            "consul-network-segment": ""
        },
        "ServiceID": "count2",
        "ServiceName": "Count",
        "ServiceTags": [
            "dev"
        ],
        "ServiceAddress": "localhost",
        "ServiceMeta": {},
        "ServicePort": 5002,
        "ServiceEnableTagOverride": false,
        "CreateIndex": 7,
        "ModifyIndex": 7
    }
]