Describe the architecture of a billing application — does it constantly calculate or only at request?

In the architecture of the accrual application, two approaches are usually used: continuous calculation (reactive) and on-demand calculation (lazy).

1. Continuous calculation (reactive) — the system continuously monitors events and changes, updating accruals in real-time or with minimal delay. This is suitable for scenarios where data relevance is important, such as in financial services with instant transactions.

2. On-demand calculation (lazy) — accruals are calculated only when necessary, for example, when a user requests a report or balance. This reduces system load but may lead to delays in displaying current data.

Often, the architecture combines both approaches: main accruals are kept up-to-date, while additional or complex calculations are performed on demand. The choice depends on requirements for performance, accuracy, and update frequency.