prometheus源码分析：scrape模块

scrape模块代码位于prometheus/scrape目录下，负责监控对象的指标拉取。

1.整体框架

整体代码框架：

• 由scrape.Manager管理所有的抓取对象；
• 所有的抓取对象按group分组，每个group是一个job_name；
• 每个group下含多个scrapeTarget，即具体的抓取目标endpoint；
• 对每个目标endpoint，启动一个抓取goroutine，按照interval间隔循环的抓取对象的指标；

假如prometheus.yaml中的抓取配置为：

scrape_configs:
  - job_name: "monitor"
    static_configs:
    - targets: ['192.168.101.9:11504']

  - job_name: 'node-exporter'
    static_configs:
    - targets: ['10.21.1.74:9100', '192.168.101.9:9100']

那么，抓取对象将按如下结构分组：

{
    "monitor": [
        {
            "Targets": [
                {
                    "__address__": "192.168.101.9:11504"
                }
            ],
            "Labels": null,
            "Source": "0"
        }
    ],
    "node-exporter": [
        {
            "Targets": [
                {
                    "__address__": "10.21.1.74:9100"
                },
                {
                    "__address__": "192.168.101.9:9100"
                }
            ],
            "Labels": null,
            "Source": "0"
        }
    ]
}

2.scrape.Manager的代码逻辑

代码入口，其中函数参数中的map[string][]*targetgroup.Group由discover组件传入：

// scrape.manager.go
func (m *Manager) Run(tsets <-chan map[string][]*targetgroup.Group) error {
    go m.reloader()
    for {
        select {
        case ts := <-tsets:
            m.updateTsets(ts)

            select {
            case m.triggerReload <- struct{}{}:    //发送数据到channel: m.triggerReload
            default:
            }
        case <-m.graceShut:
            return nil
        }
    }
}

// 等待channel:m.triggerReload上的数据，然后进行reload
// 热加载
func (m *Manager) reloader() {
    ticker := time.NewTicker(5 * time.Second)
    defer ticker.Stop()
    for {
        select {
        case <-m.graceShut:
            return
        case <-ticker.C:
            select {
            case <-m.triggerReload:
                m.reload()
            case <-m.graceShut:
                return
            }
        }
    }
}

具体的初始化动作在reload()中，对每个targetGroup：

• 创建scrapePool;
• 对scrapePool进行Sync：同步信息进行抓取；

// scrape.manager.go
//遍历每个targetGroup：创建scrapePool，然后对scrapePool进行Sync
func (m *Manager) reload() {
    m.mtxScrape.Lock()
    var wg sync.WaitGroup
    for setName, groups := range m.targetSets {
        if _, ok := m.scrapePools[setName]; !ok {
            scrapeConfig, ok := m.scrapeConfigs[setName]
            ...
            //创建scrapePool
            sp, err := newScrapePool(scrapeConfig, m.append, m.jitterSeed, log.With(m.logger, "scrape_pool", setName))
            m.scrapePools[setName] = sp
        }

        wg.Add(1)
        // Run the sync in parallel as these take a while and at high load can't catch up.
        go func(sp *scrapePool, groups []*targetgroup.Group) {
            sp.Sync(groups)        //scrapePool内的Sync，这里是groups是数组，但是一般只有1个元素
            wg.Done()
        }(m.scrapePools[setName], groups)
    }
    m.mtxScrape.Unlock()
    wg.Wait()
}

对于一个targetGroup下的所有target对象，它们共享httpclient和bufferPool:

// scrape/scrape.go
// 创建scrapePool
func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed uint64, logger log.Logger) (*scrapePool, error) {
    client, err := config_util.NewClientFromConfig(cfg.HTTPClientConfig, cfg.JobName, false)
    // bufferPool
    buffers := pool.New(1e3, 100e6, 3, func(sz int) interface{} { return make([]byte, 0, sz) })

    ctx, cancel := context.WithCancel(context.Background())
    sp := &scrapePool{
        cancel:        cancel,
        appendable:    app,
        config:        cfg,
        client:        client,
        activeTargets: map[uint64]*Target{},
        loops:         map[uint64]loop{},
        logger:        logger,
    }
    sp.newLoop = func(opts scrapeLoopOptions) loop {
        ......
        return newScrapeLoop(
            ctx,
            opts.scraper,
            log.With(logger, "target", opts.target),
            buffers,
            func(l labels.Labels) labels.Labels {
                return mutateSampleLabels(l, opts.target, opts.honorLabels, opts.mrc)
            },
            func(l labels.Labels) labels.Labels { return mutateReportSampleLabels(l, opts.target) },
            func() storage.Appender { return appender(app.Appender(), opts.limit) },
            cache,
            jitterSeed,
            opts.honorTimestamps,
        )
    }
    return sp, nil
}

3.scrape.scrapePool的代码逻辑

scrapePool为targetGroup下的每个targets，创建1个scrapeLoop，然后让scrapeLoop干活。

// scrape/scrape.go
func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
        //所有的targets
    var all []*Target
    sp.mtx.Lock()
    sp.droppedTargets = []*Target{}
    for _, tg := range tgs {    
        targets, err := targetsFromGroup(tg, sp.config)
        ......
        for _, t := range targets {
            if t.Labels().Len() > 0 {
                all = append(all, t)
            }
            ......
        }
    }
    sp.mtx.Unlock()
    //指挥target干活
    sp.sync(all)
    ......
}

对每个target，使用newLoop()创建targetLoop，然后启动1个goroutine，让targetLoop.run()循环拉取：

// scrape/scrape.go
//遍历Group下的每个target，对每个target: 创建targetScraper，创建scrapeLoop，然后scrapeLoop进行干活
func (sp *scrapePool) sync(targets []*Target) {
    for _, t := range targets {
        t := t
        hash := t.hash()
        uniqueTargets[hash] = struct{}{}

        if _, ok := sp.activeTargets[hash]; !ok {
            s := &targetScraper{Target: t, client: sp.client, timeout: timeout}    //创建targetScraper
            l := sp.newLoop(scrapeLoopOptions{    //创建scrapeLoop
                target:          t,
                scraper:         s,
                limit:           limit,
                honorLabels:     honorLabels,
                honorTimestamps: honorTimestamps,
                mrc:             mrc,
            })

            sp.activeTargets[hash] = t
            sp.loops[hash] = l

            go l.run(interval, timeout, nil)  //scrapeLoop循环拉取
        }
        ......
    }
    ...
    wg.Wait()
}

4.scrapeLoop的代码逻辑

每个scrapeLoop按抓取周期循环执行

• scrape抓取指标数据；
• append写入底层存储；
• 最后更新scrapeLoop的状态(主要是指标值)；

// scrape/scrape.go
func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
    ......
    ticker := time.NewTicker(interval)    //定时器，定时执行
    defer ticker.Stop()

    for {
        ......
        var (
            start             = time.Now()
            scrapeCtx, cancel = context.WithTimeout(sl.ctx, timeout)
        )
        ......
        contentType, scrapeErr := sl.scraper.scrape(scrapeCtx, buf)    //scrape进行抓取
        ......

        //写scrape的数据写入底层存储
        total, added, seriesAdded, appErr := sl.append(b, contentType, start)
        ......

        sl.buffers.Put(b)    //写入buffer
        //更新scrapeLoop的状态
        if err := sl.report(start, time.Since(start), total, added, seriesAdded, scrapeErr); err != nil {
            level.Warn(sl.l).Log("msg", "Appending scrape report failed", "err", err)
        }

        select {
        ......
        case <-ticker.C:       //循环执行
        }
    }
    ......
}

5.targetScrape的抓取逻辑

最终达到HTTP抓取的逻辑：

• 首先，向target的url发送HTTP Get；
• 然后，将写入io.writer(即上文中的buffers)中，待后面解析出指标：

//抓取逻辑
func (s *targetScraper) scrape(ctx context.Context, w io.Writer) (string, error) {
    if s.req == nil {
        req, err := http.NewRequest("GET", s.URL().String(), nil)    //Get /metrics接口
        if err != nil {
            return "", err
        }
        req.Header.Add("Accept", acceptHeader)
        req.Header.Add("Accept-Encoding", "gzip")
        req.Header.Set("User-Agent", userAgentHeader)
        req.Header.Set("X-Prometheus-Scrape-Timeout-Seconds", fmt.Sprintf("%f", s.timeout.Seconds()))

        s.req = req
    }

    resp, err := s.client.Do(s.req.WithContext(ctx))    //发送http GET请求
    if err != nil {
        return "", err
    }
    .......

    if resp.Header.Get("Content-Encoding") != "gzip" {
        _, err = io.Copy(w, resp.Body)      //将response body写入参数w
        if err != nil {
            return "", err
        }
        return resp.Header.Get("Content-Type"), nil
    }
    if s.gzipr == nil {
        s.buf = bufio.NewReader(resp.Body)
        s.gzipr, err = gzip.NewReader(s.buf)
        if err != nil {
            return "", err
        }
    }
    // 写入io.writer
    _, err = io.Copy(w, s.gzipr)
    s.gzipr.Close()
    return resp.Header.Get("Content-Type"), nil
}