Mission-Critical Systems

When to Use

• Classify criticality tiers (mission-critical, business-critical, important) with explicit impact criteria
• Set availability, integrity, and continuity objectives (uptime, RTO/RPO, MTPD, data loss tolerance)
• Map dependencies, failure domains, and blast radius for severe-impact services
• Select architecture patterns—active-active, geo-redundancy, quorum, deterministic execution, fail-safe defaults
• Define change and release governance—hold points, CAB tiers, emergency change, rollback criteria
• Tailor monitoring, alerting, and escalation to criticality (SLOs alone are insufficient for Tier 0)
• Align engineering with contractual SLAs, sector regulation, and continuity obligations (generic framing)
• Produce criticality registers, architecture decision records, and governance briefs for leadership

When NOT to Use

• Own SLI/SLO definitions, error-budget policy, burn-rate alerting, and toil reduction → site-reliability-engineer
• Establish failure-prevention culture, HRO principles, stop-the-line, and defect-escape metrics → zero-tolerance-for-failure
• Run live incident war room, containment, and forensic preservation → incident-responder
• Design backup/immutability, ransomware recovery, and resilience test evidence → cyber-resilience-engineer
• Own enterprise BCM program, BIA facilitation, and regulatory BCM policy → bcm-disaster-recovery-specialist
• Enterprise security reference architecture and control frameworks without tiering lens → enterprise-security-architect
• Greenfield integration patterns and ADRs without criticality classification → senior-system-architecture
• Cleared program ATO, inspection, and personnel security without operational tiering → classified-cyber-security-senior-manager
• CI compile/lint gates without release governance for critical services → build-validator

Related skills

Core Workflows

1. Scope and criticality tiering

Define what “mission-critical” means in context; classify services and data; document drivers.

See references/missioncriticalscopeandtiering.md.

2. Objectives: availability, integrity, continuity

Set measurable targets and exclusions; align tiers to RTO/RPO/MTPD and integrity classes.

See references/objectivesavailabilityintegrity_continuity.md.

3. Architecture patterns for critical systems

Choose redundancy, partitioning, determinism, and degradation modes for Tier 0/1 workloads.

See references/architecturepatternsforcriticalsystems.md.

4. Dependencies and blast radius

Map upstream/downstream, shared fate, control planes, and containment boundaries.

See references/dependenciesandblast_radius.md.

5. Change control and release governance

Tier releases, evidence gates, emergency change, and rollback for high-criticality paths.

See references/changecontrolandreleasegovernance.md.

6. Operations, monitoring, and escalation

Observability depth, synthetic probes, runbooks, and escalation matched to impact.

See references/operationsmonitoringand_escalation.md.

Outputs

• Criticality register — tier, owner, impact narrative, objectives, last review
• Objectives sheet — availability, integrity, RTO/RPO/MTPD per tier with measurement method
• Dependency / blast-radius map — failure domains, shared components, isolation notes
• Architecture decision pack — pattern choice, trade-offs, degradation and fail-safe behavior
• Release governance matrix — change type × tier → gates, approvers, evidence, rollback
• Operations brief — alert routes, escalation tree, runbook index, drill calendar hooks

Principles

• Tier before tooling — criticality drives architecture and governance; do not retrofit labels after build
• Integrity equals availability — corruption or unauthorized change can be worse than downtime
• Contain blast radius — shared control planes and “single throat to choke” are Tier 0 risks
• Govern change proportionally — higher tier → more evidence, independence, and rollback discipline
• Measure what customers and regulators care about — pair SRE SLIs with tier-specific continuity proofs
• Pair with peers — reliability math (SRE), prevention culture (zero-tolerance), recovery (resilience), incidents (IR)