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hnology “Change Bad!”

Coping with change in the data you store

Andrew J. Todd esq. OSDC – November, 2009

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What?

A journey through identifying, understanding and managing change in database schemas

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Who?

Application developers

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Who?

Database designers/developers

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Who?

Database Administrators

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Databases

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Even these ones

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Terminology   Database

  A service that allows an application to store data

  Database Schema   A logical collection of objects within a database

  Database Object   A construct within a database that stores similar items

  Domain Model   A representation of our understanding of a problem area

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Change

“Data matures like wine, applications like fish”

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The Causes of Change

User requirements maturing

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The Causes of Change

New domains

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The Causes of Change

YAGNI

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When Do Things Change?

Data models have a lower rate of change than the applications that work with them

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What Changes?

Tables

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What Changes?

Columns

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What Changes?

Views

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What Changes?

Stored Procedures

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What Changes?

Indexes

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How Do We Track Changes?

Put the DDL in version control

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How Do We Track Changes?

Put a database independent model in version control

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How Do We Track Changes?

But … Database = DDL + Data

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How Do We Track Changes?

Actually … Database = DDL + Data + Implementation

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Strategies For Coping

Do Nothing

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Strategies For Coping

Store complete DDL for each schema version

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Strategies For Coping

Store complete initial DDL and subsequent deltas

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Strategies For Coping

Store complete DDL for every version and subsequent deltas

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Strategies For Coping

Don’t forget reference data

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Strategies For Coping

Or schema (user) creation scripts

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What To Do When Things Go Wrong

You have a backup, right?

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What To Do When Things Go Wrong

The secret to success is testing

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What To Do When Things Go Wrong

Where possible complement ‘upgrade’ with ‘downgrade’

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What To Do When Things Go Wrong

Each migration should maintain data integrity

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What To Do When Things Go Wrong

  Prior to the migration take a copy of the table being updated and store it in a staging area

  Validate that the copy and the original object have the same structure and contents.

  Perform the migration on the original object   Validate that the migration is successful - by running the

migration tests   After a suitable period of time, usually not less than a week,

remove the copy of the table from the staging area

From “Database Refactoring”:

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What To Do When Things Go Wrong

High Availability is a different kettle of fish

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Finis

Version Control is your friend but you need to think about how you use it …