Fennel - A flowering plant in the carrot family

Fennel is a simple library that speaks Promethean. If can translate prometheus log strings to typed metrics, as well as create metrics in well formatted Prometheus lines.

But the noble son of Iapetus outwitted him and stole the far-seen gleam of unwearying fire in a hollow fennel stalk - Hesiod in the Theogony

The myth has it that Prometheus stole fire from Zeus by smuggling an ember in a hollow fennel stalk.

If you are wondering more about why I created this than about the name, it is simple. I was looking for something that just allowed me to create Prometheus log lines as strings. I also wanted to be able to parse Prometheus logs. Circa 2019 when I looked for this all I found were libraries that tied very deeply into ASP.NET Core. I had been looking for an excuse to play with a parsing library like FParsec... and here we are.

Available on Nuget.org

Install-Package Fennel

dotnet add package Fennel

<PackageReference Include="Fennel" />

paket add Fennel

Assuming we have the following Prometheus logs as input:

# Finally a summary, which has a complex representation, too:
# HELP rpc_duration_seconds A summary of the RPC duration in seconds.
# TYPE rpc_duration_seconds summary
rpc_duration_seconds{quantile="0.01"} 3102
rpc_duration_seconds{quantile="0.05"} 3272
rpc_duration_seconds{quantile="0.5"} 4773
rpc_duration_seconds{quantile="0.9"} 9001
rpc_duration_seconds{quantile="0.99"} 76656
rpc_duration_seconds_sum 1.7560473e+07
rpc_duration_seconds_count 2693

You can break the result down into individually parsed lines.

open Fennel

let lines = Prometheus.parseText input

Each line has multiple possibilities:

match line with
| Help (name, doc) -> printfn "Help line %A" (name, doc)
| Comment txt -> printfn "Comment line %s" txt
| Type (name, t) -> printfn "Type line %A" (name, t)
| Metric m -> printfn "Metric line %A" m
| Blank -> printfn "Blank line"

using Fennel.CSharp;

var lines = Prometheus.ParseText(input);

Or you can parse an individual line:

var input = "http_requests_total{method=\"post\",code=\"200\"} 1027 1395066363000";
var result = Prometheus.ParseLine(input);
if(result.IsMetric)
{
    var metric = result as Metric;
}

You can also create correctly formatted Prometheus strings for comment, help, and metric lines.

open Fennel

let prometheusString = Prometheus.metric "http_requests_total" 1027. [("method","post");("code","200")] DateTimeOffset.UtcNow

using Fennel.CSharp;

var labels = new Dictionary<string, string>{ {"method", "post"}, {"code", "200"} };
var prometheusString = Prometheus.Metric("http_requests_total", 1027, labels, DateTimeOffset.UtcNow);