Three Docker Gotchas

As we’re on the topic of errors, when you’re using Docker commands, make sure not to enter the wrong parameter string.

1. Docker Case-Sensitivity

For example, here in Example 2-10, the user incorrectly supplies an uppercase letter in the Container parameter.

Example 2-10. Docker is case-sensitive

C:\Users\Stephen>docker Container ls -a
unknown shorthand flag: ‘a’ in -a
See ‘docker –help’.
Usage:  docker [OPTIONS] COMMAND
A self-sufficient runtime for containers
Options:
      –config string      Location of client config files (default
                           “C:\\Users\\Stephen\\.docker”)
  -c, –context string     Name of the context to use to connect to the
                           daemon (overrides DOCKER_HOST env var and
                           default context set with “docker context use”)

This reflects the Linux heritage of Docker, where commands generally must be supplied in a case-sensitive manner. Windows traditionally tends to be a little more forgiving, though the result in Example 2-10 did occur in a Docker container on Windows.

2. Connecting to the Database Too Soon

When you’re starting up your container, it’s only natural to get a little impatient. If you try to connect (using SQL Developer) to an Oracle Database instance before the container is fully started, you might see an “Error encountered” dialog box warning you of a refused connection. It can take up to a few minutes for the Oracle Database container to reach healthy status.

The guideline is to not attempt to connect to the database until this point. Once the container is healthy, then it’s safe to connect to the Oracle Database instance using SQL Developer. During the period where the Docker container is starting up, it’s perfectly fine to run the familiar command multiple times:

docker container ls -a

One area of difficulty I’ve encountered with Docker on Windows is the impact of security updates, as discussed next.

3. A Docker Issue Caused by the Dreaded Windows Updates

It’s quite common to see anomalous platform conditions arising after applying Windows updates. Sadly, Docker is not immune to this type of problem. I experienced this myself after accepting a bunch of Windows security updates. After rebooting, I subsequently attempted to run the usual docker command:

docker container ls -a

This was greeted with a rather cryptic response:

Error
response
from
 
daemon
:
 
open
 \\
.
\
pipe
\
docker_engine_linux
:

The
 
system
 
cannot
 
find
 
the
 
file
 
specified
.

A bit of searching online reveals that one solution is to run the following two commands. Remember to substitute the location of the Docker application specific to your own setup:

cd
 
C
:
\
Program
 
Files
\
Docker
\
Docker

DockerCli
.
exe
–
SwitchLinuxEngine

After running this command pair, the original docker container ls -a command was successful. Needless to say, this applies only to the Windows platform. Other platforms might or might not suffer from a similar problem.

The next section covers a couple of alternatives to the exclusive use of the command line with Docker. This becomes increasingly relevant as the number of containers in use grows.