prometheus remote-write解析(一) -- 使用

prometheus没有提供远程存储，但提供了远程存储的接口:

• 远程存储只要实现这一接口，即可存储和读取prometheus的数据；
• 这里仅分析remote-write:

笔者的prometheus被prometheus-operator部署在kubernetes中，kubernetes使用prometheus这个CRD管理配置，prometheus-operator监听到配置变化，将新配置apply到prometheus POD上。

prometheus CRD中的remote-write配置：

remoteWrite:
  - url: "https://1.2.3.4/api/monitor/v1/prom/write"
    tlsConfig:
      insecureSkipVerify: true

apply以后，prometheus生成如下的配置：

remote_write:
- url: https://1.2.3.4/api/monitor/v1/prom/write
  remote_timeout: 30s
  tls_config:
    insecure_skip_verify: true
  queue_config:
    capacity: 500
    max_shards: 1000
    min_shards: 1
    max_samples_per_send: 100
    batch_send_deadline: 5s
    min_backoff: 30ms
    max_backoff: 100ms

可以看到，它增加了queue_config，即传输过程中的队列配置。
假设每个remoteStorage使用1个queue进行传输：

• queue中的初始shards数=min_shards，最大shards数=max_shards；
• 每个shard的容量=capacity个sample；
• 通过HTTP向remoteStorage发送数据时，若发送失败，则回退min_backoff；再次失败，则回退2*min_backoff，直到max_backoff；

prometheus的remote-write数据协议

prometheus的samples，经过protobuf的序列化，然后再经过snappy压缩，最后通过HTTP发送给remoteStorage;

对应的源代码：

// prometheus/storage/remote/queue_manager.go
func buildWriteRequest(samples []prompb.TimeSeries, buf []byte) ([]byte, int64, error) {
    var highest int64
    for _, ts := range samples {
        // At the moment we only ever append a TimeSeries with a single sample in it.
        if ts.Samples[0].Timestamp > highest {
            highest = ts.Samples[0].Timestamp
        }
    }
    req := &prompb.WriteRequest{
        Timeseries: samples,
    }

    data, err := proto.Marshal(req)
    if err != nil {
        return nil, highest, err
    }

    // snappy uses len() to see if it needs to allocate a new slice. Make the
    // buffer as long as possible.
    if buf != nil {
        buf = buf[0:cap(buf)]
    }
    compressed := snappy.Encode(buf, data)
    return compressed, highest, nil
}

remoteStorage如何实现remote-write协议接口

remoteStorage要实现remoteConfigs中定义的HTTP接口，这里主要参考influxdb的实现。
HTTP接口：

// 实现如下的API
Route{
            "prometheus-write", // Prometheus remote write
            "POST", "/api/v1/prom/write", false, true, h.servePromWrite,
        },

HTTP接口的实现：

func (h *Handler) servePromWrite(w http.ResponseWriter, r *http.Request, user meta.User) {
    ......
    var bs []byte
    if r.ContentLength > 0 {
        bs = make([]byte, 0, r.ContentLength)
    }
    body := r.Body
    buf := bytes.NewBuffer(bs)
    // 读request body
    _, err := buf.ReadFrom(body)
    // snappy解压缩
    reqBuf, err := snappy.Decode(nil, buf.Bytes())
    if err != nil {
        h.httpError(w, err.Error(), http.StatusBadRequest)
        return
    }
    // Convert the Prometheus remote write request to Influx Points
    var req remote.WriteRequest
    // protobuf反序列化
    if err := proto.Unmarshal(reqBuf, &req); err != nil {
        h.httpError(w, err.Error(), http.StatusBadRequest)
        return
    }
    ......
}

跟prometheus做的事情正好相反，这里先进行sappy的解压缩，然后再protobuf反序列化，得到真实的数据。