Prometheus源码-Relabel

基于3.5

relabel address流程

1. Prometheus读取Job中的targets，把target赋值给__address__标签，此标签代表采集地址
2. 将__address__赋值给__param_target标签，因为Promehrues访问采集地址需要传参，__param_target代表target参数
3. 将__param_target标签赋值给instance标签
4. 将__param_target替换为真实的采集地址

核心解析

Label Re

// PopulateDiscoveredLabels sets base labels on lb from target and group labels and scrape configuration, before relabeling.
// PopulateDiscoveredLabels 在重新标记之前，从目标和组标签以及抓取配置在 lb 上设置基础标签
func PopulateDiscoveredLabels(lb *labels.Builder, cfg *config.ScrapeConfig, tLabels, tgLabels model.LabelSet) {
	lb.Reset(labels.EmptyLabels())

	for ln, lv := range tLabels {
		lb.Set(string(ln), string(lv))
	}
	for ln, lv := range tgLabels {
		if _, ok := tLabels[ln]; !ok {
			lb.Set(string(ln), string(lv))
		}
	}

	// Copy labels into the labelset for the target if they are not set already.
	// 如果尚未设置，则将标签复制到目标的标签集中
	scrapeLabels := []labels.Label{
		{Name: model.JobLabel, Value: cfg.JobName},
		{Name: model.ScrapeIntervalLabel, Value: cfg.ScrapeInterval.String()},
		{Name: model.ScrapeTimeoutLabel, Value: cfg.ScrapeTimeout.String()},
		{Name: model.MetricsPathLabel, Value: cfg.MetricsPath},
		{Name: model.SchemeLabel, Value: cfg.Scheme},
	}

	for _, l := range scrapeLabels {
		if lb.Get(l.Name) == "" {
			lb.Set(l.Name, l.Value)
		}
	}
	// Encode scrape query parameters as labels.
	// 将抓取查询参数编码为标签
	for k, v := range cfg.Params {
		if name := model.ParamLabelPrefix + k; len(v) > 0 && lb.Get(name) == "" {
			lb.Set(name, v[0])
		}
	}
}

// PopulateLabels builds labels from target and group labels and scrape configuration,
// performs defined relabeling, checks validity, and adds Prometheus standard labels such as 'instance'.
// A return of empty labels and nil error means the target was dropped by relabeling.
// PopulateLabels 从目标和组标签以及抓取配置构建标签，
// 执行定义的重新标记，检查有效性，并添加 Prometheus 标准标签如 'instance'。
// 返回空标签和 nil 错误意味着目标被重新标记丢弃。
func PopulateLabels(lb *labels.Builder, cfg *config.ScrapeConfig, tLabels, tgLabels model.LabelSet) (res labels.Labels, err error) {
	PopulateDiscoveredLabels(lb, cfg, tLabels, tgLabels)
	keep := relabel.ProcessBuilder(lb, cfg.RelabelConfigs...)

	// Check if the target was dropped.
	// 检查目标是否被丢弃
	if !keep {
		return labels.EmptyLabels(), nil
	}
	if v := lb.Get(model.AddressLabel); v == "" {
		return labels.EmptyLabels(), errors.New("no address")
	}

	addr := lb.Get(model.AddressLabel)

	if err := config.CheckTargetAddress(model.LabelValue(addr)); err != nil {
		return labels.EmptyLabels(), err
	}

	interval := lb.Get(model.ScrapeIntervalLabel)
	intervalDuration, err := model.ParseDuration(interval)
	if err != nil {
		return labels.EmptyLabels(), fmt.Errorf("error parsing scrape interval: %w", err)
	}
	if time.Duration(intervalDuration) == 0 {
		return labels.EmptyLabels(), errors.New("scrape interval cannot be 0")
	}

	timeout := lb.Get(model.ScrapeTimeoutLabel)
	timeoutDuration, err := model.ParseDuration(timeout)
	if err != nil {
		return labels.EmptyLabels(), fmt.Errorf("error parsing scrape timeout: %w", err)
	}
	if time.Duration(timeoutDuration) == 0 {
		return labels.EmptyLabels(), errors.New("scrape timeout cannot be 0")
	}

	if timeoutDuration > intervalDuration {
		return labels.EmptyLabels(), fmt.Errorf("scrape timeout cannot be greater than scrape interval (%q > %q)", timeout, interval)
	}

	// Meta labels are deleted after relabelling. Other internal labels propagate to
	// the target which decides whether they will be part of their label set.
	// 元标签在重新标记后被删除。其他内部标签传播到目标，由目标决定它们是否成为其标签集的一部分
	lb.Range(func(l labels.Label) {
		if strings.HasPrefix(l.Name, model.MetaLabelPrefix) {
			lb.Del(l.Name)
		}
	})

	// Default the instance label to the target address.
	// 默认 instance 标签为目标地址
	if v := lb.Get(model.InstanceLabel); v == "" {
		lb.Set(model.InstanceLabel, addr)
	}

	res = lb.Labels()
	err = res.Validate(func(l labels.Label) error {
		// Check label values are valid, drop the target if not.
		// 检查标签值是否有效，如果不有效则丢弃目标
		if !model.LabelValue(l.Value).IsValid() {
			return fmt.Errorf("invalid label value for %q: %q", l.Name, l.Value)
		}
		return nil
	})
	if err != nil {
		return labels.EmptyLabels(), err
	}
	return res, nil
}