Docker debugging tips

Abstract: "Reuse" the container name But when we write/debug Dockerfile, we often repeat the previous command, such as this docker run --name jstorm-zookeeper zookeeper:3.4, and then the container name conflicts.

"Reuse" container names

But when we write/debug Dockerfile, we often repeat the previous command, such as this docker run --name jstorm-zookeeper zookeeper:3.4, and then the container name conflicts.

$ docker run --name jstorm-zookeeper zookeeper:3.4

...

$ docker run --name jstorm-zookeeper zookeeper:3.4

docker: Error response from daemon: Conflict. The name "/jstorm-zookeeper" is already in use by container xxxxxxxxx

docker run You can add a flag at runtime --rm , the container will be destroyed after exiting. no manualdocker rm CONTAINER

$ docker run --name jstorm-zookeeper zookeeper:3.4 --rm

# reuse

$ docker create --name jstorm-zookeeper zookeeper:3.4

$ docker start jstorm-zookeeper

# not error

debug Dockerfile

When writing Dockerfile , it is usually not done in one go. Sometimes the container crashes and exits directly when it starts, sometimes the build image fails, or to verify whether the Dockerfile meets the expectations, we often need to debug the Dockerfile.

If the build fails, you can directly view the error information of stdout, split the instructions, and rebuild.

`logs` View stdout

All content written to stdout in the container will be captured in a history file in the host, which can be docker logs CONTAINERviewed.

$ docker run -d --name jstorm-zookeeper zookeeper:3.4

$ docker logs jstorm-zookeeper

ZooKeeper JMX enabled by default

Using config: /conf/zoo.cfg

2016-12-18 05:55:27,717 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 05:55:27,725 [myid:] - INFO [main:DatadirCleanupManager@78] - autopurge.snapRetainCount set to 3

2016-12-18 05:55:27,725 [myid:] - INFO [main:DatadirCleanupManager@79] - autopurge.purgeInterval set to 0

2016-12-18 05:55:27,726 [myid:] - INFO [main:DatadirCleanupManager@101] - Purge task is not scheduled.

2016-12-18 05:55:27,728 [myid:] - WARN [main:QuorumPeerMain@113] - Either no config or no quorum defined in config, running in standalone mode

2016-12-18 05:55:27,746 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 05:55:27,747 [myid:] - INFO [main:ZooKeeperServerMain@96] - Starting server

2016-12-18 05:55:27,766 [myid:] - INFO [main:Environment@100] - Server environment:zookeeper.version=3.4.9-1757313, built on 08/23/2016 06:50 GMT

2016-12-18 05:55:27,766 [myid:] - INFO [main:Environment@100] - Server environment:host.name=dbc742dd5688

2016-12-18 05:55:27,767 [myid:] - INFO [main:Environment@100] - Server environment:java.version=1.8.0_111-internal

即使是容器已经退出的也可以看到，所以可以通过这种方式来分析非预期的退出。这些文件一直保存着，直到通过docker rm把容器删除。文件的具体路径可以通过docker inspect CONTAINER 获得。
（然后osx上你并找不到这些文件，因为其实osx的docker实际是运行在”VM”中，具体就不展开了，但是可以通过 screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/ttytouch上”VM”的tty）

在使用docker logs 的时候加一些参数来过滤log，默认输出所有log。

Options:

--details Show extra details provided to logs

-f, --follow Follow log output

--help Print usage

--since string Show logs since timestamp

--tail string Number of lines to show from the end of the logs (default "all")

-t, --timestamps Show timestamps

`attach` 实时查看stdout

如果你想实时查看容器的输出你可以用 docker attach CONTAINER 命令。

默认会绑定stdin，代理signals，所以如果你 ctrl-c 容器通常会退出。很多时候大家并不想这样，只是想分离开，可以ctrl-p ctrl-q。

执行任意command

可以通过docker exec CONTAINER COMMAND，来在容器内执行任意 command，比如 cat 一些东西来debug。

$ docker run -d --name jstorm-zookeeper zookeeper:3.4

$ docker exec jstorm-zookeeper java -version

openjdk version "1.8.0_111-internal"

OpenJDK Runtime Environment (build 1.8.0_111-internal-alpine-r0-b14)

OpenJDK 64-Bit Server VM (build 25.111-b14, mixed mode)

也可以直接通过 exec 在容器内启动一个 shell 更方便地调试容器，不必一条条执行docker exec。

$ docker exec -it jstorm-zookeeper /bin/bash

bash-4.3# pwd

/zookeeper-3.4.9

docker exec 只能在正在运行的容器上使用，如果已经停止了退出了就不行了，就只好用 docker logs了。

重写`entrypoint`和`cmd`

每个Docker镜像都有 entrypoint 和 cmd , 可以定义在 Dockerfile 中，也可以在运行时指定。这两个概念很容易混淆，而且它们的试用方式也不同。

entrypoint 比 cmd 更”高级”，entrypoint 作为容器中pid为1的进程运行（docker不是虚拟机，只是隔离的进程。真正的linux中pid为1的是init）。
cmd 只是 entrypoint的参数:

1	<ENTRYPOINT> "<CMD>"

当我们没有指定 entrypoint 时缺省为 /bin/sh -c。所以其实 entrypoint 是真正表达这个docker应该干什么的，通常大家有一个shell 脚本来代理。
entrypoint 和 cmd 都可以在运行的时候更改，通过更改来看这样设置entrypoint是否优雅合理。

$ docker run -it --name jstorm-zookeeper --entrypoint /bin/bash zookeeper:3.4

bash-4.3# top

Mem: 320212K used, 1725368K free, 89112K shrd, 35532K buff, 130532K cached

CPU: 0% usr 0% sys 0% nic 100% idle 0% io 0% irq 0% sirq

Load average: 0.20 0.06 0.02 5/195 7

PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND

1 0 root S 6220 0% 0 0% /bin/bash

7 1 root R 1516 0% 2 0% top

任何 docker run 命令中在image名后的内容都作为cmd的内容传给 entrypoint当参数。

暂停容器

使用 docker pause 可以暂停容器中所有进程。这非常有用。

$ docker run -d --name jstorm-zookeeper zookeeper:3.4 && sleep 0.1 && docker pause jstorm-zookeeper && docker logs jstorm-zookeeper

a24405a53ddd9b7d94d9e77fe2b5a67639a251d681aa2f34fcb0cc96f347ba48

jstorm-zookeeper

ZooKeeper JMX enabled by default

Using config: /conf/zoo.cfg

2016-12-18 16:17:47,720 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@78] - autopurge.snapRetainCount set to 3

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@79] - autopurge.purgeInterval set to 0

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@101] - Purge task is not scheduled.

2016-12-18 16:17:47,731 [myid:] - WARN [main:QuorumPeerMain@113] - Either no config or no quorum defined in config, running in standalone mode

2016-12-18 16:17:47,757 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 16:17:47,757 [myid:] - INFO [main:ZooKeeperServerMain@96] - Starting server

$ docker unpause jstorm-zookeeper && docker logs jstorm-zookeeper

jstorm-zookeeper

ZooKeeper JMX enabled by default

Using config: /conf/zoo.cfg

2016-12-18 16:17:47,720 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@78] - autopurge.snapRetainCount set to 3

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@79] - autopurge.purgeInterval set to 0

2016-12-18 16:17:47,730 [myid:] - INFO [main:DatadirCleanupManager@101] - Purge task is not scheduled.

2016-12-18 16:17:47,731 [myid:] - WARN [main:QuorumPeerMain@113] - Either no config or no quorum defined in config, running in standalone mode

2016-12-18 16:17:47,757 [myid:] - INFO [main:QuorumPeerConfig@124] - Reading configuration from: /conf/zoo.cfg

2016-12-18 16:17:47,757 [myid:] - INFO [main:ZooKeeperServerMain@96] - Starting server

2016-12-18 16:18:09,039 [myid:] - INFO [main:Environment@100] - Server environment:zookeeper.version=3.4.9-1757313, built on 08/23/2016 06:50 GMT

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:host.name=a24405a53ddd

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:java.version=1.8.0_111-internal

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:java.vendor=Oracle Corporation

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:java.home=/usr/lib/jvm/java-1.8-openjdk/jre

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:java.class.path=/zookeeper-3.4.9/bin/../build/classes:/zookeeper-3.4.9/bin/../build/lib/*.jar:/zookeeper-3.4.9/bin/../lib/slf4j-log4j12-1.6.1.jar:/zookeeper-3.4.9/bin/../lib/slf4j-api-1.6.1.jar:/zookeeper-3.4.9/bin/../lib/netty-3.10.5.Final.jar:/zookeeper-3.4.9/bin/../lib/log4j-1.2.16.jar:/zookeeper-3.4.9/bin/../lib/jline-0.9.94.jar:/zookeeper-3.4.9/bin/../zookeeper-3.4.9.jar:/zookeeper-3.4.9/bin/../src/java/lib/*.jar:/conf:

2016-12-18 16:18:09,040 [myid:] - INFO [main:Environment@100] - Server environment:java.library.path=/usr/lib/jvm/java-1.8-openjdk/jre/lib/amd64/server:/usr/lib/jvm/java-1.8-openjdk/jre/lib/amd64:/usr/lib/jvm/java-1.8-openjdk/jre/../lib/amd64:/usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib

2016-12-18 16:18:09,041 [myid:] - INFO [main:Environment@100] - Server environment:java.io.tmpdir=/tmp

2016-12-18 16:18:09,041 [myid:] - INFO [main:Environment@100] - Server environment:java.compiler=<NA>

2016-12-18 16:18:09,043 [myid:] - INFO [main:Environment@100] - Server environment:os.name=Linux

2016-12-18 16:18:09,043 [myid:] - INFO [main:Environment@100] - Server environment:os.arch=amd64

2016-12-18 16:18:09,044 [myid:] - INFO [main:Environment@100] - Server environment:os.version=4.4.27-moby

2016-12-18 16:18:09,044 [myid:] - INFO [main:Environment@100] - Server environment:user.name=zookeeper

2016-12-18 16:18:09,044 [myid:] - INFO [main:Environment@100] - Server environment:user.home=/home/zookeeper

2016-12-18 16:18:09,044 [myid:] - INFO [main:Environment@100] - Server environment:user.dir=/zookeeper-3.4.9

2016-12-18 16:18:09,057 [myid:] - INFO [main:ZooKeeperServer@815] - tickTime set to 2000

2016-12-18 16:18:09,057 [myid:] - INFO [main:ZooKeeperServer@824] - minSessionTimeout set to -1

2016-12-18 16:18:09,058 [myid:] - INFO [main:ZooKeeperServer@833] - maxSessionTimeout set to -1

2016-12-18 16:18:09,076 [myid:] - INFO [main:NIOServerCnxnFactory@89] - binding to port 0.0.0.0/0.0.0.0:2181

`top` 和 `stats` 获得容器中进程的状态

docker top CONTAINER 和在容器里执行 top 的效果类似。

$ docker top jstorm-zookeeper

PID USER TIME COMMAND

24593 dockrema 0:01 /usr/lib/jvm/java-1.8-openjdk/jre/bin/java -Dzookeeper.log.dir=. .....

$ docker stats jstorm-zookeeper

CONTAINER CPU % MEM USAGE / LIMIT MEM % NET I/O BLOCK I/O PIDS

jstorm-zookeeper 0.00% 24.86 MiB / 1.951 GiB 1.24% 648 B / 648 B 0 B / 0 B 20

通过 `inspect` 查看容器的详细信息

docker inspect CONTAINER 饭后镜像和容器的详细信息。比如：

State —— 容器的当先状态
LogPath —— history(stdout) file 的路径
Config.Env —— 环境变量
NetworkSettings.Ports —— 端口的映射关系

环境变量非常有用很多问题都是环境变量引起的。

`history` 查看 image layers

可以看到各层创建的指令，大小和哈希。可以用来检查这个image是否符合你的预期。

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