Non-Functional Requirements (NFRs) Framework for Software Systems

Overview

Availability Non-Functional Requirements (NFRs) define when a software system, service, application, platform, integration, or capability must be accessible and usable. Availability expectations should specify who depends on the system, when it must be available, which transactions or capabilities are included, what downtime is allowed, how availability is measured, and what evidence proves the requirement has been satisfied.

Availability should not be expressed only as a general desire for a system to be available. It should be defined through measurable targets, measurement windows, maintenance expectations, monitoring coverage, incident handling, and governance response when targets are not met.

Best Practice: Define uptime and service availability non-functional requirements

Description

Uptime and service availability NFRs define the percentage of time a system or service must be usable during a defined measurement window. These requirements should clarify whether the target applies to the whole system, a specific customer journey, a critical API, a business process, an operational dashboard, or another defined service boundary.

Benefits

Measurable availability NFRs help stakeholders make realistic architecture, cost, resilience, support, and service-level decisions. They also give operations teams and governance stakeholders a clear basis for monitoring performance against agreed expectations.

Example non-functional requirements

• The customer-facing portal shall maintain at least 99.9% monthly availability for authenticated users, excluding approved maintenance windows.

• Validation method: Validate through production availability monitoring, synthetic transactions, approved maintenance records, and monthly service reporting.

• Example validation evidence: Availability dashboard, synthetic monitoring report, incident records, maintenance calendar, and approved monthly service report.

• The order-submission API shall maintain at least 99.95% monthly availability during approved business-critical operating periods.

• Validation method: Validate through API health checks, error-rate monitoring, uptime monitoring, and incident correlation for the defined operating period.

• Example validation evidence: API uptime report, health-check dashboard, error-rate dashboard, incident summary, and service review record.

Related stakeholders

Typical stakeholders include product owners, business owners, service owners, SRE teams, operations teams, support teams, architects, and customer-facing business stakeholders.

Related lifecycle phases

Availability targets are defined during requirements and architecture; validated during performance and resiliency testing, production monitoring, service reviews, incident reviews, and governance reporting.

Best Practice: Define planned and unplanned downtime non-functional requirements

Description

Downtime NFRs define which outages are acceptable, which are unacceptable, how downtime is classified, how planned maintenance is communicated, and how unplanned downtime is measured. These requirements should specify whether maintenance windows are excluded from availability calculations and what stakeholder approvals are required.

Benefits

Defining downtime expectations prevents disagreement about whether service interruptions are acceptable. It also helps delivery and operations teams plan maintenance, patching, upgrades, recovery activities, customer communication, and service-level reporting.

Example non-functional requirements

• Planned maintenance shall occur only within approved maintenance windows and shall be communicated to affected stakeholders at least five business days before the maintenance event unless an emergency change is approved.

• Validation method: Review the maintenance calendar, communication records, change approvals, and post-maintenance validation results.

• Example validation evidence: Maintenance schedule, stakeholder notification, approved change record, emergency change approval if applicable, and post-maintenance validation report.

• Unplanned downtime affecting critical business transactions shall be recorded as an incident and included in monthly availability calculations and service review reporting.

• Validation method: Compare monitoring alerts, incident records, availability calculations, and service-review reports for the measurement window.

• Example validation evidence: Monitoring alert history, incident tickets, availability report, root-cause analysis, and service review minutes.

Related stakeholders

Typical stakeholders include product owners, service owners, change managers, operations teams, incident managers, customer support, business continuity teams, and communications teams.

Related lifecycle phases

Downtime NFRs are defined during service planning and release planning; validated during change management, maintenance execution, incident management, availability reporting, and service-level review.

Best Practice: Define redundancy and failover non-functional requirements

Description

Redundancy and failover NFRs define whether critical components, services, databases, regions, networks, or integrations require duplicated capacity or alternate processing paths. These requirements should specify failover triggers, failover time expectations, manual versus automated failover, failback expectations, and how failover will be tested.

Benefits

Redundancy and failover requirements help teams design systems that can continue operating when individual components fail. They also prevent expensive over-engineering by aligning redundancy design to business criticality and explicit availability and recovery targets.

Example non-functional requirements

• The software system shall provide redundant production application instances across at least two availability zones for critical user-facing capabilities.

• Validation method: Inspect deployment topology and run zone-failure or instance-failure tests in a production-like environment.

• Example validation evidence: Architecture diagram, infrastructure inventory, availability zone mapping, failover test results, and monitoring screenshots.

• Critical integrations shall have documented failover or fallback behavior when the primary integration endpoint is unavailable.

• Validation method: Test endpoint unavailability scenarios and verify fallback behavior, user impact, alerting, and recovery procedures.

• Example validation evidence: Integration failure test report, fallback workflow evidence, alert record, runbook, and incident simulation results.

Related stakeholders

Typical stakeholders include solution architects, platform engineers, SRE teams, operations teams, network teams, database teams, integration teams, product owners, and business continuity stakeholders.

Related lifecycle phases

Redundancy and failover NFRs are defined during architecture; validated during environment build, resiliency testing, disaster recovery exercises, release readiness, and recurring operational drills.

Best Practice: Define maintenance window non-functional requirements

Description

Maintenance window NFRs define when planned service interruptions, platform updates, patching, release deployments, database maintenance, vendor maintenance, and operational changes may occur. These requirements should clarify approved windows, blackout periods, business-critical periods, communication expectations, and validation after maintenance.

Benefits

Maintenance window requirements help balance operational work with business availability expectations. They also reduce the risk that teams perform planned work during critical business periods or without appropriate communication, approvals, rollback plans, and post-change validation.

Example non-functional requirements

• Production maintenance for the software system shall be scheduled outside defined business-critical processing windows unless an approved emergency change is required.

• Validation method: Compare production change records against the approved maintenance calendar and business-critical processing calendar.

• Example validation evidence: Maintenance calendar, business-critical calendar, change record, emergency approval if applicable, and post-change validation results.

• Each planned maintenance event shall include a rollback plan, validation checklist, communication plan, and named owner responsible for declaring service restoration.

• Validation method: Review change records for required artifacts before the maintenance event and verify completion afterward.

• Example validation evidence: Change plan, rollback plan, validation checklist, stakeholder communication, restoration confirmation, and post-implementation review.

Related stakeholders

Typical stakeholders include change managers, release managers, operations teams, service owners, product owners, business stakeholders, and communications teams.

Related lifecycle phases

Maintenance window NFRs are defined during service design and release planning; validated during change approval, maintenance execution, post-change testing, incident review, and service reporting.

Best Practice: Define availability monitoring and reporting non-functional requirements

Description

Availability monitoring and reporting NFRs define how availability is measured, which signals are used, how outages are detected, how false positives are handled, how reports are generated, and who reviews results. Monitoring may include synthetic transactions, health checks, endpoint monitoring, dependency monitoring, customer journey monitoring, and incident correlation.

Benefits

Availability monitoring provides the evidence needed to know whether availability requirements are being met. Reporting turns monitoring data into operational and governance insight that supports incident response, continuous improvement, service-level review, and stakeholder accountability.

Example non-functional requirements

• The software system shall monitor availability using synthetic transactions that represent critical user journeys and critical integration paths.

• Validation method: Review synthetic transaction configuration and compare monitored paths to approved critical journeys and integrations.

• Example validation evidence: Synthetic monitoring configuration, critical journey mapping, dashboard screenshot, alert history, and monitoring review approval.

• Monthly availability reports shall include target availability, actual availability, excluded planned maintenance, unplanned downtime, major incidents, and corrective actions.

• Validation method: Review the monthly service report and reconcile reported downtime against monitoring and incident records.

• Example validation evidence: Monthly availability report, monitoring export, incident report, corrective-action backlog, and service review minutes.

Related stakeholders

Typical stakeholders include SRE teams, operations teams, service owners, product owners, customer support, incident managers, and governance stakeholders.

Related lifecycle phases

Monitoring requirements are defined during design and operational planning; validated during monitoring setup, testing, release readiness, production operation, service reviews, and incident retrospectives.

Best Practice: Define availability validation and evidence non-functional requirements

Description

Availability validation NFRs define how availability expectations will be proven before and after release. Validation should combine design review, monitoring configuration review, failure testing, incident correlation, service reporting, and operational review.

Benefits

Explicit availability validation requirements prevent availability from being asserted without proof. They also ensure that stakeholders can see whether availability targets are being achieved and what actions are required when they are not.

Example non-functional requirements

• Availability NFRs shall identify the service boundary, Service Level Indicator (SLI), Service Level Objective (SLO), measurement window, exclusions, validation method, evidence source, and approving stakeholder.

• Validation method: Review availability requirements for the required fields and confirm approval before production readiness signoff.

• Example validation evidence: Approved availability requirement, SLI/SLO definition, monitoring design, service boundary definition, and production readiness approval.

• Availability evidence shall be retained for each agreed reporting period and shall be available for operational review, governance review, and audit review where applicable.

• Validation method: Inspect availability report retention, dashboard history, monitoring exports, incident history, and evidence repository permissions.

• Example validation evidence: Evidence repository record, dashboard history, monitoring export, incident history, report archive, and access-control review.

Related stakeholders

Typical stakeholders include service owners, SRE teams, operations teams, product owners, auditors, compliance stakeholders, architecture governance, and executive stakeholders.

Related lifecycle phases

Availability validation occurs during requirements review, architecture review, monitoring implementation, testing, production readiness, production operations, service review, and audit/governance review.