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: , 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
  2011-11-13-third-migration/
    migration.ini
    1-pre.py
    2-up.sql
    3-post.sql.j2

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

There are three types of migration files that nomad supports:

Plain SQL files with the extension .sql. Just put SQL commands you need to execute in the migration folder and they will be executed.
Executable files. All file extensions are supported as long as the file is executable. These files must contain everything necessary to migrate your data, including setting up a database connection. nomad will pass all of the Configuration variables as environmental variables, prefixed with their section.
Template files with the extension .j2. These templates will be passed through the Jinja2 templating library. You must install the jinja2 library for this functionality. The entire Configuration is available to the template files as a single dictionary. These could be useful if you are distributing an application where the end user needs to control some aspects of the migrations (ie. additional database users and passwords, additonal database names, etc.).
```
# nomad.ini
[db]
another_user = reader
another_pass = pass
```
```
# migrations/0001-initial/up.sql.j2
CREATE ROLE {{ db.another_user }};
ALTER ROLE {{ db.another_user }} WITH NOSUPERUSER LOGIN PASSWORD '{{ db.another_pass }}' VALID UNTIL 'infinity';
```

Files inside of each migration folder are executed in lexicographical order.

Configuration

Nomad reads database connection information from the [nomad] section of the nomad.ini file.

[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, by default, has a single configuration option, nomad.dependencies, defining which migration (or migrations) this one depends.

You may add your own configuration variables to either the nomad.ini or migration.ini files and they will be available in your jinja2 templates as a single dictionary and your executable files as environmental variables.

Note that ini-files are parsed with extended interpolation (use it like ${var} or ${section.var}).

A few predefined variables are provided to every migration:

confpath - path to nomad.ini
confdir - path to directory, containing nomad.ini
dir - path to directory of migration

Example configuration:

configration	executable	template
[nomad] engine = sqla url = someurl [foo] bar = zeta	NOMAD_ENGINE = sqla NOMAD_URL = someurl FOO_BAR = zeta NOMAD_CONFPATH = path NOMAD_CONFDIR = dir1 NOMAD_DIR = dir2	nomad.engine = sqla nomad.url = someurl foo.bar = zeta nomad.confpath = path nomad.confdir = dir1 nomad.dir = dir2

configration

executable

template

[nomad]
engine = sqla
url = someurl

[foo]
bar = zeta

NOMAD_ENGINE = sqla
NOMAD_URL = someurl

FOO_BAR = zeta

NOMAD_CONFPATH = path
NOMAD_CONFDIR = dir1
NOMAD_DIR = dir2

nomad.engine = sqla
nomad.url = someurl

foo.bar = zeta

nomad.confpath = path
nomad.confdir = dir1
nomad.dir = dir2

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

Notice that in all cases in the end you have to return URL to a database in normal format, i.e. dbtype://user:pass@host:port/dbname?options.

options are supported only by pgsql right now, whatever you put there, nomad will do set ... before every migration. Note that if you do not supply anything there, nomad sets statement_timeout to 1000 ms and lock_timeout to 500 ms by default.

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, .j2, 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).