data_experts-1

Supporting historical data directly within an operational system will make your application much more complex than it would otherwise be. Generally, I would not recommend doing it unless you have a hard requirement to manipulate historical versions of a record within the system.

If you look closely, most requirements for historical data fall into one of two categories:

  • Audit logging: This is better off done with audit tables. It’s fairly easy to write a tool that generates scripts to create audit log tables and triggers by reading metadata from the system data dictionary. This type of tool can be used to retrofit audit logging onto most systems. You can also use this subsystem for changed data capture if you want to implement a data warehouse (see below).
  • Historical reporting: Reporting on historical state, ‘as-at’ positions or analytical reporting over time. It may be possible to fulfil simple historical reporting requirements by quering audit logging tables of the sort described above. If you have more complex requirements then it may be more economical to implement a data mart for the reporting than to try and integrate history directly into the operational system.Slowly changing dimensions are by far the simplest mechanism for tracking and querying historical state and much of the history tracking can be automated. Generic handlers aren’t that hard to write. Generally, historical reporting does not have to use up-to-the-minute data, so a batched refresh mechanism is normally fine. This keeps your core and reporting system architecture relatively simple.

If your requirements fall into one of these two categories, you are probably better off not storing historical data in your operational system. Separating the historical functionality into another subsystem will probably be less effort overall and produce transactional and audit/reporting databases that work much better for their intended purpose.