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Non-Functional Requirements

Non-functional requirements (NFRs) specify the quality attributes and constraints that the system must satisfy. These requirements are derived from business goals and operational needs rather than specific features.

1. Performance - Responsive User Experience​

Fast response times improve user experience, increase conversion rates, and reduce cart abandonment. Response times are critical during promotional events when user engagement is highest.

  • NFR-1.1: Product search, browsing, and category navigation shall complete within 500-800ms on average. Occasional delays up to 2-3 seconds are acceptable for power users during high-load periods.
  • NFR-1.2: Adding items to cart and checkout operations shall complete within 1 second on average to minimize user frustration at critical transaction points.
  • NFR-1.3: Product detail page loading (including images, descriptions, reviews, and variants) shall complete within 1 second on average.
  • NFR-1.4: Initial page load for the homepage and category pages shall complete within 2 seconds including all static assets.

2. Availability & Reliability - High System Availability During Business Hours​

The system must be available during peak shopping times, especially during promotional events, to maximize revenue and customer satisfaction. E-commerce platforms lose revenue during outages. Promotional events are high-profit periods, making availability during these times critical. Graceful degradation ensures users can continue shopping even when some features are unavailable.

  • NFR-2.1: The system shall achieve 99%+ uptime during business hours (06:00 - 23:59 daily). Downtime during off-peak hours (02:00 - 06:00) has lower priority.
  • NFR-2.2: The system shall handle promotional events and flash sales without outages or degraded performance, even with significant traffic spikes (estimated 5-10x normal load).
  • NFR-2.3: The system shall automatically recover from temporary failures (e.g., database connection timeouts, service crashes) within 2-5 minutes without manual intervention.
  • NFR-2.4: If a service becomes unavailable, the system shall gracefully degrade by either serving cached data, showing appropriate error messages, or disabling specific features rather than failing completely. Especially critical functions (product browsing, searching, adding to cart, checkout, payment processing) should remain operational when non-critical services (reviews, wishlists, user profiles) experience temporary issues.

3. Data Integrity & Consistency - Reliable Order Processing​

Users and the business must trust that orders, payments, and inventory updates are processed correctly and consistently. Incorrect orders, duplicate charges, or lost inventory information damage reputation and revenue.

  • NFR-3.1: All orders placed by users shall be recorded exactly once in the system. Lost orders are unacceptable. Duplicate orders need to be prevented or detected and resolved automatically.
  • NFR-3.2: Inventory updates (after an order is placed) shall be reflected consistently across the system within 10 minutes at maximum. Overbooking products should be prevented, thus inventory updates shall be updated in the checkout process within 30 seconds to avoid overselling.
  • NFR-3.3: Payment transactions shall be atomic: either an order is fully confirmed with payment, or the transaction is completely rolled back. Partial order states are unacceptable.
  • NFR-3.4: User shopping carts and wishlist contents shall remain consistent and not be lost during system failures or deploys.

4. Security & Data Privacy - Protect User Data and Payment Information​

The system must comply with data protection regulations and protect sensitive customer information. Data breaches result in regulatory fines, customer trust loss, and reputational damage. Compliance is mandatory in EU markets.

  • NFR-4.1: The system shall comply with GDPR and EU data protection regulations, including user consent management, data subject rights (access, deletion, portability), and data breach notification procedures.
  • NFR-4.2: All payment information (credit card numbers, bank details) shall be processed securely via PCI-DSS compliant payment gateways. The system shall never store raw payment card data.
  • NFR-4.3: User authentication shall use strong, industry-standard methods (e.g., bcrypt, Argon2) to hash passwords. Passwords shall never be transmitted or logged in plain text.
  • NFR-4.4: All communication between users and the system, and between internal services, shall use encrypted channels (HTTPS/TLS).
  • NFR-4.5: User data (addresses, order history, reviews) shall be protected from unauthorized access. Access to sensitive data shall be logged and auditable.
  • NFR-4.6: The system shall have clear data retention policies and automatically delete personal data when no longer needed (e.g., after user account deletion or after a retention period).

5. Scalability - Handling User Spikes and Business Growth​

The system must accommodate increased traffic, data volume, and concurrent users without requiring rewrites or extended downtime. The system should be able to handle slow, sustained business growth.

  • NFR-5.1: The system shall support 10x increase in concurrent users and transactions through horizontal scaling (adding more servers) rather than requiring hardware upgrades or architectural changes.
  • NFR-5.2: The system shall automatically scale up resources during traffic spikes (e.g., promotional events) and scale down during low-traffic periods to optimize costs.
  • NFR-5.3: The system architecture shall allow independent scaling of different services. For example, if product browsing is slow, that service can be scaled without scaling payment processing.
  • NFR-5.4: The system shall remain responsive and reliable with future business growth. A slow, sustained increase in users should not cause problems with appropriate handling in operations, and should not cause a need for big refactorings or rewrites.

6. Maintainability & Observability - Understand and Troubleshoot the System​

Operations and development teams must be able to monitor, debug, and improve the system in production. Microservices add operational complexity. Without good observability, debugging is time-consuming and expensive. Poor observability increases mean-time-to-recovery during incidents.

  • NFR-6.1: The system shall provide comprehensive logging for all significant operations (user actions, service calls, errors, transactions). Logs shall be centralized and searchable.
  • NFR-6.2: The system shall expose detailed metrics (response times, error rates, resource usage) that are collected automatically and visualized in dashboards. Teams shall be able to identify bottlenecks and anomalies.
  • NFR-6.3: The system shall enable distributed request tracing across microservices. When a user complains about slow checkout, teams shall be able to see which service was the bottleneck.
  • NFR-6.4: System errors shall include sufficient context for debugging (stack traces, relevant state, request details) without exposing sensitive data to end users.
  • NFR-6.5: Teams shall be able to deploy new versions without downtime or customer impact (e.g., via blue-green deployments or gradual rollouts).
  • NFR-6.6: The system shall automatically detect an increase of slow responses, both in case of a small increase for a long time and a sudden spike.