Context

It is generally not possible to 100% ensure consistency between independent systems. Even in the best-designed systems, bugs can exist or be introduced. In some cases it can be important to know when any inconsistencies appear so they can be corrected.

Example

A e-wallet company stores users’ e-money transactions and balances in their own systems. These are linked to underlying transactions and balances with their banking partner in fiat currency. It is essential that these match.

Problem

How do we detect and fix inconsistencies between disparate systems?

Solution

Compare state between the two systems. There will be three outcomes for any inconsistency found:

• Pending – the system has not had long enough to become consistent. Wait and do nothing.
• Resolvable – the reconciliation system can automatically resolve the inconsistency. Apply the change.
• Unresolvable – the inconsistency needs manual intervention. Raise an alert.

It is worth considering how to appropriately limit the amount of data being checked for consistency, e.g. by only checking data that has changed more recently than the previous reconciliation.