Relational databases
• Two dimensional
• Rows and columns
• Only know about NOW
• Has no concept of ”history”
What if we can add a
third dimension?
What if we can add the notion of
TIME
Data Versioning
Data Versioning
• ”Time Machine” for data
• Snapshot of any point in time
• Immediately and continously available
• No rollback or restore of database
• Demo:
http://johan.solve.se/dataversioning/
Data Versioning
• Consistent across tables
• Solution for MySQL
• Uses VIEW to access data
• Data access it transparent to the application
• Unique for each database session/visitor
Views
• Views are like search macros
• They are created as a saved SELECT statement
• Performs good if indexed properly
Data Versioning
• Each table is replaced by a ”data” table that maintains every version of each row/record
• Common ”version” table
• Points at current row in data table
• Keeps meta data about changes
• Each table is completed by a VIEW to
Never Change
• Most important:
• Never update or delete a row in the data table
• All changes add a new row in the data table
Time
Row 1Row 2Row 3Row 4Row 5
Table
Time
Row 1Row 2Row 3Row 4Row 5
Table
Table 2
Row 1Row 2Row 3Row 4
Database operations– CRUD
• SELECT from a view - just as usual
• INSERT creates a row in data table and verison table
• UPDATE updates a timestamp in the version table and creates a row in the data table and version table
• DELETE updates a timestamp in the version table
Database operations– CRUD
• SELECT – no change
• INSERT, UPDATE, DELETE handles specially
• Triggers and Stored procedures
• Or in the application’s database abstraction layer
INSERT
INSERT INTO version (guid, changedby_start) VALUES ("gweyufghkj", "JS");
INSERT INTO people_data (id_version, firstname, lastname) VALUES(LAST_INSERT_ID(), "Nisse", "Hult");
UPDATE
UPDATE version SET changedby_end="JS", dt_end=NOW() WHERE guid="gweyufghkj" AND dt_end="3000-01-01 00:00:00";
INSERT INTO version (guid, changedby_start) VALUES ("gweyufghkj", "JS");
DELETE
UPDATE version SET changedby_end="JS", dt_end=NOW() WHERE guid="gweyufghkj" AND dt_end="3000-01-01 00:00:00";
SELECT
SELECT * FROM people;
View
CREATE VIEW people AS
SELECT people_data.*, version.guid
FROM people_data
JOIN version ON
people_data.id_version=version.id
WHERE dt_start <= history()
AND dt_end > history();
Go Back in Time
SET @history="2010-02-22 10:00:00";
SELECT * FROM people;
Only affects this database session
Uses
• Content Management
• Definitions for dynamic forms
• Data entry must match form definition at any point in time
Considerations• Carefully evaluate for each table if it’s
relevant to include it in data versioning
Example:
• Permission settings
• What if the table structure changes?
• Performance
• Consider using emulated
Implementation
• Modified knop_database
• Methods for:
-> addrecord
-> saverecord
-> deleterecord
Read more
• Description:
http://re.solve.se/database-versioning
• Demo:
http://johan.solve.se/dataversioning/
Johan Sölve
