Mechanisms for tracking DB schema changes [closed]

By : pix0r
Source: Stackoverflow.com
Question!

What are the best methods for tracking and/or automating DB schema changes? Our team uses Subversion for version control and we've been able to automate some of our tasks this way (pushing builds up to a staging server, deploying tested code to a production server) but we're still doing database updates manually. I would like to find or create a solution that allows us to work efficiently across servers with different environments while continuing to use Subversion as a backend through which code and DB updates are pushed around to various servers.

Many popular software packages include auto-update scripts which detect DB version and apply the necessary changes. Is this the best way to do this even on a larger scale (across multiple projects and sometimes multiple environments and languages)? If so, is there any existing code out there that simplifies the process or is it best just to roll our own solution? Has anyone implemented something similar before and integrated it into Subversion post-commit hooks, or is this a bad idea?

While a solution that supports multiple platforms would be preferable, we definitely need to support the Linux/Apache/MySQL/PHP stack as the majority of our work is on that platform.

By : pix0r


Answers

In the Rails world, there's the concept of migrations, scripts in which changes to the database are made in Ruby rather than a database-specific flavour of SQL. Your Ruby migration code ends up being converted into the DDL specific to your current database; this makes switching database platforms very easy.

For every change you make to the database, you write a new migration. Migrations typically have two methods: an "up" method in which the changes are applied and a "down" method in which the changes are undone. A single command brings the database up to date, and can also be used to bring the database to a specific version of the schema. In Rails, migrations are kept in their own directory in the project directory and get checked into version control just like any other project code.

This Oracle guide to Rails migrations covers migrations quite well.

Developers using other languages have looked at migrations and have implemented their own language-specific versions. I know of Ruckusing, a PHP migrations system that is modelled after Rails' migrations; it might be what you're looking for.



We use a very simple but yet effective solution.

For new installs, we have a metadata.sql file in the repository which holds all the DB schema, then in the build process we use this file to generate the database.

For updates, we add the updates in the software hardcoded. We keep it hardcoded because we don't like solving problems before it really IS a problem, and this kind of thing didn't prove to be a problem so far.

So in our software we have something like this:

RegisterUpgrade(1, 'ALTER TABLE XX ADD XY CHAR(1) NOT NULL;');

This code will check if the database is in version 1 (which is stored in a table created automatically), if it is outdated, then the command is executed.

To update the metadata.sql in the repository, we run this upgrades locally and then extract the full database metadata.

The only thing that happens every so often, is to forget commiting the metadata.sql, but this isn't a major problem because its easy to test on the build process and also the only thing that could happen is to make a new install with an outdated database and upgraded it on the first use.

Also we don't support downgrades, but it is by design, if something breaks on an update, we restored the previous version and fix the update before trying again.



We use something similar to bcwoord to keep our database schemata synchronized across 5 different installations (production, staging and a few development installations), and backed up in version control, and it works pretty well. I'll elaborate a bit:


To synchronize the database structure, we have a single script, update.php, and a number of files numbered 1.sql, 2.sql, 3.sql, etc. The script uses one extra table to store the current version number of the database. The N.sql files are crafted by hand, to go from version (N-1) to version N of the database.

They can be used to add tables, add columns, migrate data from an old to a new column format then drop the column, insert "master" data rows such as user types, etc. Basically, it can do anything, and with proper data migration scripts you'll never lose data.

The update script works like this:

  • Connect to the database.
  • Make a backup of the current database (because stuff will go wrong) [mysqldump].
  • Create bookkeeping table (called _meta) if it doesn't exist.
  • Read current VERSION from _meta table. Assume 0 if not found.
  • For all .sql files numbered higher than VERSION, execute them in order
  • If one of the files produced an error: roll back to the backup
  • Otherwise, update the version in the bookkeeping table to the highest .sql file executed.

Everything goes into source control, and every installation has a script to update to the latest version with a single script execution (calling update.php with the proper database password etc.). We SVN update staging and production environments via a script that automatically calls the database update script, so a code update comes with the necessary database updates.

We can also use the same script to recreate the entire database from scratch; we just drop and recreate the database, then run the script which will completely repopulate the database. We can also use the script to populate an empty database for automated testing.


It took only a few hours to set up this system, it's conceptually simple and everyone gets the version numbering scheme, and it has been invaluable in having the ability to move forward and evolving the database design, without having to communicate or manually execute the modifications on all databases.

Beware when pasting queries from phpMyAdmin though! Those generated queries usually include the database name, which you definitely don't want since it will break your scripts! Something like CREATE TABLE mydb.newtable(...) will fail if the database on the system is not called mydb. We created a pre-comment SVN hook that will disallow .sql files containing the mydb string, which is a sure sign that someone copy/pasted from phpMyAdmin without proper checking.

By : rix0rrr


This video can help you solving your question :)
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