2024-02-17

源码分析 PromQL 中的 rate 与 irate 方法

家庭网络质量，如同水电燃气一般，对生活幸福感至关重要。突发奇想为家中的网络中枢 HTTP proxy 编写 Prometheus exporter 并配置可观测大盘。

趁此机会学习如何编写一个自定义的 exporter，以及 PromQL 中 rate/irate 函数的实现原理。

Workflow

Exporter (0.0.0.0:8000) ---pull--> Prometheus -> Grafana

Step1: 编写 HTTP proxy exporter

Prometheus 中共存在四种 metrics 类型：

Counter: 累计计数的指标，随着时间的推移递增，例如请求数量、错误次数等。
Gauge: 适用于需要实时测量的指标，例如CPU 使用率、内存占用、网络延迟等。
Histogram: 用于度量和统计数据分布的指标类型，例如默认的 bucket 使用的 Web/RPC 请求耗时范围，从毫秒到秒：DEFAULT_BUCKETS = (.005, .01, .025, .05, .075, .1, .25, .5, .75, 1.0, 2.5, 5.0, 7.5, 10.0, INF)。
Summary: 在不同百分位数上对观测值进行摘要统计的指标类型。

最终决定使用 Counter 统计请求总数与失败数，Histogram 统计请求耗时：proxy_monitor/exporter.py

Step2: 接入 Prometheus

修改 yaml 配置文件，重启后，稍等片刻即可生效。

Step3: 配置 Grafana 大盘

针对 Counter 指标，通常使用 rate/increase 函数查询每分钟的请求量。但该函数背后的实现原理是？以及 rate 与 irate 函数的区别？

代码是我们最好的朋友，尝试通过源码一探究竟 :)

rate & increase

Source code: promql/functions.go

1）计算平均增长率

首先计算首尾两个 sample 的差值，例如下图 1 1 1 2 3，则 resultFloat = 3 - 1 = 2 rate-1.drawio

关键代码如下：

numSamplesMinusOne = len(samples.Floats) - 1
firstT = samples.Floats[0].T
lastT = samples.Floats[numSamplesMinusOne].T
resultFloat = samples.Floats[numSamplesMinusOne].F - samples.Floats[0].F
if !isCounter {
  break
}

2）推测拟合

现实世界中，虽然样本间隔时间大概率接近，但首尾样本（firstT & lastT）并不会刚好落在 range 的开始/结束位置（rangeStart & rangeEnd）

所以需要进一步推断（extrapolate） rangeStart 位置的 counter 数值（i.e. 黄色虚线延伸）。

从而尝试估算更加真实的 rate 变化率（允许一定误差）。

rate-4.drawio

如上图，假如不做推断，increase = 3 - 1 = 2，尝试推断后 increase = 4 - 0 = 4

显然后置更加准确，同时也不难理解：

为什么 rate 函数计算的结果会出现小数点
为什么推荐针对平稳型数据使用 rate 计算变化率

// p.s. extrapolate 推测的源码逻辑，请参考下文 “边界情况3”

3）边界情况处理

边界情况1 -> 处理计数器重置（e.g. exporter 重启等情况）

例如 1 2 3 1 2，则 resultFloat = 2 - 1 + 3 = 4，个人理解等同于 (3 - 1) + (2 - 0) rate-2.drawio

关键代码如下：

// Handle counter resets:
prevValue := samples.Floats[0].F
for _, currPoint := range samples.Floats[1:] {
  if currPoint.F < prevValue {
    resultFloat += prevValue
  }
  prevValue = currPoint.F
}

边界情况2 -> 推断范围限制（估计的起始时间，对应计数不得为负数）。

简而言之，下图黄线向左持续延伸时，不得低于 x 轴 rate-3.drawio

关键代码如下：

// Duration between first/last samples and boundary of range.
durationToStart := float64(firstT-rangeStart) / 1000
durationToEnd := float64(rangeEnd-lastT) / 1000

sampledInterval := float64(lastT-firstT) / 1000
averageDurationBetweenSamples := sampledInterval / float64(numSamplesMinusOne)

// TODO(beorn7): Do this for histograms, too.
if isCounter && resultFloat > 0 && len(samples.Floats) > 0 && samples.Floats[0].F >= 0 {
  // Counters cannot be negative. If we have any slope at all
  // (i.e. resultFloat went up), we can extrapolate the zero point
  // of the counter. If the duration to the zero point is shorter
  // than the durationToStart, we take the zero point as the start
  // of the series, thereby avoiding extrapolation to negative
  // counter values.
  durationToZero := sampledInterval * (samples.Floats[0].F / resultFloat)
  if durationToZero < durationToStart {
    durationToStart = durationToZero
  }
}

边界情况3 -> 推断范围限制（根据样本平均间隔）

以下图为例，sample 只有两个（A & B），所以平均间隔 averageDurationBetweenSamples = (75s - 45s) / (2 - 1) = 30s，阈值extrapolationThreshold = 30s * 1.1 = 33s

由于 durationToStart = 45s - 30s = 15s < 33s，未超过阈值，所以黄线可以扩展至 rangeStart
反之若超出阈值，则最多延伸样本平均间隔的一半（首尾相加刚好凑齐一个整的间隔）

rate-4.drawio

关键代码如下：

// If the first/last samples are close to the boundaries of the range,
// extrapolate the result. This is as we expect that another sample
// will exist given the spacing between samples we've seen thus far,
// with an allowance for noise.
extrapolationThreshold := averageDurationBetweenSamples * 1.1
extrapolateToInterval := sampledInterval

if durationToStart < extrapolationThreshold {
  extrapolateToInterval += durationToStart
} else {
  extrapolateToInterval += averageDurationBetweenSamples / 2
}
if durationToEnd < extrapolationThreshold {
  extrapolateToInterval += durationToEnd
} else {
  extrapolateToInterval += averageDurationBetweenSamples / 2
}
factor := extrapolateToInterval / sampledInterval

4）`increase` vs `rate`

参考如下代码，不难理解 rate 每秒增长率，等同于 increase / range seconds

if isRate {
  factor /= ms.Range.Seconds()
}

irate

不同于 rate，irate 函数仅取最后两个样本进行计算，以获取更快的响应速度，所以推荐有时效性需求的场景使用。

The source code below is straightforward:

lastSample := samples.Floats[len(samples.Floats)-1]
previousSample := samples.Floats[len(samples.Floats)-2]

var resultValue float64
if isRate && lastSample.F < previousSample.F {
  // Counter reset.
  resultValue = lastSample.F
} else {
  resultValue = lastSample.F - previousSample.F
}

sampledInterval := lastSample.T - previousSample.T
if sampledInterval == 0 {
  // Avoid dividing by 0.
  return out, nil
}

if isRate {
  // Convert to per-second.
  resultValue /= float64(sampledInterval) / 1000
}

大功告成，大盘 instant snapshot & 配置：

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源码分析 PromQL 中的 rate 与 irate 方法

Workflow

Step1: 编写 HTTP proxy exporter

Step2: 接入 Prometheus

Step3: 配置 Grafana 大盘

rate & increase

1）计算平均增长率

2）推测拟合

3）边界情况处理

4）increase vs rate

irate

参考：

4）`increase` vs `rate`