一、docker compose 服务依赖
- 使用depends_on关键字
二、docker compose 健康检查
- 容器的健康检查
- docker-compose 健康检查

一、docker compose 服务依赖

使用depends_on关键字

version: "3.8"
services:
  flask:
    build:
      context: ./flask
      dockerfile: Dockerfile
    image: flask-demo:latest
    environment:
      - REDIS_HOST=redis-server
      - REDIS_PASS=${REDIS_PASSWORD}
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:5000"]
      interval: 30s
      timeout: 3s
      retries: 3
      start_period: 40s
    depends_on:
      - redis-server
    networks:
      - backend
      - frontend
  redis-server:
    image: redis:latest
    command: redis-server --requirepass ${REDIS_PASSWORD}
    networks:
      - backend
  nginx:
    image: nginx:stable-alpine
    ports:
      - 8000:80
    depends_on:
      flask:
        condition: service_healthy
    volumes:
      - ./nginx/nginx.conf:/etc/nginx/conf.d/default.conf:ro
      - ./var/log/nginx:/var/log/nginx
    networks:
      - frontend
networks:
  backend:
  frontend:

二、docker compose 健康检查

Dockerfile healthcheck https://docs.docker.com/engine/reference/builder/#healthcheck
docker compose https://docs.docker.com/compose/compose-file/compose-file-v3/#healthcheck
健康检查是容器运行状态的高级检查，主要是检查容器所运行的进程是否能正常的对外提供“服务”，比如一个数据库容器，我们不光需要这个容器是up的状态，我们还要求这个容器的数据库进程能够正常对外提供服务，这就是所谓的健康检查。

容器的健康检查

容器本身有一个健康检查的功能，但是需要在Dockerfile里定义，或者在执行docker container run 的时候，通过下面的一些参数指定

--health-cmd string              Command to run to check health
--health-interval duration       Time between running the check
                                (ms|s|m|h) (default 0s)
--health-retries int             Consecutive failures needed to
                                report unhealthy
--health-start-period duration   Start period for the container to
                                initialize before starting
                                health-retries countdown
                                (ms|s|m|h) (default 0s)
--health-timeout duration        Maximum time to allow one check to

示例源码

我们以下面的这个flask容器为例，相关的代码如下

PS C:\Users\Peng Xiao\code-demo\compose-env\flask> dir
    目录: C:\Users\Peng Xiao\code-demo\compose-env\flask
Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----         2021/7/13     15:52            448 app.py
-a----         2021/7/14      0:32            471 Dockerfile
PS C:\Users\Peng Xiao\code-demo\compose-env\flask> more .\app.py
from flask import Flask
from redis import StrictRedis
import os
import socket
app = Flask(__name__)
redis = StrictRedis(host=os.environ.get('REDIS_HOST', '127.0.0.1'),
                    port=6379, password=os.environ.get('REDIS_PASS'))
@app.route('/')
def hello():
    redis.incr('hits')
    return f"Hello Container World! I have been seen {redis.get('hits').decode('utf-8')} times and my hostname is {socket.gethostname()}.\n"
PS C:\Users\Peng Xiao\code-demo\compose-env\flask> more .\Dockerfile
FROM python:3.9.5-slim
RUN pip install flask redis && \
    apt-get update && \
    apt-get install -y curl && \
    groupadd -r flask && useradd -r -g flask flask && \
    mkdir /src && \
    chown -R flask:flask /src
USER flask
COPY app.py /src/app.py
WORKDIR /src
ENV FLASK_APP=app.py REDIS_HOST=redis
EXPOSE 5000
HEALTHCHECK --interval=30s --timeout=3s \
    CMD curl -f http://localhost:5000/ || exit 1
CMD ["flask", "run", "-h", "0.0.0.0"]

上面Dockerfili里的HEALTHCHECK 就是定义了一个健康检查。会每隔30秒检查一次，如果失败就会退出，退出代码是1

构建镜像和创建容器

构建镜像，创建一个bridge网络，然后启动容器连到bridge网络

$ docker image build -t flask-demo .
$ docker network create mybridge
$ docker container run -d --network mybridge --env REDIS_PASS=abc123 flask-demo

查看容器状态

$ docker container ls
CONTAINER ID   IMAGE        COMMAND                  CREATED       STATUS                            PORTS      NAMES
059c12486019   flask-demo   "flask run -h 0.0.0.0"   4 hours ago   Up 8 seconds (health: starting)   5000/tcp   dazzling_tereshkova

也可以通过docker container inspect 059 查看详情，其中有有关health的

"Health": {
"Status": "starting",
"FailingStreak": 1,
"Log": [
    {
        "Start": "2021-07-14T19:04:46.4054004Z",
        "End": "2021-07-14T19:04:49.4055393Z",
        "ExitCode": -1,
        "Output": "Health check exceeded timeout (3s)"
    }
]
}

经过3次检查，一直是不通的，然后health的状态会从starting变为 unhealthy

docker container ls
CONTAINER ID   IMAGE        COMMAND                  CREATED       STATUS                     PORTS      NAMES
059c12486019   flask-demo   "flask run -h 0.0.0.0"   4 hours ago   Up 2 minutes (unhealthy)   5000/tcp   dazzling_tereshkova

启动redis服务器

启动redis，连到mybridge上，name=redis，注意密码

$ docker container run -d --network mybridge --name redis redis:latest redis-server --requirepass abc123

经过几秒钟，我们的flask 变成了healthy

$ docker container ls
CONTAINER ID   IMAGE          COMMAND                  CREATED          STATUS                   PORTS      NAMES
bc4e826ee938   redis:latest   "docker-entrypoint.s…"   18 seconds ago   Up 16 seconds            6379/tcp   redis
059c12486019   flask-demo     "flask run -h 0.0.0.0"   4 hours ago      Up 6 minutes (healthy)   5000/tcp   dazzling_tereshkova

docker-compose 健康检查

示例代码下载(flask healthcheck) [本节源码](https://dockertips.readthedocs.io/en/latest/_downloads/df3430ebd5e1f962f0c10136565257e1/compose-healthcheck-flask.zip)
示例代码下载(flask + redis healthcheck) [本节源码](https://dockertips.readthedocs.io/en/latest/_downloads/529c888c2faf46a0906548ed7510d12b/compose-healthcheck-redis.zip)
一个healthcheck不错的例子 https://gist.github.com/phuysmans/4f67a7fa1b0c6809a86f014694ac6c3a

容器技术

十一、Docker-Compose-服务依赖和健康检查