Schema Evolution Manager Docker Image

Available on Docker Hub as alandipert/sem:0.9.12

Schema Evolution Manager (sem) is a Ruby-based tool for developing, deploying, and otherwise managing PostgreSQL schema. In a nutshell, sem supports the idea that you should manage your database schema like a library dependency, with its own repository, version number, and deployment process. The sem Project Goals elaborate further.

This project is a Docker image containing a sem installation. It is useful primarily because Ruby setup can be fickle, and because sem releases aren't distributed as gems. It also includes Ubuntu's postgresql-client so you can easily run psql to experiment.

At Adzerk we use sem together with fig.sh to develop, deploy, and manage applications atop Docker.

Usage

The image provides the sem scripts: sem-init, sem-add, sem-apply, etc. However, they are linked to shorter names for docker run usage.

Docker run

Initialize sem in the sample repository:

git init sample
cd sample
docker run -t -i -v $PWD:/app alandipert/sem:0.9.12 init --dir . --name sample_db --user postgres

Add a new SQL script, new.sql:

echo "create table tmp_table (id integer)" > new.sql
docker run -t -i -v $PWD:/app alandipert/sem:0.9.12 add new.sql

fig.sh

sem is much more useful when you have a local development database to run its scripts against. You could achieve this yourself by running PostgreSQL separately via docker run and linking the sem and PostgreSQL containers explicitly, or you could use fig.sh.

fig is a tool that lets you run sem and PostgreSQL together as linked Docker containers very easily.

After installing fig, copy the included fig.yml to your sem-based schema repository.

Then, create a local database to work with:

fig run sem psql -h db_1 -U postgres sample_db

Next, apply schema to the fresh database:

fig run sem apply --host db_1 --name sample_db --user postgres

Differences from native sem

Publishing releases using this image must be done slightly differently than with a native sem installation. You shouldn't run sem-dist without a --tag argument because git in the container won't be able to see the remote git repository if it is private.

Instead, to publish a release, you should create a tag on the host:

git tag -a 0.0.1 -m 'schema release 0.0.1'

And then supply a --tag argument to sem-dist:

fig run sem dist --tag 0.0.1