Prometheus源码系列：指标缓存(scrapeCache)

　　Prometheus通过scrapeManager抓取的指标(metrics)可通过本地TSDB时序数据库存储，简单高效，但无法持久化数据．所以，可根据需求，选择本地存储或远端存储．本文不涉及存储的具体实现，而是分析指标(metrics)在存储前合法性校验，即指标缓存层(scrapeCache)．

　　由上文Prometheus源码系列：指标采集(scrapeManager)可知，scrapeLoop结构体包含scrapeCache，调用scrapeLoop.append方法处理指标(metrics)存储．在方法append中，把每个指标(metirics)放到结构体scrapeCache对应的方法中进行合法性验证，过滤和存储．

• scrapeCache结构及实例化如下：

prometheus/scrape/scrape.go

// scrapeCache tracks mappings of exposed metric strings to label sets and
// storage references. Additionally, it tracks staleness of series between
// scrapes.
type scrapeCache struct {
	iter uint64 // Current scrape iteration. //被缓存的迭代次数

	// Parsed string to an entry with information about the actual label set
	// and its storage reference.
	series map[string]*cacheEntry  //map类型,key是metric,value是cacheEntry结构体

	// Cache of dropped metric strings and their iteration. The iteration must
	// be a pointer so we can update it without setting a new entry with an unsafe
	// string in addDropped().
	droppedSeries map[string]*uint64 //缓存不合法指标(metrics)

	// seriesCur and seriesPrev store the labels of series that were seen
	// in the current and previous scrape.
	// We hold two maps and swap them out to save allocations.
	seriesCur  map[uint64]labels.Labels //缓存本次scrape的指标(metrics)
	seriesPrev map[uint64]labels.Labels  //缓存上次scrape的指标(metrics)

	metaMtx  sync.Mutex //同步锁
	metadata map[string]*metaEntry //元数据
}


func newScrapeCache() *scrapeCache {
	return &scrapeCache{
		series:        map[string]*cacheEntry{},
		droppedSeries: map[string]*uint64{},
		seriesCur:     map[uint64]labels.Labels{},
		seriesPrev:    map[uint64]labels.Labels{},
		metadata:      map[string]*metaEntry{},
	}
}

• scrapeCache结构体包含多种方法（按照在方法append出现的顺序列出）

　　(1)  判断指标(metrics)的合法性

prometheus/scrape/scrape.go

func (c *scrapeCache) getDropped(met string) bool {
    // 判断metric是否在非法的dropperSeries的map类型里, key是metric, value是迭代版本号
	iterp, ok := c.droppedSeries[met]
	if ok {
		*iterp = c.iter
	}
	return ok
}

　　(2) 根据指标(metrics)信息获取结构体cacheEntry

prometheus/scrape/scrape.go

func (c *scrapeCache) get(met string) (*cacheEntry, bool) {
    ser