Friday, September 7, 2012

Database migrations using DbDeploy & Phing

Install Phing

The best way to download and install phing is using PEAR. This can be done on Linux or Windows assuming you have the pear script in your PATH with three shell commands.

shell> pear channel-discover
shell> pear config-set preferred_state beta
shell> pear install [--alldeps] phing/phing

Example Application structure

As an example, we’re going to develop a simple application with the following directory structure.
 |-- db/
 |   `-- deltas/
 |-- deploy/
 |   `-- scripts/  
 |-- library/
 `-- public/
The db directory contains sql files for using and manipulating our database and the deploy directory contains our build scripts that set the migrations in motion. The library directory contains our application code and the public folder will contain scripts and files accessible directly from the web, but will not be the focus of this article.

Build scripts

This section shows you how to develop the build scripts that will run the database migrations. The first file we need to create is a simple configuration file and should be fairly self explanatory. The file is written as key=value, lines beginning with a # are comments. Open your editor and save the following text as deploy/
# Property files contain key/value pairs

# This dir must contain the local application

# Credentials for the database migrations

# paths to programs
The next file we are going to create is the deploy/build.xml file. This is the file that tells Phing what we want it to do. I’m not going to go into too much detail describing each part of the build file, there are some comments, but you should consult the Phing Documentation for further details and enhancements.
<?xml version="1.0" ?>
<project name="PurpleMonkey" basedir="." default="build">

<!-- Sets the DSTAMP, TSTAMP and TODAY properties -->

<!-- Load our configuration -->
<property file="./" />

<!-- create our migration task -->
<target name="migrate" description="Database Migrations">  

<!-- load the dbdeploy task -->
<taskdef name="dbdeploy" classname="phing.tasks.ext.dbdeploy.DbDeployTask"/>

<!-- these two filenames will contain the generated SQL to do the deploy and roll it back-->
<property name="build.dbdeploy.deployfile" value="deploy/scripts/deploy-${DSTAMP}${TSTAMP}.sql" />
<property name="build.dbdeploy.undofile" value="deploy/scripts/undo-${DSTAMP}${TSTAMP}.sql" />

<!-- generate the deployment scripts -->
undooutputfile="${build.dir}/${build.dbdeploy.undofile}" />

<!-- execute the SQL - Use mysql command line to 
avoid trouble with large files or many statements and PDO -->
command="${progs.mysql} -h${} -u${db.user}
-p${db.pass} ${} &lt; ${build.dbdeploy.deployfile}" dir="${build.dir}" checkreturn="true" /> </target> </project>
That’s essentially all the magic we need. Now we just need to create our database.

Writing dbdeploy delta scripts

We haven’t actually created our database, so rather than create it the traditional way, we will actually use the migrations to create the initial schema. We’ve not actually decided what our example application does yet, but seeing as most tutorials make blogs, why don’t we give that a bash. We’ll start simple, one table with three columns called post.
Field Type Comment
title VARCHAR(255) The title of our post
time_created DATETIME The time we created our post
content MEDIUMTEXT The content of our post
Dbdeploy works by creating numbered delta files. Each delta files contains simple SQL to both deploy the change and roll it back. The basic layout of a delta file is like so.

-- //

-- Run SQL to do the changes

-- //@UNDO

-- RUN SQL to undo the changes

We are creating our initial schema, so put the following content in db/deltas/1-create_initial_schema.sql

    `title` VARCHAR(255),
    `time_created` DATETIME,
    `content` MEDIUMTEXT


DROP TABLE `post`;

Note: recent versions of mysql require space between -- and // 

Migrating the database

We are one step away from running our first migration. To keep track of the current version of the database, dbdeploy requires a table in the database. This is the only time we will have to interact with the mysql client directly.
shell> mysql -hlocalhost -uroot -ppassword example
mysql> CREATE TABLE changelog (
  change_number BIGINT NOT NULL,
  delta_set VARCHAR(10) NOT NULL,
  complete_dt TIMESTAMP NULL,
  applied_by VARCHAR(100) NOT NULL,
  description VARCHAR(500) NOT NULL
mysql> ALTER TABLE changelog ADD CONSTRAINT Pkchangelog PRIMARY KEY (change_number,delta_set);
We are now ready to run our first migration and create the initial schema for our application.
shell>cd deploy
shell>phing migrate
All being well, we now have a posts table in our database. But what about an author for our blog posts? We’ll have to add another table and a foreign key from the post table to author table. To do this we create another delta, we call this one db/deltas/2-create_author_and_link_to_post.sql

CREATE TABLE `author` (
    `author_id` INT(10) unsigned auto_increment,
    `name` VARCHAR(255),
    PRIMARY KEY (`author_id`)

ALTER TABLE `post` ADD `author_id` INT(10) unsigned NULL;


ALTER TABLE `post` DROP `author_id`;

DROP TABLE `author`;

shell> cd deploy
shell> phing migrate

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