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Spotting errors earlier in Docker Compose files

An easy first step with CUE is shown in Checking existing Docker Compose files using CUE. There, we see the happy path of using cue vet to confirm that one of our existing compose files is valid. But what does it look like when one of our manually maintained compose files isn’t valid? How does CUE help us catch errors early, before they’ve had the chance to move further downstream and waste time, resources, and focus?

This guide shows what happens when the cue vet command finds a problem with a compose file using a curated module from the CUE Central Registry, and how it highlights the data we need to fix.

Login to the Central Registry

$ cue login # only during beta

The Central Registry requires authentication while it’s in beta testing, so we need to login before using its modules.

A broken compose file

This example is based on one from the docker/awesome-compose repository, – but the deliberate errors introduced here aren’t part of the original file, of course!

# filepath: compose.yml

services:
  postgres:
    container_name: postgres
    environment:
      - POSTGRES_USER=${POSTGRES_USER}
      - POSTGRES_PASSWORD=${POSTGRES_PW}
      - POSTGRES_DB=${POSTGRES_DB}
    image: postgres:latest
    port:
      - "5432:5432"
    restart: always
  pgadmin:
    container_name: pgadmin
    environment:
      - PGADMIN_DEFAULT_EMAIL=${PGADMIN_MAIL}
      - PGADMIN_DEFAULT_PASSWORD=${PGADMIN_PW}
    image: dpage/pgadmin4:latest
    port:
      - "5050:80"
    restart: always

Validate the compose file

We use cue vet to validate compose.yml against the dockercompose package’s #Schema definition:

$ cue vet -c -d '#Schema' cue.dev/x/dockercompose@latest compose.yml
services.pgadmin.port: field not allowed:
    ./compose.yml:18:5
services.postgres.port: field not allowed:
    ./compose.yml:9:5

The cue vet command shows us two errors (...port: field not allowed) and tells us the locations of each problem (lines 18 and 9 of our data file).

Fix the compose file

From the cue vet error message we know that the port field isn’t permitted. We check the Docker Compose documentation (or perhaps we review the most recent change made to the compose file in our version control system) and discover that the field should actually be named ports.

We fix our compose file by updating the highlighted line that cue vet mentioned, so that the file becomes:

# filepath: compose.yml

services:
  postgres:
    container_name: postgres
    environment:
      - POSTGRES_USER=${POSTGRES_USER}
      - POSTGRES_PASSWORD=${POSTGRES_PW}
      - POSTGRES_DB=${POSTGRES_DB}
    image: postgres:latest
    ports:
      - "5432:5432"
    restart: always
  pgadmin:
    container_name: pgadmin
    environment:
      - PGADMIN_DEFAULT_EMAIL=${PGADMIN_MAIL}
      - PGADMIN_DEFAULT_PASSWORD=${PGADMIN_PW}
    image: dpage/pgadmin4:latest
    ports:
      - "5050:80"
    restart: always

Re-validate the compose file

$ cue vet -c -d '#Schema' cue.dev/x/dockercompose@latest compose.yml

We know that the curated module has validated our file this time because cue vet doesn’t display any errors.

Next steps

Validating your existing configuration files with CUE can help make development and deployments safer, but defining those same files in CUE lets you build on its first-class templating, referencing, and policy features. Take the first step with Getting started with Docker Compose + CUE …

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