How Can I Efficiently Parse Prometheus Data in Go?

Parsing Prometheus Data

Obtaining metrics from Prometheus using an HTTP GET request is just the first step. The next challenge lies in parsing the data and extracting its components. This article explores how to parse Prometheus data, focusing on the EBNF package and alternative solutions.

Parsing with EBNF

The EBNF (Extended Backus-Naur Form) package in Go offers a way to define and parse grammars. While it could be used to parse Prometheus data, it requires significant effort and manual labor. Exponents of this method must meticulously create the grammar, anticipating all possible data variations to ensure accurate parsing.

Leveraging Prometheus's expfmt

Instead of relying on complex grammar definitions, you can take advantage of a package developed by the Prometheus authors themselves - expfmt. This Go library specializes in encoding and decoding the Prometheus Exposition Format (EBNF-based). Its ease of use and out-of-the-box functionality make it an ideal choice for parsing Prometheus data.

An Example with expfmt

Consider the following sample Prometheus data:

# HELP net_conntrack_dialer_conn_attempted_total
# TYPE net_conntrack_dialer_conn_attempted_total untyped
net_conntrack_dialer_conn_attempted_total{dialer_name="federate",instance="localhost:9090",job="prometheus"} 1 1608520832877

The following Go code demonstrates how to parse this data using expfmt:

package main

import (
    "flag"
    "fmt"
    "log"
    "os"

    dto "github.com/prometheus/client_model/go"
    "github.com/prometheus/common/expfmt"
)

func main() {
    f := flag.String("f", "", "set filepath")
    flag.Parse()

    mf, err := parseMF(*f)
    fatal(err)

    for k, v := range mf {
        fmt.Println("KEY: ", k)
        fmt.Println("VAL: ", v)
    }
}

func parseMF(path string) (map[string]*dto.MetricFamily, error) {
    reader, err := os.Open(path)
    if err != nil {
        return nil, err
    }

    var parser expfmt.TextParser
    mf, err := parser.TextToMetricFamilies(reader)
    if err != nil {
        return nil, err
    }
    return mf, nil
}

func fatal(err error) {
    if err != nil {
        log.Fatalln(err)
    }
}

Running this program produces the following output:

KEY:  net_conntrack_dialer_conn_attempted_total
VAL:  name:"net_conntrack_dialer_conn_attempted_total" type:UNTYPED metric:<label:<name:"dialer_name" value:"federate" > label:<name:"instance" value:"localhost:9090" > label:<name:"job" value:"prometheus" > untyped:<value:1 > timestamp_ms:1608520832877 >

Troubleshooting Formatting Issues

Ensure that the Prometheus data adheres to the proper formatting requirements. Each line must conclude with a line-feed character 'n'. Deviations from this format, including 'r' or 'rn', will trigger protocol errors.

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