• 目錄
  • Introduce and analysis
  • cluster level ELK
  • @ sidecar mode @
  • integrate log collecting component into an app

HI 大家今天過得好嗎？
今天要來動手做個sidecar

以下這張來自官網的圖很清楚地解釋了sidecar on K8s的運作模式

大部分的pod只運行一個container, 但在某些情況下
我們必須要在一個pod運行兩個以上的containers, 這種multi-container的模式又可以衍伸至三種design patterns:

• sidecar pattern: 最簡單的方式， 兩個container利用volume的方式share同一個檔案目錄
• adapter pattern: 利用另外一個container做接口， 把要輸出的資料格式化， 例如同一套log system便可以處理不同pods的log
• ambassador pattern: 假若開發環境是在local, 此種模式可以將pod要輸出的資料直接寫在local的資料庫等

此圖出自這裡, 這張圖蠻清楚的描述了三種模式的不同，但由於篇幅以及時間關係，我就先講sidecar mode


這邊就利用上面文件裡的範例寫一個帶有nginx sidecar的pod yaml.

apiVersion: v1
kind: Pod
metadata:
  name: pod-with-sidecar
spec:
  # Create a volume called 'shared-logs' that the
  # app and sidecar share.
  volumes:
  - name: shared-logs 
    emptyDir: {}

  # In the sidecar pattern, there is a main application
  # container and a sidecar container.
  containers:

  # Main application container
  - name: app-container
    # Simple application: write the current date
    # to the log file every five seconds
    image: alpine # alpine is a simple Linux OS image
    command: ["/bin/sh"]
    args: ["-c", "while true; do date >> /var/log/app.txt; sleep 5;done"]

    # Mount the pod's shared log file into the app 
    # container. The app writes logs here.
    volumeMounts:
    - name: shared-logs
      mountPath: /var/log

  # Sidecar container
  - name: sidecar-container
    # Simple sidecar: display log files using nginx.
    # In reality, this sidecar would be a custom image
    # that uploads logs to a third-party or storage service.
    image: nginx:1.7.9
    ports:
      - containerPort: 80

    # Mount the pod's shared log file into the sidecar
    # container. In this case, nginx will serve the files
    # in this directory.
    volumeMounts:
    - name: shared-logs
      mountPath: /usr/share/nginx/html # nginx-specific mount path

在這邊請注意 volumes 裡的 emptyDir 是建立pod時會自動生成的一個目錄，該pod下所有container都可以讀取這目錄的內容

直接創建：

跑起來之後下這個指令查看pod的詳細內容：

接著透過詳細內容中的訊息分別進入兩個containers中查看紀錄logs的目錄:

可以看到在app-container產生的時間log被同步進nginx這個container了

這就是最簡單的sidecar mode
下篇來講app level的log收集

Reference

官方文件-Pod Overview:
https://kubernetes.io/docs/concepts/workloads/pods/pod-overview/
Multi-container design pattern:https://matthewpalmer.net/kubernetes-app-developer/articles/multi-container-pod-design-patterns.html