System design: three calculation services, each calls a data service, results need to be saved and quickly displayed in a web interface. How to build such a system with guaranteed calculation execution?

To build a system with three calculation services, each accessing a data service, and ensuring calculation completion, the following approach can be used:

1. Message Queue — when starting calculations, each service places a task in the queue (e.g., RabbitMQ, Kafka). This guarantees that tasks are not lost and will be processed.

2. Idempotency and retries — calculation services should be idempotent so that in case of failures, tasks can be retried without errors.

3. Data service — should provide fast and reliable data access, possibly with caching (Redis, Memcached) to speed up reads.

4. Result storage — results are stored in a database with quick read capabilities (e.g., NoSQL database or a relational one with indexes).

5. Web interface — for quick display of results, WebSocket or long polling can be used to receive real-time updates.

6. Monitoring and logging — tracking task statuses and errors to ensure completion.

Architecture example:

[Web Interface] <---> [API Gateway] <---> [Message Queue] <---> [Calculation Services] <---> [Data Service]
                                          |
                                          +--> [Results Database]

This approach ensures reliability, scalability, and guaranteed calculation execution.