Lab 03 - Persistency in containers
On this lab you’ll use volumes to share data between containers. Then we’ll use environment variables to execute containers with different values.
On this lab
Create volume
Before sharing data between container, you need a volume. Let’s create a volume.
docker volume create lab03-volume
Getting a list of volumes allow us to confirm that the volume was created.
docker volume ls --filter name=lab03-volume
You may get a list like this.
DRIVER VOLUME NAME
local lab03-volume
Create Scripts
Now let’s create the scripts and the images to use persistent storage on containers.
First, create a folder to keep all your files.
Then, create a bash script called run-reader.sh with the following content.
#!/bin/sh
echo "Log: $LOG_FILE";
touch $LOG_FILE
tail -f $LOG_FILE
This script will run command tail on a file to show the changes on that file.
The path for that file is available on environment variable LOG_FILE.
Now let’s create another bash script called run-writer.sh with following content.
#!/bin/sh
echo "Log: $LOG_FILE";
touch $LOG_FILE
while $true
do
echo $(date +"%Y-%m-%d %H:%M:%S") >> $LOG_FILE
sleep 1
done
This script will create a new entry on a file every second.
The path for that file is available on environment variable LOG_FILE.
Create Images
Now it’s time to create the Dockerfiles for each image.
Create a new file called Dockerfile.reader for reader image with this content.
FROM busybox
ARG LOG_FILE
ENV LOG_FILE=$LOG_FILE
WORKDIR /script
COPY run-reader.sh .
RUN chmod +x run-reader.sh
CMD ["/script/run-reader.sh" ]
Create a new file called Dockerfile.writer for writer image with this content.
FROM busybox
ARG LOG_FILE
ENV LOG_FILE=$LOG_FILE
WORKDIR /script
COPY run-writer.sh .
RUN chmod +x run-writer.sh
CMD ["/script/run-writer.sh" ]
Let’s look to the Dockerfiles and check that you have a line with the following content.
ARG LOG_FILE
ENV LOG_FILE=$LOG_FILE
This code means that you may define the default value for the environment variable LOG_FILE during image build.
To create the reader image use the next docker build command.
docker build -f Dockerfile.reader --build-arg "LOG_FILE=/share/data.log" -t busybox-logger:reader .
To create the writer image use the next docker build command.
docker build -f Dockerfile.writer --build-arg "LOG_FILE=/share/data.log" -t busybox-logger:writer .
Run docker images command to check if the images were created properly.
Run Containers
On a command line, run docker run command to run reader container and block your terminal to check the changes on the file.
docker run -it --rm -v lab03-volume:/share busybox-logger:reader
On another command line, run docker run command to run writer command.
docker run -d -v lab03-volume:/share --name busybox-writer busybox-logger:writer
Now check on the first terminal that you’re seeing the changes on the file that the writer is doing.
Stop the writer container to check that the reader stops to show changes.
docker stop busybox-writer
On your first terminal you don’t see any updates anymore.
Now let’s stop the reader container doing CTRL+c on the first terminal.
Let’s start a new reader container. tail command by default shows the last 10 lines from the file that you run the command.
Since we’re using a docker volume, starting another command you may see the last 10 lines previously written by writer container.
docker run -it --rm -v lab03-volume:/share busybox-logger:reader
Finally, let’s start again writer container to see new lines on reader container terminal.
docker start busybox-writer
We’re done! You can clean all containers on your machine.
docker rm -f $(docker ps -aq)