Effective strategy for leaving an audit trail/change history for DB applications?
What are some strategies that people have had success with for maintaining a change history for data in a fairly complex database. One of the applications that I frequently use and develop for could really benefit from a more comprehensive way of tracking how records have changed over time. For instance, right now records can have a number of timestamp and modified user fields, but we currently don't have a scheme for logging multiple change, for instance if an operation is rolled back. In a perfect world, it would be possible to reconstruct the record as it was after each save, etc.
Some info on the DB:
- Needs to have the capacity to grow by thousands of records per week
- 50-60 Tables
- Main revisioned tables may have several million records each
- Reasonable amount of foreign keys and indexes set
- Using PostgreSQL 8.x
Answer
One strategy you could use is MVCC, Multi-Value Concurrency Control. In this scheme, you never do updates to any of your tables, you just do inserts, maintaining version numbers for each record. This has the advantage of providing an exact snapshot from any point in time, and it also completely sidesteps the update lock problems that plague many databases.
But it makes for a huge database, and selects all require an extra clause to select the current version of a record.