Credit system: history based or balance based?

Question

I am going to write a simple credit system that user can "add", "deduct" credits in the system. Currently I am thinking of two approaches.

1. Simple one: Store the user' credit as balance field in the database, and all actions ("add", "deduct") are logged but not used to compute the latest balance.

2. History based: Don't store the balance in database. The balance is computed by looking at the history of transactions, e.g. ("add", "deduct")

Both case would works I think, but I am looking to see if any caveat when designing such a system, particularly I am favoring the History based system.

Or, are there any reference implementation or open source module I am use?

Update: Or are there any Ruby/Rail based module like AuthLogic so I can plug and play into my existing code without reinventing the wheel (e.g. transaction, rollback, security etc)?

Answer 1

Absolutely use both.

• The balance-based way gives you fast access to the current amount.

• The history-based way gives you auditing. The history table should store the transaction (as you describe), a timestamp, the balance before the transaction happened, and ideally a way to track the funds' source/destination.

See the Ruby Toolbox for bookkeeping and Plutus double-entry bookkeeping gem.

In addition, if your credit system may affect users, then I recommend also using logging, and ideally read about secure log verification and provable timestamp chaining.

• For logging details see: techniques for ensuring verifiability of event log files.

• For open source code that does credit, you may want to look into: http://www.gnucash.org/