Kubernetes ConfigMaps: Nuances to Know

Background: From rsync to ... Kubernetes

Simple ConfigMaps

apiVersion: v1
kind: ConfigMap
metadata:
  name: app
data:
  config.json: |
    {
      "welcome": {{ pluck .Values.global.env .Values.welcome | quote }},
      "name": {{ pluck .Values.global.env .Values.name | quote }}
    }

$ cat .helm/values.yaml 
welcome:
  production: "Hello"
  test: "Hey"
name:
  production: "Bob"
  test: "Mike"

data:
  config.json: |
    {{ pluck .Values.global.env .Values.data | first | toJson | indent 4 }}

data:
  production:
    welcome: "Hello"
    name: "Bob"

When there are a lot of configs

data:
  config.json: |
    {
      "welcome": {{ pluck .Values.global.env .Values.welcome | first | quote }},
      "name": {{ pluck .Values.global.env .Values.name | first | quote }}
    }
  database.yml: |
    host: 127.0.0.1
    db: app
    user: app
    password: app

        volumeMounts:
        - name: app-conf
          mountPath: /app/configfiles/config.json
          subPath: config.json
        - name: app-conf
          mountPath: /app/configfiles/database.yml
          subPath: database.yml

        volumeMounts:
        - name: app-conf
          mountPath: /app/configfiles

ConfigMap Change Response

1. : ConfigMap, subPath.
   : .
2. : ConfigMap, pod.
   : pod .
3. : ConfigMap Deployment -.
   : , ConfigMap’, Deployment, pod , — .
4. : ConfigMap, .
   : pod’ / pod’.

Scenario 1

Scenario 2

Scenario 3

kind: Deployment
spec:
  template:
    metadata:
      annotations:
        checksum/config: {{ include (print $.Template.BasePath "/configmap.yaml") . | sha256sum }}
[...]

Scenario 4

• Such an updated config file inside the container is updated with some delay. This is due to the fact that the file is not mounted exactly, but the Kubernetes object.
• The file inside is a symlink. Example with subPath:
  
  

  $ kubectl -n production exec go-conf-example-6b4cb86569-22vqv -- ls -lha /app/configfiles 
  total 20K    
  drwxr-xr-x    1 root     root        4.0K Mar  3 19:34 .
  drwxr-xr-x    1 app      app         4.0K Mar  3 19:34 ..
  -rw-r--r--    1 root     root          42 Mar  3 19:34 config.json
  -rw-r--r--    1 root     root          47 Mar  3 19:34 database.yml

  
  And what will happen without subPathwhen mounted by the directory?
  
  

  $ kubectl -n production exec go-conf-example-67c768c6fc-ccpwl -- ls -lha /app/configfiles 
  total 12K    
  drwxrwxrwx    3 root     root        4.0K Mar  3 19:40 .
  drwxr-xr-x    1 app      app         4.0K Mar  3 19:34 ..
  drwxr-xr-x    2 root     root        4.0K Mar  3 19:40 ..2020_03_03_16_40_36.675612011
  lrwxrwxrwx    1 root     root          31 Mar  3 19:40 ..data -> ..2020_03_03_16_40_36.675612011
  lrwxrwxrwx    1 root     root          18 Mar  3 19:40 config.json -> ..data/config.json
  lrwxrwxrwx    1 root     root          19 Mar  3 19:40 database.yml -> ..data/database.yml

  
  Update the config (via deploy or kubectl edit), wait 2 minutes (apiserver caching time) - and voila:
  
  

  $ kubectl -n production exec go-conf-example-67c768c6fc-ccpwl -- ls -lha --color /app/configfiles 
  total 12K    
  drwxrwxrwx    3 root     root        4.0K Mar  3 19:44 .
  drwxr-xr-x    1 app      app         4.0K Mar  3 19:34 ..
  drwxr-xr-x    2 root     root        4.0K Mar  3 19:44 ..2020_03_03_16_44_38.763148336
  lrwxrwxrwx    1 root     root          31 Mar  3 19:44 ..data -> ..2020_03_03_16_44_38.763148336
  lrwxrwxrwx    1 root     root          18 Mar  3 19:40 config.json -> ..data/config.json
  lrwxrwxrwx    1 root     root          19 Mar  3 19:40 database.yml -> ..data/database.yml

  
  Note the changed timestamp in the directory created by Kubernetes.

Change tracking

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"os"
	"time"

	"github.com/fsnotify/fsnotify"
)

// Config fo our application
type Config struct {
	Welcome string `json:"welcome"`
	Name    string `json:"name"`
}

var (
	globalConfig *Config
)

// LoadConfig - load our config!
func LoadConfig(path string) (*Config, error) {
	configFile, err := os.Open(path)

	if err != nil {
		return nil, fmt.Errorf("Unable to read configuration file %s", path)
	}

	config := new(Config)

	decoder := json.NewDecoder(configFile)
	err = decoder.Decode(&config)
	if err != nil {
		return nil, fmt.Errorf("Unable to parse configuration file %s", path)
	}

	return config, nil
}

// ConfigWatcher - watches config.json for changes
func ConfigWatcher() {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		log.Fatal(err)
	}
	defer watcher.Close()

	done := make(chan bool)
	go func() {
		for {
			select {
			case event, ok := <-watcher.Events:
				if !ok {
					return
				}
				log.Println("event:", event)
				if event.Op&fsnotify.Write == fsnotify.Write {
					log.Println("modified file:", event.Name)
				}
				globalConfig, _ = LoadConfig("./configfiles/config.json")
				log.Println("config:", globalConfig)
			case err, ok := <-watcher.Errors:
				if !ok {
					return
				}
				log.Println("error:", err)
			}
		}
	}()

	err = watcher.Add("./configfiles/config.json")
	if err != nil {
		log.Fatal(err)
	}
	<-done
}

func main() {
	log.Println("Start")
	globalConfig, _ = LoadConfig("./configfiles/config.json")
	go ConfigWatcher()
	for {
		log.Println("config:", globalConfig)
		time.Sleep(30 * time.Second)
	}
}

$ cat configfiles/config.json 
{
  "welcome": "Hello",
  "name": "Alice"
}

2020/03/03 22:18:22 config: &{Hello Alice}
2020/03/03 22:18:52 config: &{Hello Alice}

helm install -n habr-configmap --namespace habr-configmap ./habr-configmap --set 'name.production=Alice' --set 'global.env=production'

-  production: "Alice"
+  production: "Bob"

helm upgrade habr-configmap ./habr-configmap --set 'name.production=Bob' --set 'global.env=production'

• Applications v1 and v2 do not restart, because for them, no change has occurred in Deployment - they still welcome Alice.
• The v3 application restarted, re-read the config, and greeted Bob.
• The v4 application did not restart. Since ConfigMap is mounted as a directory, changes in the config were noticed and the config changed on the fly, without restarting the pod. Yes, the application noticed changes in our simple example - see the event message from fsnotify :
  
  

  2020/03/03 22:19:15 event: "configfiles/config.json": CHMOD
  2020/03/03 22:19:15 config: &{Hello Bob}
  2020/03/03 22:19:22 config: &{Hello Bob}

Bonus! Third Party Solutions

• jimmidyson / configmap-reload - Sends an HTTP request if the file has changed. The developer also plans to teach SIGHUP to send, but the lack of commits from October 2019 leave these plans in question;
• stakater / Reloader - monitors ConfigMap / Secrets and performs rolling upgrade (as its author calls it) on the resources associated with them.

Conclusion

PS

• “ Local files when porting an application to Kubernetes ”;
• « Kubernetes Helm: »;
• «SSL- Let's Encrypt cert-manager Kubernetes».