SHMIG

A database migration tool written in BASH consisting of just one file - shmig.

Automated Tests

Shell	DB	Result
/bin/bash	sqlite3
/bin/bash	mysql:5.7
/bin/bash	postgres:9.6

See https://github.com/mbucc/shmig_test.

Quick Start

  $ cd shmig
  $ make install
  $ cd $HOME
  $ mkdir migrations
  $ shmig -t sqlite3 -d test.db create mytable
  generated ./migrations/1470490964-mytable.sql
  $ cat ./migrations/1470490964-mytable.sql
  -- Migration: mytable
  -- Created at: 2016-08-06 09:42:44
  -- ====  UP  ====

  PRAGMA foreign_keys = ON;

  BEGIN;

  COMMIT;

  -- ==== DOWN ====

  PRAGMA foreign_keys = ON;

  BEGIN;

  COMMIT;
  $ # In normal usage, you would add SQL to this migration file.
  $ shmig -t sqlite3 -d test.db migrate
  shmig: creating migrations table: shmig_version
  shmig: applying  'mytable'    (1470490964)... done
  $ ls -l test.db
  -rw-r--r--  1 mark  staff  12288 Aug  6 09:41 test.db
  $ shmig -t sqlite3 -d test.db rollback
  shmig: reverting 'mytable'    (1470490964)... done
  $ shmig -h | wc -l
  73
  $

Edit the function sqlite3_up_text() and sqlite3_down_text() in shmig if you don't like the default SQL template.

Why?

Currently there are lots of database migration tools such as DBV, Liquibase, sqitch, Flyway and other framework-specific ones (for Ruby on Rails, Yii, Laravel, ...). But they all are pretty heavy, with lots of dependencies (or even unusable outside of their stack), some own DSLs...

I needed some simple, reliable solution with minimum dependencies and able to run in pretty much any POSIX-compatible environment against different databases (PostgreSQL, MySQL, SQLite3).

And here's the result.

Idea

RDMS'es are bundled along with their console clients. MySQL has mysql, PostgreSQL has psql and SQLite3 has sqlite3. And that's it! This is enough for interacting with database in batch mode w/o any drivers or connectors.

Using client options one can make its output suitable for batch processing with standard UNIX text-processing tools (sed, grep, awk, ...). This is enough for implementing simple migration system that will store current schema version information withing database (see SCHEMA_TABLE variable in shmig.conf.example).

Usage

SHMIG tries to read configuration from the configuration file shmig.conf in the current working directory. A sample configuration file is shmig.conf.example.

You can also provide an optional config override file by creating the file shmig.local.conf. This allows you to provide a default configuration which is version-controlled with your project, then specify a non-version-controlled local config file that you can use to provide instance-specific config. (An alternative is to use envrionment variables, though some people prefer concrete files to nebulous environment variables.) This works even with custom config files specified with the -c option.

You can also configure SHMIG from command line, or by using environmental variables. The command line settings have higher priority than configuration files or environment settings.

Required options are:

1. TYPE or -t - database type
2. DATABASE or -d - database to operate on
3. MIGRATIONS or -m - directory with migrations

All other options (see shmig.conf.example and shmig -h) are not necessary.

To simplify usage, create shmig.conf in your project root directory with your configuration directives. When you shmig <action> ... in that directory, shmig will use the configuration in that file.

For detailed information see shmig.conf.example and shmig -h.

Migrations

Migrations are SQL files whose name starts with "<UNIX TIMESTAMP>-" and end with ".sql". The order that new migrations are applied is determined by the seconds-since-epoch time stamp in the filename, with the oldest migration going first.

Each migration contains two special markers: -- ==== UP ==== that marks start of section that will be executed when migration is applied and -- ==== DOWN ==== that marks start of section that will be executed when migration is reverted.

For example:

-- Migration: create users table
-- Created at: 2013-10-02 07:03:11
-- ====  UP  ====
CREATE TABLE `users`(
  id int not null primary key auto_increment,
  name varchar(32) not null,
  email varchar(255) not null
);

CREATE UNIQUE INDEX `users_email_uq` ON `users`(`email`);
-- ==== DOWN ====
DROP TABLE `users`;

Everything between -- ==== UP ==== till -- ==== DOWN ==== will be executed when migration is applied and everything between -- ==== DOWN ==== till the end of file will be executed when migration is reverted. If migration is missing marker or contents of marker is empty then appropriate action will fail (i.e. if you're trying to revert migration that has no or empty -- ==== DOWN ==== marker you'll get an error and script won't execute any migrations following script with error). Also note those semicolons terminating statements. They're required because you're basically typing that into your database CLI client.

SHMIG can generate skeleton migration for you, see create action.

Migrations with test data

One nice feature of Liquibase is contexts, which are used to implement different behavior based on environment; for example, in a development environment you can insert test data.

shmig can support this with symbolic links. For example, say your production migrations are in prod and test data in test:

.
└── migrations
    ├── prod
    │   └── 1485643154-create_table.sql
    └── test
        └── 1485648520-testdata.sql

To create a test environment context, link the prod SQL in test directory:

$ cd migrations/test/
$ ln -s ../prod/1485643154-create_table.sql

.
└── migrations
    ├── prod
    │   └── 1485643154-create_table.sql
    └── test
        ├── 1485643154-create_table.sql -> ../prod/1485643154-create_table.sql
        └── 1485648520-testdata.sql

When applying migrations to test, point shmig to the test directory either via the command line or using the local config override file.

Since migrations are applied in order of epoch seconds in the file name, this works.

Current state

Stable and maintained. Pull requests welcome.

Security considerations

Password is passed to mysql and psql via environment variable. This can be a security issue if your system allows other users to read environment of process that belongs to another user. In most Linux distributions with modern kernels this is forbidden. You can check this (on systems supporting /proc file system) like this: cat /proc/1/env - if you get permission denied error then you're secure.

Efficiency

Because SHMIG is just a shell script it's not a speed champion. Every time a statement is executed new client process is spawned. I didn't experience much issues with speed, but if you'll have then please file an issue and maybe I'll get to that in detail.

Usage with Docker

Shmig can be used and configured with env vars

docker run -e PASSWORD=root -e HOST=mariadb -v $(pwd)/migrations:/sql --link mariadb:mariadb mkbucc/shmig:latest -t mysql -d db-name up

This will attempt 3 retries of the command before exiting. This can be useful in a docker-compose set up where the SHMIG container depends_on another DB service within the docker-compose configuration.

OS Packaging

A Debian package is available for shmig at https://packages.kaelshipman.me.

NixOS supports shmig on Linux and Darwin at the moment, the package can be installed into the user's profile by running nix-env -iA nixos.shmig since 18.03.

*Contributions for other systems would be greatly welcomed, and can be submitted via PR to this repo.

Todo

1. Speed. Some optimizations are definitely possible to speed things up.
2. A way to spawn just one CLI client. Maybe something with FIFOs and SIGCHLD handler.
3. Better documentation :\