用于 docker 的 Cron 容器 - 它们实际上是如何工作的? [英] Cron containers for docker - how do they actually work?

问题描述

我已经使用 docker 几个月了，并且正在对各种不同的服务器镜像进行 dockerizing.一个一致的问题是许多服务器需要运行 cron 作业.网上对此有很多讨论(包括在 Stackoverflow 上)，但我并不完全了解它的机制.

目前，我在每个容器中使用主机的 cron 和 docker exec 来运行脚本.我创建了一个关于脚本名称和位置的约定；我所有的容器都有相同的脚本.这避免了主机的 cron 依赖于容器.

基本上，主机的 cron 每分钟执行一次:

对于每个容器docker exec -it <容器名称>/cronscript/分钟脚本

这可行，但使容器依赖于主机.

我想做的是创建一个 cron 容器，该容器在每个其他容器中启动一个脚本 - 但我不知道从一个容器到另一个容器的等效于docker exec".

我现在遇到的具体情况是在 MySQL 容器中运行备份，并且运行 Moodle 需要每分钟运行一次的 cron 作业.最终，我需要通过 cron 做一些额外的事情.Moodle 使用命令行 PHP 脚本.

从另一个容器中的一个容器启动脚本的正确"dockerized 方式是什么?

更新:也许它有助于提及我的具体用例，尽管随着时间的推移会有更多.

目前cron需要做的有:

• 从 MySQL 执行数据库转储.我可以通过 cron 容器中的 mysqldump TCP 链接来做到这一点；这里的缺点是我不能将备份用户限制为主机 127.0.0.1.我还可以通过一个卷以某种方式将 MySQL 套接字编入 cron 容器.
• 对 Moodle 安装执行定期维护.Moodle 包含一个运行所有维护任务的 php 命令行脚本.这对我来说很重要.我可能可以在一个卷中运行这个脚本，但是 Moodle 的设计并没有考虑到这种情况，我不排除竞争条件.此外，我不希望将我的moodle 安装在一个卷中，因为它使更新容器变得更加困难(请记住，在Docker 中，当您使用新映像更新容器时，卷不会重新初始化).
• 未来:对我的许多其他服务器进行日常维护，例如清理电子邮件队列等.

解决方案

我的解决方案是:

• 在容器内安装 crond
• 安装你的软件
• 将 cron 作为守护进程运行
• 运行你的软

我的Dockerfile

的一部分

来自 debian:jessie运行 mkdir -p/usr/src/app工作目录/usr/src/app复制 .crontab/usr/src/app# 设置时区运行回声欧洲/华沙">/etc/时区&&dpkg-reconfigure --frontend 非交互式 tzdata# Cron，邮件运行设置 -x &&apt-get 更新 &&apt-get install -y cron rsyslog mailutils --no-install-recommends &&rm -rf/var/lib/apt/lists/*CMD rsyslogd &&环境 >/tmp/crontab &&猫.crontab >>/tmp/crontab &&crontab/tmp/crontab &&cron -f

说明

1. 设置时区，因为 cron 需要它来正确运行任务
2. 安装 cron 包 - 带有 cron 守护进程的包
3. 安装 rsyslog 包以记录 cron 任务输出
4. 如果您想从 cron 任务发送电子邮件，请安装 mailutils 包
5. 运行rsyslogd
6. 将 ENV 变量复制到 tmp 文件，因为 cron 以最小的 ENV 运行任务，而您的任务可能需要访问容器 ENV 变量
7. 将您的 .crontab 文件(带有您的任务)附加到 tmp 文件
8. 从 tmp 文件设置 root crontab
9. 运行 cron 守护进程

我在我的容器中使用它并且效果很好.

一个进程一个容器

如果您喜欢这种范例，那么为每个 cron 任务制作一个 Dockerfile.例如

• Dockerfile - 主程序
• Dockerfile_cron_task_1 - cron 任务 1
• Dockerfile_cron_task_1 - cron 任务 2

并构建所有容器:

docker build -f Dockerfile_cron_task_1 ...

I have been using docker for a couple of months now, and am working on dockerizing various different server images. One consistent issue is that many servers need to run cron jobs. There is a lot of discussion about that online (including on Stackoverflow), but I don't completely understand the mechanics of it.

Currently, I am using the host's cron and docker exec into each container to run a script. I created a convention about the script's name and location; all my containers have the same script. This avoids having the host's cron depending on the containers.

Basically, once a minute, the host's cron does this:

for each container
   docker exec -it <containername> /cronscript/minute-script

That works, but makes the containers depend on the host.

What I would like to do is create a cron container that kicks off a script within each of the other containers - but I am not aware of an equivalent to "docker exec" that works from one container to the other.

The specific situations I have right now are running a backup in a MySQL container, and running the cron jobs Moodle requires to be run every minute. Eventually, there will be additional things I need to do via cron. Moodle uses command-line PHP scripts.

What is the "proper" dockerized way to kick off a script from one container in another container?

Update: maybe it helps to mention my specific use cases, although there will be more as time goes on.

Currently, cron needs to do the following:

• Perform a database dump from MySQL. I can do that via mysqldump TCP link from a cron container; the drawback here is that I can't limit the backup user to host 127.0.0.1. I might also be able to somehow finagle the MySQL socket into the cron container via a volume.
• Perform regular maintenance on a Moodle installation. Moodle includes a php command line script that runs all of the maintenance tasks. This is the biggie for me. I can probably run this script through a volume, but Moodle was not designed with that situation in mind, and I would not rule out race conditions. Also, I do not want my moodle installation in a volume because it makes updating the container much harder (remember that in Docker, volumes are not reinitialized when you update the container with a new image).
• Future: perform routine maintenance on a number of other of my servers, such as cleaning out email queues, etc.

解决方案

My solution is:

• install crond inside container
• install Your soft
• run cron as a daemon
• run Your soft

Part of my Dockerfile

FROM debian:jessie

RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY .crontab /usr/src/app

# Set timezone
RUN echo "Europe/Warsaw" > /etc/timezone 
    && dpkg-reconfigure --frontend noninteractive tzdata

# Cron, mail
RUN set -x 
    && apt-get update 
    && apt-get install -y cron rsyslog mailutils --no-install-recommends 
    && rm -rf /var/lib/apt/lists/*

CMD rsyslogd && env > /tmp/crontab && cat .crontab >> /tmp/crontab && crontab /tmp/crontab && cron -f

Description

1. Set timezone, because cron need this to proper run tasks
2. Install cron package - package with cron daemon
3. Install rsyslog package to log cron task output
4. Install mailutils package if You want to send e-mails from cron tasks
5. Run rsyslogd
6. Copy ENV variables to tmp file, because cron run tasks with minimal ENV and You tasks may need access to containers ENV variables
7. Append Your .crontab file (with Your tasks) to tmp file
8. Set root crontab from tmp file
9. Run cron daemon

I use this in my containers and work very well.

one-process-per-container

If You like this paradigm, then make one Dockerfile per cron task. e.g.

• Dockerfile - main program
• Dockerfile_cron_task_1 - cron task 1
• Dockerfile_cron_task_1 - cron task 2

and build all containers:

docker build -f Dockerfile_cron_task_1 ...

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