Multi-Tenant SaaS Architecture

Use this skill to join product tenancy semantics to system boundaries and operating decisions. It owns the cross-cutting architecture choice, not the implementation or specialist evidence behind each choice.

Workflow

1. Frame the service promise. Identify customer/account shape, users and operators, subscription commitments, regions, data classes, lifecycle states, support model, scale assumptions, and unknowns. Separate contractual promises from aspirations.
2. Define tenant semantics. Load references/tenant-domain-model-and-boundaries.md. Establish tenant, account, organization, user, membership, subscription, entitlement, resource, and environment meanings, ownership, cardinality, and lifecycle authority.
3. Separate planes. Load references/control-and-application-planes.md. Draw the control-plane policy and lifecycle responsibilities apart from application-plane request and data serving, including asynchronous handoffs and failure behavior.
4. Choose isolation per resource. Load references/tenancy-and-data-partitioning-models.md. Compare pooled, bridge, silo, and hybrid choices by data, compute, cache, queue, search, and operational resource. Record why the choice meets the promise and what can trigger a change.
5. Connect the commercial path. Load references/onboarding-identity-entitlements-metering-billing.md. Trace signup, identity, provisioning, configuration, entitlement evaluation, usage capture, invoice authority, suspension, support, and reactivation without making billing or security policy implicit.
6. Design lifecycle and operations. Load references/tenant-aware-operations-capacity-and-cost.md. Cover deployment, observability, quotas, hot tenants, restore scope, export/deletion, cost attribution, and escalation. Route evidence-heavy decisions to the specialist owners.
7. Record and challenge. Use templates/saas-architecture-brief.md, templates/tenancy-decision-record.md, and templates/tenant-lifecycle-checklist.md. Test cross-tenant safety, lifecycle idempotency, entitlement lag, billing disagreement, restore scope, and noisy-neighbor behavior.

Output Contract

Produce a SaaS architecture brief with: service promise; domain vocabulary and authorities; control/application plane boundary; resource-by-resource tenancy choices; lifecycle state transitions; entitlement, metering, and billing handoffs; data and recovery boundaries; tenant-aware operational scenarios; explicit assumptions and evidence gaps; specialist handoffs; and decisions with owners and review triggers.

Ownership Boundaries

• Threat modeling, authorization, tenant isolation controls, secrets, and security evidence belong to secure-software-engineering.
• Capacity models, load or soak evidence, quotas as capacity controls, unit cost, and SLO-cost tradeoffs belong to capacity-and-cost-engineering.
• Pricing, ARR/MRR, churn, retention, margin, and SaaS financial outcomes belong to financial-modeling.
• Infrastructure, networking, deployment, observability substrate, and secret-management implementation belong to platform-engineering.
• API, event, webhook, schema, compatibility, and deprecation contracts belong to api-design-and-evolution.
• Service code, data access, jobs, integrations, and application-level tests belong to backend-engineering.
• Storage-platform, data-model, data-product, and governance decisions belong to data-architect; lifecycle deletion requirements and verification belong to privacy-engineering.
• Cross-system tenant moves, partition migrations, cutovers, reconciliation, and deprecation execution belong to migration-engineering.
• General system boundary and architecture-quality decisions belong to software-architecture; this skill adds SaaS tenant and commercial lifecycle semantics to that decision.

When Not To Use

Do not use this skill for an isolated security review, API contract, data-platform choice, service implementation, migration plan, privacy artifact, capacity study, financial model, or platform runbook. Start with the narrower owner when the SaaS context is incidental. For a general architecture decision with no tenant lifecycle or subscription boundary, use software-architecture.

Reference Guide

Load when	Reference or template
Defining tenant/account/subscription terms and ownership	references/tenant-domain-model-and-boundaries.md
Splitting lifecycle policy from request serving	references/control-and-application-planes.md
Comparing pooled, bridge, silo, or hybrid isolation	references/tenancy-and-data-partitioning-models.md
Connecting signup, identity, entitlements, usage, and billing	references/onboarding-identity-entitlements-metering-billing.md
Planning tenant-aware operations, restore, fairness, and unit-cost inputs	references/tenant-aware-operations-capacity-and-cost.md
Producing the overall architecture artifact	templates/saas-architecture-brief.md
Capturing a tenancy choice and trigger to revisit it	templates/tenancy-decision-record.md
Checking tenant lifecycle completeness	templates/tenant-lifecycle-checklist.md
Reviewing public provenance and transformation limits	references/source-index.md

Completion

Stop when each material SaaS decision has an accountable owner, alternatives and consequences, a lifecycle and failure path, evidence or a named gap, and an explicit specialist handoff. Escalate unresolved tenant authority, isolation, commercial, privacy, or recovery questions rather than inventing policy.