This post is a reminder for either myself or other people wanting to use Common Lisp applications in Docker containers.

The end goal

At the end of this post, there will be:

• 2 Docker containers: web and postgres
• The db container will run a postgresql instance
• The web container will run a Hunchentoot web server
• The web container will run a swank server

The 2 last points mean that:

• Going to http://localhost:4242 will hit Hunchentoot
• Running M-x slime-connect 127.0.0.1 5555 will connect to the container’s swank server

The required Lisp code

To have a Hunchentoot instance as well as a swank server, the following Lisp code is needed, somewhere in the application:

(defun main (&rest args)
  (declare (ignore args))
  (hunchentoot:start
    (make-instance 'hunchentoot:easy-acceptor
                   :port 4242
                   :address "localhost"))
  (setf swank::*loopback-interface* "0.0.0.0")
  (swank-loader:init)
  (swank:create-server :port 4005
                       :style swank:*communication-style*
                       :dont-close t))

Note that the swank server’s loopback interface must be 0.0.0.0. This is required so that the host can connect to the container. 127.0.0.1 wouldn’t work and building an ssh tunnel is more complicated than it’s worth. This is not a security issue, since the container will be on a network shared between the host and the container only.

This code can be compiled to a binary using something similar to this:

$ buildapp --load-system foo --entry foo::main

The containers

Since 2 containers will be needed, their specification is needed.

• The db container will just be the official postgres image with a persistent folder. (So that the data in the databases are persisted between each container’s restart.)
• The web container will be based on a custom Docker image.

The db container

Here is the command to create the db container:

$ sudo docker run \
    --volume=/var/lib/postgresql/data \
    --env="POSTGRES_PASSWORD=password" \
    --name=db postgres

This will create a container named db, based on the postgres image. The image will be downloaded from dockerhub if you don’t have it locally. The POSTGRES_PASSWORD environment variable allows you to define the password for the postgres database, usable by the postgres user in the container.

To start and stop the container later on, the following commands can be used:

$ sudo docker start db
$ sudo docker stop db

As simple as that!

The web container

This container needs a special image. For that, a Dockerfile is needed. Before this, a simple Makefile should be used to simplify life.

Here is a sample Makefile (let’s assume the project name is foo):

all:
        buildapp --load-system foo --entry foo::main --output foo.bin

With this done, here is a sample Dockerfile:

# This image is available on dockerhub, automatically downloaded
FROM dparnell/sbcl-1.2.5

# 4242 is for hunchentoot, 4005 is for swank
EXPOSE 4242 4005

# Going to this folder means ASDF automatically picks up the project
WORKDIR /root/common-lisp/foo

# The command to run every time the container is started
CMD make && ./foo.bin

To build this custom image:

$ sudo docker build -t my/foo .

Now that the custom image (named my/foo) is ready, let’s create a container:

$ sudo docker run \
    --volume="$PWD:/root/common-lisp/foo
    --link="db:dbhost"
    --publish="4242:4242"
    --publish="5555:4005"
    --name="web"
    my/foo

Here is what each argument means:

• The --volume argument defines a folder mapping between the host and the container.
• The --link argument adds a link between 2 containers. Here, the db container’s IP will be added to the web container’s /etc/hosts, under the name dbhost. This will let you connect to the database using the following parameters:
  • username: postgres
  • password: password
  • database: postgres
  • host: dbhost
• The --publish argument defines a port forwarding between the host and the container. In this case, host:4242 forwards to container:4242, and host:5555 forwards to container:4005.
• The --name argument gives a name to the container.
• The last argument is the image that the container is based on.

Same as earlier, to start/stop the container:

$ sudo docker start web
$ sudo docker stop web

Conclusion

So, there it is. There now is 2 containers, and they:

• Run a postgresql instance
• Run a hunchentoot instance
• Run a swank-server instance

Which means:

• You can go on http://localhost:4242
• You can M-x slime-connect on 127.0.0.1:5555