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Docker shell vs. exec form

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21 Jan 2019

I containerize… Why I containerize you might ask? Because of reasons:

As it is quite common in modern world, containerizing comes with a price to pay. And the price is called: abstraction. I remember someone said that

Every abstraction layer solves one problem, by introducing ten different ones.

Or something quite close to that. Of course, that is an oversimplication (still being true), but I wouldn’t change the docker abstraction for now due to the benefits it gives us. This post, however, is not about the benefits but about a specific issue, which I was not aware of for quite a long time. It’s related to running apps, commands inside containers - and more closely - about stopping them.

How to specify run commands in docker?

I have created a sample project for this post, which can be found at https://github.com/gmaslowski/docker-shell-vs-exec. This project has a simple Spring based app and some Docker descriptor files, for building images and setting up container with docker-compose (please note, that described issues should correspond to any form of starting a docker container).

The simple snippet project focuses on two ways of executing commands inside a docker container:

Both will have the same effect, at least when it comes to running containers on top of those images. If we build the application and the docker images, as specified in the README, like this:

./gradlew clean build
cp build/libs/docker-shell-vs-exec-0.0.1-SNAPSHOT.jar docker-shell/app.jar
cp build/libs/docker-shell-vs-exec-0.0.1-SNAPSHOT.jar docker-exec/app.jar
docker build --build-arg JAR_FILE=build/libs/docker-shell-vs-exec-0.0.1-SNAPSHOT.jar docker-exec -t dsve:exec
docker build --build-arg JAR_FILE=build/libs/docker-shell-vs-exec-0.0.1-SNAPSHOT.jar docker-shell -t dsve:shell

We would create two images:

By running the following script, we would deploy and run our containers with docker-compose:

docker-compose -f deployment/docker-compose.yml up -d

The actual docker process runtime should look similar to that:

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED              STATUS              PORTS               NAMES
bd9e3f85a7b0        dsve:shell          "/bin/sh -c 'java -X…"   About a minute ago   Up 56 seconds                           deployment_dsve-shell_1
f0ae8ce0cbc8        dsve:exec           "java -XX:+ExitOnOut…"   About a minute ago   Up 56 seconds                           deployment_dsve-exec_1

In the COMMAND section one can already see both ways (shell and exec) of executing the java app inside the container. Let us have a quick look into the containers to list the processes.

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker exec -it bd9 sh
/ # ps uxa
PID   USER     TIME   COMMAND
    1 root       0:00 /bin/sh -c java -XX:+ExitOnOutOfMemoryError -Djava.securi
    5 root       0:08 java -XX:+ExitOnOutOfMemoryError -Djava.security.egd=file
   33 root       0:00 sh
   37 root       0:00 ps uxa

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker exec -it f0a sh
/ # ps uxa
PID   USER     TIME   COMMAND
    1 root       0:08 java -XX:+ExitOnOutOfMemoryError -Djava.security.egd=file
   27 root       0:00 sh
   32 root       0:00 ps uxa

The difference is easy to spot, the same java command, one started with /bin/sh and the other without it.

What Docker documentation says about those two forms?

Well, it says many things, and it also describes the difference between shell and exec form. In my opinion, such “details” are often in places which are easy to overlook, and if you’re as impatient and careless :) as I am - you probably will overlook them as well. Careful reading of the docker documentation is strongly advised - https://docs.docker.com/engine/reference/builder/#entrypoint.

Yes, ok, but what are those forms implying?

Evironment variables substitution

In a shell form, all environment variables will be evaluated as the actual provided command will be run within a shell by prepending /bin/sh -c before it, which can also be observed in the snippet from previous section. In the exec form, however, there is no shell processing involved and the executable is being called directly. So please make sure that your env vars are being substituted before or that the executable you invoke does it.

RUN, ENTRYPOINT and CMD

I don’t want to focus on explaining the differences in much detail.

In this article http://goinbigdata.com/docker-run-vs-cmd-vs-entrypoint/ you can find a really great explanation of the difference and it really wouldn’t make sense to duplicate the content. Additionally, the difference has also been explained quite well in the Docker documentation in the section understand-how-cmd-and-entrypoint-interact.

Gracefully stopping a container

But here we can get into troubles. If we try to stop a container with the shell form

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker stop bd9

there’s a significant time, which we might notice before the container stops. That’s because we extended the stop_grace_period from the default 10s to 30s - mainly for the presentation purposes. But if you look closely into the logs, you won’t find any information from the Spring application notifying that the system sent a SIGTERM signal. That’s due to the fact that this signal was send actually to the shell, which doesn’t pass any signals to the process it started. It is described in Docker documentation, however it is quite easy to miss that - I know I was myself not aware of those implications for a long time. And hence, after the stop_grace_period passes, docker daemon sends a SIGKILL signal causing the container to stop, forcefully.

On the other hand, the exec form stops almost immediately

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker stop f0a

and in the logs we cas spot that Sring based application handled the SIGTERM command allowing to close all obtained resources:

/c/dev_env/projects/private/docker-shell-vs-exec (master)
$ docker logs f0a --tail=10
2019-01-21 17:44:39.089  INFO 1 --- [           main] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization completed in 28 ms
2019-01-21 17:44:39.163  INFO 1 --- [           main] o.e.jetty.server.AbstractConnector       : Started ServerConnector@7a3d45bd{HTTP/1.1,[http/1.1]}{0.0.0.0:8080}
2019-01-21 17:44:39.164  INFO 1 --- [           main] .s.b.c.e.j.JettyEmbeddedServletContainer : Jetty started on port(s) 8080 (http/1.1)
2019-01-21 17:44:39.174  INFO 1 --- [           main] com.gmaslowski.dsve.SampleApplication    : Started SampleApplication in 6.492 seconds (JVM running for 7.867)
2019-01-21 18:21:42.328  INFO 1 --- [      Thread-11] ationConfigEmbeddedWebApplicationContext : Closing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@2401f4c3: startup date [Mon Jan 21 17:44:33 GMT 2019]; root of context hierarchy
2019-01-21 18:21:42.341  INFO 1 --- [      Thread-11] o.s.j.e.a.AnnotationMBeanExporter        : Unregistering JMX-exposed beans on shutdown
2019-01-21 18:21:42.394  INFO 1 --- [      Thread-11] o.e.jetty.server.AbstractConnector       : Stopped ServerConnector@7a3d45bd{HTTP/1.1,[http/1.1]}{0.0.0.0:8080}
2019-01-21 18:21:42.395  INFO 1 --- [      Thread-11] org.eclipse.jetty.server.session         : Stopped scavenging
2019-01-21 18:21:42.412  INFO 1 --- [      Thread-11] o.e.j.s.h.ContextHandler.application     : Destroying Spring FrameworkServlet 'dispatcherServlet'
2019-01-21 18:21:42.425  INFO 1 --- [      Thread-11] o.e.jetty.server.handler.ContextHandler  : Stopped o.s.b.c.e.j.JettyEmbeddedWebAppContext@50d0686{/,[file:///tmp/jetty-docbase.3963833647300409511.8080/],UNAVAILABLE}

And that’s the crucial part. In best case scenario, the problems will only cause longer waits for the container to stop. But in worst case scenario, if the application doesn’t free any used resources (like database connections, locks etc.)… yeah, you can imagine the consequences.

I spotted similar issues while working with k8s as the container orchestrator. And this should be fully understandable. The container in the pod tries to handle the SIGTERM signal, and if it doesn’t, the orchestrator will SIGKILL it.

Extra

In my current project we use, amongst others, Sbt. It has its own plugin for creating docker images - sbt-native-packager, please be careful when choosing Cmd over ExecCmd :D.

I’m curious about, What other things are commonly overlooked while using docker? If you have an example, just comment or send an email.