Eureka服务发现协议允许使用Eureka Rest API检索出Prometheus需要监控的targets，Prometheus会定时周期性的从Eureka调用Eureka Rest API，并将每个应用实例创建出一个target。
Eureka服务发现协议支持对如下元标签进行relabeling：
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__meta_eureka_app_name: the name of the app__meta_eureka_app_instance_id: the ID of the app instance__meta_eureka_app_instance_hostname: the hostname of the instance__meta_eureka_app_instance_homepage_url: the homepage url of the app instance__meta_eureka_app_instance_statuspage_url: the status page url of the app instance__meta_eureka_app_instance_healthcheck_url: the health check url of the app instance__meta_eureka_app_instance_ip_addr: the IP address of the app instance__meta_eureka_app_instance_vip_address: the VIP address of the app instance__meta_eureka_app_instance_secure_vip_address: the secure VIP address of the app instance__meta_eureka_app_instance_status: the status of the app instance__meta_eureka_app_instance_port: the port of the app instance__meta_eureka_app_instance_port_enabled: the port enabled of the app instance__meta_eureka_app_instance_secure_port: the secure port address of the app instance__meta_eureka_app_instance_secure_port_enabled: the secure port of the app instance__meta_eureka_app_instance_country_id: the country ID of the app instance__meta_eureka_app_instance_metadata_: app instance metadata__meta_eureka_app_instance_datacenterinfo_name: the datacenter name of the app instance__meta_eureka_app_instance_datacenterinfo_: the datacenter metadataeureka_sd_configs配置可选项如下：
# The URL to connect to the Eureka server.server: # Sets the `Authorization` header on every request with the# configured username and password.# password and password_file are mutually exclusive.basic_auth: [ username: ] [ password: ] [ password_file: ]# Optional `Authorization` header configuration.authorization: # Sets the authentication type. [ type: | default: Bearer ] # Sets the credentials. It is mutually exclusive with # `credentials_file`. [ credentials: ] # Sets the credentials to the credentials read from the configured file. # It is mutually exclusive with `credentials`. [ credentials_file: ]# Optional OAuth 2.0 configuration.# Cannot be used at the same time as basic_auth or authorization.oauth2: [ ]# Configures the scrape request"s TLS settings.tls_config: [ ]# Optional proxy URL.[ proxy_url: ]# Configure whether HTTP requests follow HTTP 3xx redirects.[ follow_redirects: | default = true ]# Refresh interval to re-read the app instance list.[ refresh_interval: | default = 30s ] 通过前面分析的Prometheus服务发现原理以及基于文件方式服务发现协议实现的分析，Eureka服务发现大致原理如下图：
通过解析配置中eureka_sd_configs协议的job生成Config，然后NewDiscovery方法创建出对应的Discoverer，最后调用Discoverer.Run()方法启动服务发现targets。
1、基于文件服务发现配置解析
假如我们定义如下job:
- job_name: "eureka" eureka_sd_configs: - server: http://localhost:8761/eureka 会被解析成eureka.SDConfig如下：
eureka.SDConfig定义如下：
type SDConfig struct { // eureka-server地址 Server string `yaml:"server,omitempty"` // http请求client配置，如:认证信息 HTTPClientConfig config.HTTPClientConfig `yaml:",inline"` // 周期刷新间隔，默认30s RefreshInterval model.Duration `yaml:"refresh_interval,omitempty"`}2、Discovery创建
func NewDiscovery(conf *SDConfig, logger log.Logger) (*Discovery, error) { rt, err := config.NewRoundTripperFromConfig(conf.HTTPClientConfig, "eureka_sd", config.WithHTTP2Disabled()) if err != nil { return nil, err } d := &Discovery{ client: &http.Client{Transport: rt}, server: conf.Server, } d.Discovery = refresh.NewDiscovery( logger, "eureka", time.Duration(conf.RefreshInterval), d.refresh, ) return d, nil}3、Discovery创建完成，最后会调用Discovery.Run()启动服务发现：
和上一节分析的服务发现之File机制类似，执行Run方法时会执行tgs, err := d.refresh(ctx)，然后创建定时周期触发器，不停执行tgs, err := d.refresh(ctx)，将返回的targets结果信息通过channel传递出去。
4、上面Run方法核心是调用d.refresh(ctx)逻辑获取targets，基于Eureka发现协议主要实现逻辑就在这里：
func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error) { // 通过Eureka REST API接口从eureka拉取元数据：http://ip:port/eureka/apps apps, err := fetchApps(ctx, d.server, d.client) if err != nil { return nil, err } tg := &targetgroup.Group{ Source: "eureka", } for _, app := range apps.Applications {//遍历app // targetsForApp()方法将app下每个instance部分转成target targets := targetsForApp(&app) //假如到 tg.Targets = append(tg.Targets, targets...) } return []*targetgroup.Group{tg}, nil}refresh方法主要有两个流程：
1、fetchApps()：从eureka-server的/eureka/apps接口拉取注册服务信息；
2、targetsForApp()：遍历app下instance，将每个instance解析出一个target，并添加一堆元标签数据。
如下就是从eureka-server的/eureka/apps接口拉取的注册服务信息：
1 UP_1_ SERVICE-PROVIDER-01 localhost:service-provider-01:8001 192.168.3.121 SERVICE-PROVIDER-01 192.168.3.121 UP UNKNOWN 8001 443 1 MyOwn 30 90 1629385562130 1629385682050 0 1629385562132 8001 true 8080 http://192.168.3.121:8001/ http://192.168.3.121:8001/actuator/info http://192.168.3.121:8001/actuator/health service-provider-01 service-provider-01 false 1629385562132 1629385562039 ADDED 5、instance信息解析target：
func targetsForApp(app *Application) []model.LabelSet { targets := make([]model.LabelSet, 0, len(app.Instances)) // Gather info about the app"s "instances". Each instance is considered a task. for _, t := range app.Instances { var targetAddress string // __address__取值方式：instance.hostname和port，没有port则默认port=80 if t.Port != nil { targetAddress = net.JoinHostPort(t.HostName, strconv.Itoa(t.Port.Port)) } else { targetAddress = net.JoinHostPort(t.HostName, "80") } target := model.LabelSet{ model.AddressLabel: lv(targetAddress), model.InstanceLabel: lv(t.InstanceID), appNameLabel: lv(app.Name), appInstanceHostNameLabel: lv(t.HostName), appInstanceHomePageURLLabel: lv(t.HomePageURL), appInstanceStatusPageURLLabel: lv(t.StatusPageURL), appInstanceHealthCheckURLLabel: lv(t.HealthCheckURL), appInstanceIPAddrLabel: lv(t.IPAddr), appInstanceVipAddressLabel: lv(t.VipAddress), appInstanceSecureVipAddressLabel: lv(t.SecureVipAddress), appInstanceStatusLabel: lv(t.Status), appInstanceCountryIDLabel: lv(strconv.Itoa(t.CountryID)), appInstanceIDLabel: lv(t.InstanceID), } if t.Port != nil { target[appInstancePortLabel] = lv(strconv.Itoa(t.Port.Port)) target[appInstancePortEnabledLabel] = lv(strconv.FormatBool(t.Port.Enabled)) } if t.SecurePort != nil { target[appInstanceSecurePortLabel] = lv(strconv.Itoa(t.SecurePort.Port)) target[appInstanceSecurePortEnabledLabel] = lv(strconv.FormatBool(t.SecurePort.Enabled)) } if t.DataCenterInfo != nil { target[appInstanceDataCenterInfoNameLabel] = lv(t.DataCenterInfo.Name) if t.DataCenterInfo.Metadata != nil { for _, m := range t.DataCenterInfo.Metadata.Items { ln := strutil.SanitizeLabelName(m.XMLName.Local) target[model.LabelName(appInstanceDataCenterInfoMetadataPrefix+ln)] = lv(m.Content) } } } if t.Metadata != nil { for _, m := range t.Metadata.Items { // prometheus label只支持[^a-zA-Z0-9_]字符，其它非法字符都会被替换成下划线_ ln := strutil.SanitizeLabelName(m.XMLName.Local) target[model.LabelName(appInstanceMetadataPrefix+ln)] = lv(m.Content) } } targets = append(targets, target) } return targets}解析比较简单，就不再分析，解析后的标签数据如下图：
标签中有两个特别说明下：
1、__address__：这个取值instance.hostname和port(默认80)，所以要注意注册到eureka上的hostname准确性，不然可能无法抓取；
2、metadata-map数据会被转成__meta_eureka_app_instance_metadata_格式标签，prometheus进行relabeling一般操作metadata-map，可以自定义metric_path、抓取端口等；
3、prometheus的label只支持[a-zA-Z0-9_]，其它非法字符都会被转换成下划线，具体参加：strutil.SanitizeLabelName(m.XMLName.Local)；但是eureka的metadata-map标签含有下划线时，注册到eureka-server上变成双下划线，如下配置:
eureka: instance: metadata-map: scrape_enable: true scrape.port: 8080通过/eureka/apps获取如下：
基于Eureka方式的服务原理如下图：
大概说明：Discoverer启动后定时周期触发从eureka server的/eureka/apps接口拉取注册服务元数据，然后通过targetsForApp遍历app下的instance，将每个instance解析成target，并将其它元数据信息转换成target原标签可以用于target抓取前relabeling操作。
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