Trying out Prometheus

2021-06-27 trying-out

This time I’m trying out a monitoring solution: Prometheus.

All I know about it so far:

pull based, server scrapes /metrics on workloads, human readable format
scales well because of this
popular in kuberetes space

Let’s try running the docker iamge based on the README.

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docker run --rm -it -p9090:9090 prom/prometheus:main

prometheus dashboard

We’ve got lift off! And it’s monitoring itself right out the gate:

prometheus monitoring itself

docker-compose

I want to add more services now and I don’t want to fiddle with docker run so time for a compose file.

Consulting with the docs I managed this

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version: '3.7'
volumes:
    prometheus_data:
    grafana_data:
services:
  prometheus:
    image: prom/prometheus:v2.28.0
    volumes:
      - ./prometheus_config/:/etc/prometheus/
      - prometheus_data:/prometheus
    command:
      - '--config.file=/etc/prometheus/prometheus.yml'
      - '--storage.tsdb.path=/prometheus'
    ports:
      - 9090:9090
  grafana:
    image: grafana/grafana:8.0.3
    ports:
      - 3000:3000
    volumes:
      - grafana_data:/var/lib/grafana

Yes I threw in a Grafana instance as well.

I based prometheus_config/prometheus.yml of of getting started

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global:
  scrape_interval:     15s # By default, scrape targets every 15 seconds.

scrape_configs:
  - job_name: 'prometheus'
    scrape_interval: 5s
    static_configs:
      - targets: ['localhost:9090']

Running docker-compose up -d and we’re off to the races. http://localhost:9090 gives the same interface (but now with persistent storage) but there is now Grafana as well (on port 3000)

And it can easily be configured to point to the Prometheus instance.

Yaay, metrics in Grafana.

My app

For the next step I want to run my own app and have Prometheus scrape it. To get started I add gow-based service to my compose file to have a development friendly environment with auto-recompilation.

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  app:
    image: golang:1.16-alpine
    volumes:
      - go:/go
      - ./app:/go/src/app
    working_dir: /go/src/app
    command: go run github.com/mitranim/gow run .
    ports:
      - 2112:2112

And run with docker-compose up app

Init the app with go mod init app and get something up based on the example from the docs: https://prometheus.io/docs/guides/go-application/

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package main

import (
	"log"
	"net/http"

	"github.com/prometheus/client_golang/prometheus/promhttp"
)

func main() {
	addr := ":2112"
	log.Println("Starting server on", addr)
	http.Handle("/metrics", promhttp.Handler())
	http.ListenAndServe(addr, nil)
}

I can now curl for metrics from my laptop:

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 $ curl -s http://localhost:2112/metrics | head -n5
 # HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles.
 # TYPE go_gc_duration_seconds summary
 go_gc_duration_seconds{quantile="0"} 0
 go_gc_duration_seconds{quantile="0.25"} 0
 go_gc_duration_seconds{quantile="0.5"} 0

Great. Now I only need to inform Prometheus about the existence of my app.

Back to getting started to configure scraping:

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  - job_name: 'app'
    scrape_interval: 5s
    static_configs:
      - targets: ['app:2112']

Apparently SIGHUP to reload config should do the trick

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dc logs -f prometheus

And in another window

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 $ dc kill -s SIGHUP prometheus
Killing prometheus_prometheus_1 ... done

Apparently it worked

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prometheus_1  | level=info ts=2021-06-27T09:05:57.511Z caller=main.go:964 msg="Loading configuration file" filename=/etc/prometheus/prometheus.yml
prometheus_1  | level=info ts=2021-06-27T09:05:57.511Z caller=main.go:995 msg="Completed loading of configuration file" filename=/etc/prometheus/prometheus.yml totalDuration=645.968µs remote_storage=2.492µs web_handler=602ns query_engine=1.266µs scrape=120.724µs scrape_sd=82.519µs notify=1.101µs notify_sd=1.398µs rules=1.343µs

and now “app” shows up in metrics in Grafana!

Request counter

This is still only showing preconfigured metrics. How do I do my custom metrics? e.g. a request counter.

Based on the same Go docs I put this together

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package main

import (
	"log"
	"net/http"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

func main() {
	addr := ":2112"
	log.Println("Starting server on", addr)
	http.Handle("/metrics", promhttp.Handler())
	http.Handle("/", newHandleReq())
	http.ListenAndServe(addr, nil)
}

type handleReq struct {
	reqCounter prometheus.Counter
}

func newHandleReq() http.Handler {
	return &handleReq{
		reqCounter: promauto.NewCounter(prometheus.CounterOpts{
			Name: "app_req_counter",
			Help: "The total number of processed requests",
		}),
	}
}

func (h *handleReq) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	h.reqCounter.Inc()
	w.Write([]byte("ok\n"))
}

gow picks up changes, and my curl requests do indeed show up in Grafana.

Kuberetes

Now how do I get this running in kubernetes? :D How does this work in context of pods? Is there some autodetection of scraping targets?

First of all I need a fresh cluster. This time in minikube

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$ minikube start
$ eval $(minikube docker-env)

Google quickly finds helm charts

Let’s try it out

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$ helm repo add prometheus-community https://prometheus-community.github.io/helm-charts
$ helm repo update
$ helm repo add prometheus-community https://prometheus-community.github.io/helm-charts

then

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minikube dashboard

and wait for all green

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 $ k get service
NAME                               TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)    AGE
kubernetes                         ClusterIP   10.96.0.1        <none>        443/TCP    11m
my-prometheus-alertmanager         ClusterIP   10.110.118.230   <none>        80/TCP     4m45s
my-prometheus-kube-state-metrics   ClusterIP   10.102.204.67    <none>        8080/TCP   4m45s
my-prometheus-node-exporter        ClusterIP   None             <none>        9100/TCP   4m45s
my-prometheus-pushgateway          ClusterIP   10.100.67.158    <none>        9091/TCP   4m45s
my-prometheus-server               ClusterIP   10.104.36.146    <none>        80/TCP     4m45s

The chart includes services as well, let’s use port forwarding to access it

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 $ k port-forward service/my-prometheus-server 9090:80
Forwarding from 127.0.0.1:9090 -> 9090
Forwarding from [::1]:9090 -> 9090

And the regular UI is available again on localhost:9090

Let’s do Grafana as well

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$ helm repo add grafana https://grafana.github.io/helm-charts
$ helm install my-grafana grafana/grafana
NAME: my-grafana
LAST DEPLOYED: Sun Jun 27 13:37:33 2021
NAMESPACE: default
STATUS: deployed
REVISION: 1
NOTES:
1. Get your 'admin' user password by running:

   kubectl get secret --namespace default my-grafana -o jsonpath="{.data.admin-password}" | base64 --decode ; echo

2. The Grafana server can be accessed via port 80 on the following DNS name from within your cluster:

   my-grafana.default.svc.cluster.local

   Get the Grafana URL to visit by running these commands in the same shell:

     export POD_NAME=$(kubectl get pods --namespace default -l "app.kubernetes.io/name=grafana,app.kubernetes.io/instance=my-grafana" -o jsonpath="{.items[0].metadata.name}")
     kubectl --namespace default port-forward $POD_NAME 3000

3. Login with the password from step 1 and the username: admin
#################################################################################
######   WARNING: Persistence is disabled!!! You will lose your data when   #####
######            the Grafana pod is terminated.                            #####
#################################################################################

Following instructions (but forwading the sirevice directly)

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kubectl get secret --namespace default my-grafana -o jsonpath="{.data.admin-password}" | base64 --decode ; echo
k port-forward service/my-grafana 3000:80

Packaging the app

Time to package up my app

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FROM golang:1.16-alpine as builder
COPY . /src
WORKDIR /src
RUN go build .

FROM alpine
WORKDIR /bin/
COPY --from=builder /src/app .
EXPOSE 2112
CMD /bin/app

and build (inside minikube’s docker daemon!)

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docker build . -t app:latest

k8s manifests are the usual boilerplate

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apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
  labels:
    component: web
spec:
  replicas: 1
  selector:
    matchLabels:
      component: web
  template:
    metadata:
      labels:
        component: web
    spec:
      containers:
      - name: web
        image: app:latest
        imagePullPolicy: Never # side-load via minikube
        ports:
        - containerPort: 2112
---
apiVersion: v1
kind: Service
metadata:
  name: web
  labels:
    component: web
spec:
  selector:
    component: web
  ports:
    - protocol: TCP
      port: 2112

and apply

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k apply -f ./app.yaml

forward it too

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k port-forward service/web 2112

Now I can call it from my laptop

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 $ curl http://localhost:2112/
ok

now how do I get this into prometheus?

Scraping in k8s

After quite a bit of stumped googling I checked out values.yaml of the char I applied and found this explanation

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# Scrape config for service endpoints.
#
# The relabeling allows the actual service scrape endpoint to be configured
# via the following annotations:
#
# * `prometheus.io/scrape`: Only scrape services that have a value of `true`
# * `prometheus.io/scheme`: If the metrics endpoint is secured then you will need
# to set this to `https` & most likely set the `tls_config` of the scrape config.
# * `prometheus.io/path`: If the metrics path is not `/metrics` override this.
# * `prometheus.io/port`: If the metrics are exposed on a different port to the
# service then set this appropriately.

So I figured to add prometheus.io/scrape to my manifests. But where? Service or pod spec? I Prometheus should scrape pods in my case since I want to get counter values per-pod.

Updated deployment:

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apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
  labels:
    component: web
spec:
  replicas: 1
  selector:
    matchLabels:
      component: web
  template:
    metadata:
      labels:
        component: web
      annotations:
        prometheus.io/scrape: "true"
    spec:
      containers:
      - name: web
        image: app:latest
        imagePullPolicy: Never # side-load via minikube
        ports:
        - containerPort: 2112

My pod is picked up on localhost:9090/targets, great success.

Naturally metrics also show up in Grafana

Killing pods/scaling deployment is nicely visible in metrics now.

Conclusion

Setting up Prometheus was actually quite straightforward. So is configuring the actual scraping. Application codebase doesn’t need to know anything about my infrastructure, no configuration needed - it simply exposes a /metrics/ endpoint. Then scraping is enabled in my deployment manifest.

For me the part that actually needs some getting used to is the query language, but I think the infrastructure niceties are well worth it.

Last modified on 2021-06-27

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