Including Data in a MySQL Docker Image

It’s common practice for Docker-powered database images to use volumes to persist data between container restarts. But if you want to minimize the initial setup time for your local development environment or if you need to run an automated end-to-end test suite in the cloud, you should consider building a baseline set of test data into the image itself.

MySQL and Postgres images both offer a /docker-entrypoint-initdb.d directory in which the images’ entrypoint script will look for and execute SQL files when the container is started for the first time. If your SQL files need to be changed frequently or if you don’t mind the extra startup time needed to execute these files, then I’d recommend using this method to load your initial data.

If you need the database container to start up and be available right away (and you don’t mind a larger image size) then you should consider building your own database image that includes the initial data. The database container will still function the same as a container whose image contained no data (in other words, the database will be writable and it’s state will be preserved between container restarts). But when you start the container it will be ready to accept connections more quickly than a container that needs to execute SQL files.

Building the Image

The following Dockerfile will use an intermediate container to build a MySQL image containing the data imported by the intermediate container.

# Use an intermediate container to restore from SQL dump files
FROM mysql:5.7 AS intermediate
ARG container_entrypoint=/usr/local/bin/docker-entrypoint.sh
ARG mysql_config_file=/etc/mysql/my.cnf
ARG sql_file_source=storage/
ENV MYSQL_ROOT_PASSWORD ${MYSQL_ROOT_PASSWORD:-root}
ENV MYSQL_DATABASE ${MYSQL_DATABASE:-some_database_name}
ENV MYSQL_USER ${MYSQL_USER:-some_username}
ENV MYSQL_PASSWORD ${MYSQL_PASSWORD:-some_password}

COPY ${sql_file_source} /docker-entrypoint-initdb.d/

# Move the data directory to a non-volume'd path
RUN mkdir /temp_data_directory
RUN echo '[mysqld]\ndatadir=/temp_data_directory/' >> ${mysql_config_file}

# Prevent mysqld from starting (`exec "$@"`) after initialization
RUN sed -i '$d' ${container_entrypoint}

RUN ${container_entrypoint} --max-allowed-packet=1GB


# Build the final image using the data directory from the intermediate container
FROM mysql:5.7 AS final
ARG final_data_directory=/var/lib/mysql
COPY --chown=mysql:mysql --from=intermediate /temp_data_directory ${final_data_directory}
VOLUME ${final_data_directory}

Assuming a build context that looks like this:

.
|-- Dockerfile
`-- storage
    `-- import.sql

Then a relatively simple build command can be used to create the image:

$ docker build \
    --rm \
    --tag mysql_with_data \
    .

If you want to do this with a Postgres image, I recommend this article which describes a similar build process for Postgres using an intermediate container.

Why do we need an intermediate container?

The intermediate container performs steps (in this case the SQL import) during the build process that we would normally only be able perform after the build process. The intermediate container achieves the state that we want our container to be in immediately after startup.

Where do I put my SQL files?

In the <build-context>/storage directory. By default, the Dockerfile instructs Docker to copy the contents of ./storage (relative to the build context) into the intermediate container:

ARG sql_file_source=storage/
# ...
COPY ${sql_file_source} /docker-entrypoint-initdb.d/

After they’re copied, the files will be processed by the MySQL image’s entrypoint script. At the time of writing, the documentation for the MySQL image states that the image’s entrypoint script “…will execute files with extensions .sh, .sql and .sql.gz … in alphabetical order.”

If you’d like to use a different directory or if you’d like to specify a single SQL file, you can override sql_file_source by passing a --build-arg flag to the docker build command:

$ docker build \
    --rm \
    --build-arg sql_file_source=import_this.sql \
    --tag mysql_with_data \
    .

Why did you pass --max-allowed-packet=1GB to the intermediate container’s entrypoint script?

To reduce the likelihood that you’ll encounter an error during the intermediate container’s SQL import step. According to this StackExchange answer, setting --max-allowed-packet to the highest setting (1GB) will not cause MySQL to immediately allocate query packets that are 1GB large. The query packets will only be allowed to grow to 1GB only if they need to.

It’s also important to note that the --max-allowed-packet flag only affects the intermediate container. When the intermediate container is finished running the SQL import, we shut it down and discard it.

Why did you use a temp_data_directory in the intermediate container?

The mysql:5.7 image declares a volume in its Dockerfile for the default data directory (/var/lib/mysql). Because Docker handles volume data in a special way, we wouldn’t be able to copy the data directory from the intermediate container if we left it in the default location under the intermediate container’s declared volume. Docker doesn’t provide a warning or error message during the build, the final container merely starts up like a fresh database without any data.

Do you have to use sed to edit the intermediate container’s entrypoint script?

Unfortunately, yes. I believe so. Editing the intermediate container’s entrypoint script prevents the intermediate container from starting the mysqld daemon which will simply sit and waiting for incoming connections. My choices seemed to be either edit the entrypoint script before starting the intermediate container, or attempt to connect to the intermediate container withing the build process in order to shut down mysqld. Please let me know if you have an alternative though because I’m not thrilled about editing the entrypoint file.

Author: Travis Hohl

Published: Oct 21st, 2019