wiki:ChangingTheDatabaseStructure

Changing Vespa's Database Structure

As of this writing (January 2013), Vespa's database format has changed seven times. This document explains how to change the database in the future and points to some of the old changes as examples.

We've use a free tool called SQL Designer to edit our database definition. It's helpful but not necessary. We have a document that tells you what you need to know about SQL Designer before you get started.

First Steps

Vespa's database is very forgiving in that it's easy to recreate locally. This allows for lots of experimentation before you share your changes with others.

Vespa's database is defined by create_tables.sql. It creates the database structure that's later populated by Python code. Your first step in modifying the database will be to change that file.

There are two other files in that same directory -- create_indices.sql and create_views.sql -- that build on the output of create_tables.sql. Creating indices speeds up access to frequently-accessed items at the expense of insert speed, and views are sometimes useful but at present we're not using any so create_views.sql is empty.

You probably won't need to alter create_indices.sql nor create_views.sql.

Once you've modified create_tables.sql, you can use recreate_database.py to rebuild the database or just delete your existing database and then run a Vespa app. This is sufficient for local changes (i.e. testing on your machine) and you'll probably repeat this cycle many times.

The Database and Vespa Init

Every Vespa app calls init_app() in util_init.py very early in the app's invocation. This function does a number of things, including upgrading or (re)creating Vespa's database if necessary. You can trigger a database recreation by 1) deleting or 2) renaming the database file. This can be useful for testing new code. When you are done testing, you can (if you want to) re-rename the original database back to its original name and Vespa will use that file going forward. This can be especially useful while testing database "upgrade" code, more on this below.

The file create_database.py handles the work of building and populating a database from scratch. You might want to read this; it's pretty straightforward and shows how the database is constructed. You might also need to modify it if you're adding tables to the database that need to be populated.

Upgrading the database is handled by the db_upgrader module. This is explained in detail below.

Triggering an Update

When you're ready to make your changes part of an official Vespa release, you'll need to tell Vespa to upgrade the database, and you'll need to write the code to do it.

Upgrading a database occurs when init_app() notices that the value of DATABASE_VERSION in constants.py is greater than the value of the field database_version in the table vespa in the database. (This is an administrative table; it doesn't contain any user data. It only contains one row and one column.) When those values are different, the db_upgrader calls a function to upgrade that version.

Upgrade Functions

All of the code to upgrade a database is in the DatabaseUpgrader class. That class knows how to upgrade based on the value of the database version field.

Upgrades are always stepwise. If DATABASE_VERSION in constants.py is 7, and the value of database_version in the database is 5, DatabaseUpgrader will call upgrade_5() to upgrade the database from version 5 to 6, and then it will call upgrade_6() to upgrade it from 6 to 7.

The method upgrade_N() is what you'll write, where N is the number of the current database (i.e. the one you're updating FROM, not to). Your code can be simple like _upgrade_3() which merely added a table, or complex like _upgrade_2() which broke the old simulations table into several parts and replaced it with a different table of the same name.

The code in DatabaseUpgrader contains comments to help you on your way, and you can use the existing code as a template.

Summary

I put the summary last so you won't be tempted to skip the details above. They're important, please read them.

  1. When working locally, there's no need to change version numbers anywhere. Just alter create_tables.sql as you see fit, and delete/rename your database to force Vespa to recreate it.
  2. Repeat step 1 as necessary.

When you're ready to include your changes in a new Vespa version --

  1. Increment DATABASE_VERSION in constants.py
  2. Write an upgrade_N() method for DatabaseUpgrader in db_upgrader.py.

Last modified 4 years ago Last modified on Jul 27, 2015, 9:41:34 AM