How to use boot-clj and nREPL with Docker

There’s a great post about using clojure with docker. I followed the post with a twist. I used boot-alpine image[¹] and docker-compose instead of Vagrant.

Here’s the final configuration that worked.

# file docker-compose.yml
version: '2'
services:
  bootclj:
    command: boot dev
    image: clojure:boot-alpine
    ports:
      - 9007:9007
      - 9009:9009
      - 9011:9011
      - 3000:3000
    volumes_from:
      - m2
    working_dir: /opt/src
  m2:
    command: echo "m2 container ready!"
    image: alpine
    volumes:
      - /root/.m2
      - ./:/opt/src

; file build.boot
(deftask development []
  (task-options!
    cljs {:optimizations    :none
          :source-map       true
          :compiler-options {:devcards true}}
    start-repl {:ip "0.0.0.0" :port 9007}
    cljs-repl {:nrepl-opts {:bind "0.0.0.0" :port 9009}}
    reload {:on-jsload 'timer.app/reload
            :port      9011}
    serve {:port 3000})
  identity)

Challenges I faced

Being a beginner in both boot-clj and Docker, I bumped into the following issues.

I couldn’t connect to nREPL running in Docker[²].

I got the following error when I used Cursive remote repl to connect.

Connecting to remote nREPL server...
Error connecting to localhost:9009 - class java.net.ConnectException: Connection refused

By going through document and looking at the source code[³], I found out that I need to run two tasks to have browser repl working. The task cljs-repl starts a repl server and the task start-repl connects the repl server to browser.

The repl server listens to loopback interface[⁴] by default. I used the bind nrepl option to override this setting like the following:

cljs-repl {:nrepl-opts {:bind "0.0.0.0" :port 9009}}

Boot downloaded dependencies everytime I rebuilt docker images.

I used data container to store .m2 repositories. I just use the following command to rebuild and start the image:

docker-compose stop bootclj 
docker-compose rm bootclj
docker-compose create bootclj
docker-compose start bootclj

Footnote:

3: boot_cljs_repl.clj and built_in.clj

4: https://clojure.org/reference/repl_and_main#_launching_a_socket_server