OCaml client library for Prometheus monitoring

To run services reliably, it is useful if they can report various metrics (for example, heap size, queue lengths, number of warnings logged, etc).

A monitoring service can be configured to collect this data regularly. The data can be graphed to help understand the performance of the service over time, or to help debug problems quickly. It can also be used to send alerts if a service is down or behaving poorly.

This repository contains code to report metrics to a Prometheus monitoring server.

Use by libraries

Library authors should define a set of metrics that may be useful. For example, the DataKitCI cache module defines several metrics like this:

module Metrics = struct
  open Prometheus

  let namespace = "DataKitCI"
  let subsystem = "cache"

  let builds_started_total =
    let help = "Total number of builds started" in
    Counter.v_label ~help ~label_name:"name" ~namespace ~subsystem "builds_started_total"

  let builds_succeeded_total =
    let help = "Total number of builds that succeeded" in
    Counter.v_label ~help ~label_name:"name" ~namespace ~subsystem "builds_succeeded_total"

  let builds_failed_total =
    let help = "Total number of builds that failed" in
    Counter.v_label ~help ~label_name:"name" ~namespace ~subsystem "builds_failed_total"

  [...]
end

Each of these metrics has a name label, which allows the reports to be further broken down by the type of thing being built.

When (for example) a build succeeds, the CI does:

Prometheus.Counter.inc_one (Metrics.builds_succeeded_total build_type)

Use by applications

Applications can enable metric reporting using the prometheus-app opam package. This depends on cohttp and can serve the metrics collected above over HTTP.

The prometheus-app.unix ocamlfind library provides the Prometheus_unix module, which includes a cmdliner option and pre-configured web-server. See the examples/example.ml program for an example, which can be run as:

$ dune exec -- examples/example.exe --listen-prometheus=9090
If run with the option --listen-prometheus=9090, this program serves metrics at
http://localhost:9090/metrics
Tick!
Tick!
...

--listen-prometheus either takes a port number or an address such as tcp:127.0.0.1:9090, tcp:[::1]:9090, tcp:localhost or unix:/run/metrics.sock. The interface and port defaults to 0.0.0.0:9090.

Applications can also build the same configuration directly:

let listen_address = Prometheus_unix.Listen_address.tcp ~port:9090 ()
let threads = Prometheus_unix.serve (Prometheus_unix.config ~listen_address ())

Unikernels can use Prometheus_app instead of Prometheus_unix to avoid the Unix dependency.

The prometheus-reporter opam package provides the parts of prometheus-app that do not depend on cohttp. Prometheus_reporter renders a registry snapshot in the Prometheus text format and registers the GC collectors, so applications can serve metrics with any web server such as Dream or Piaf. Prometheus_reporter_unix adds the process start-time metric and a Logs reporter that counts logged messages.

Custom registries

Metrics go to Prometheus.CollectorRegistry.default unless a ~registry is supplied to allow an application to serve its own custom metrics:

let registry = Prometheus.CollectorRegistry.create ()
let requests = Prometheus.Counter.v ~registry ~help:"Requests handled" "requests_total"

let threads =
  let registry = Prometheus_lwt.CollectorRegistry.of_registry registry in
  Prometheus_unix.serve ~registry prometheus_config

Lwt collectors

The prometheus-lwt opam package provides Prometheus_lwt with collectors that may suspend before producing their samples, and also Lwt versions of the timing helpers.

The prometheus core does not depend on Lwt, so libraries that only define and record metrics do not pull in a concurrency library. Code that used the core's Lwt-typed functions should use Prometheus_lwt, or the synchronous variants such as Gauge.set_time.

Eio collectors

An Eio application uses the prometheus-eio opam package. Prometheus_eio.callback is a cohttp-eio handler for /metrics:

Eio_main.run @@ fun env ->
Eio.Switch.run @@ fun sw ->
let addr = `Tcp (Eio.Net.Ipaddr.V4.loopback, 9090) in
let socket = Eio.Net.listen ~sw (Eio.Stdenv.net env) ~backlog:5 addr in
let server = Cohttp_eio.Server.make ~callback:Prometheus_eio.callback () in
let log_warning ex = Logs.warn (fun f -> f "%a" Eio.Exn.pp ex) in
Cohttp_eio.Server.run socket server ~on_error:log_warning

API docs

Generated API documentation is available at https://mirage.github.io/prometheus/.

Licensing

This code is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.