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simple sql migration tool to save you from going mad
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README.rst

Nomad

Nomad is a simple migration application, which specifically takes into account properties of development with DVCS and is completely agnostic from ORM or whatever you are using to access your database. It uses simple SQL scripts to migrate and can run pre- and post-processing routines written in any language (Python, Ruby or whatever do you use for your application).

Tests status: travis, changelog

Layout

Nomad's migration store is a directory with nomad.ini and directories with migrations inside. Each such directory must contain migration.ini to be recognized as a migration and this directory name is an unique identifier of a migration.

Your directory tree thus will look like this:

migrations/
  nomad.ini
  2011-11-11-first-migration/
    migration.ini
    up.sql
  2011-11-12-second-migration/
    migration.ini
    1-pre.py
    2-up.sql
    3-post.py

Nomad uses table called nomad to track what was applied already. It's just a list of applied migrations and dates when they were applied.

Interface

To start working, create nomad.ini, and initialize your database (I assume it already exists):

$ nomad init

Then you can start creating your first migration:

$ nomad create 0-initial

Put your ALTERs and CREATEs in 0-initial/up.sql and apply a migration:

$ nomad apply -a # or nomad apply 0-initial

Nomad should report which migrations it applied successfully, but you can check status of that with nomad ls -a (or nomad ls to see only unapplied migrations).

I guess it's time to create new migration:

$ nomad create 1-next -d 0-initial

-d 0-initial means you want your 1-next to depend on 0-initial. This means Nomad will never apply 1-next without applying 0-initial first. You usually want to depend on migrations which created tables you're going to alter, or just to make it easier - on the latest available migration.

Usage

Idea is that you put your migrations in .sql files (name does not matter, and if there are few - they are executed in order), or in some executable file. It seems that first case is self-explanatory - you just put SQL commands you need to execute there and they will be executed.

In second case it's your job to do whatever is necessary to migrate your data, starting with establishing connection. To facilitate this, Nomad will pass everything you define in Configuration as environment variables, prefixed with NOMAD_, so at least you will get NOMAD_ENGINE and NOMAD_URL - feel free to add more configuration there.

Configuration

Nomad reads configuration from nomad.ini, here is an example:

[nomad]
engine = sqla
url = pgsql://user:password@host:port/db

Possible configuration options:

  • engine (required) - SQL engine to use, possible options:
    • sqla - use SQLAlchemy as an adapter, supports everything SQLAlchemy supports
    • dbapi - use regular DB API, supports sqlite, mysql and pgsql
  • url (required) - URL to database, takes multiple options, see format below
  • path - path to migrations (default: directory with nomad.ini)

Each migration has its own migration.ini file, which at the moment has single configuration option, nomad.dependencies, defining which migration (or migrations) this one depends.

Note that ini-files are parsed with extended interpolation (use it like ${var} or ${section.var}), few predefined variables are provided:

  • confpath - path to nomad.ini
  • confdir - path to directory, containing nomad.ini
  • dir (migration only) - path to directory of migration

URL format

Nomad can read connection url to database in a few various ways. nomad.url configuration option is a space separated list of descriptions of how Nomad can obtain database connection url.

The easiest one is simply an url (like in config example). The others are:

  • file:<path-to-file> - a path to file containing connection url
  • env:<var-name> - an environment variable (do not prefix with $)
  • py:<python.mod>:<variable.name> - a Python path to a module, containing a variable with connection url
  • cmd:<cmd-to-execute> - command to execute to get connection url
  • json:<path-to-file>:key.0.key - path to file with JSON and then path to a connection url within JSON object
  • yaml:<path-to-file>:key.0.key - path to file with YAML and then path to a connection url within YAML object
  • ini:<path-to-file>:<section.key> - path to INI file (parsed by configparser with extended interpolation) and then path to a connection url within this file

An example:

[nomad]
url =
  ini:${confdir}/../settings.ini:db.url
  json:${confdir}/../settings.json:db.url
  sqlite:///${confdir}/../local.db

Main ideas

  • There are no downgrades - nobody ever tests them, and they are rarely necessary. Just write an upgrade if you need to cancel something.
  • You can write migration in whatever language you want, Nomad only helps you track applied migrations and dependencies.
  • .sql is treated differently and executed against database, configured in nomad.ini.
  • Only .sql and executable files (sorry, Windows! - though I am eager to hear ideas how to support it) are executed. You can put READMEs, pieces of documentation, whatever you want alongside your migrations.
  • Name matters - everything is executed in order. Order is determined by using human sort (so that x-1.sql is earlier than x-10.sql, you can always check sorting with ls --sort=version).
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