Tenancy Models

Multi-Tenancy Models

Architecture patterns for deploying multi-tenant SaaS applications, from fully shared to fully isolated.

When to Use This Skill

• Designing a new SaaS application
• Choosing between shared and dedicated infrastructure
• Evaluating isolation vs cost trade-offs
• Planning tenant tiering strategy
• Migrating from single-tenant to multi-tenant

Core Concept

Multi-tenancy spectrum:

Fully Shared (Pool)              Hybrid (Bridge)              Fully Isolated (Silo)
     ◄──────────────────────────────────────────────────────────────►
   Low Cost                                                      High Cost
   Low Isolation                                                 High Isolation
   High Density                                                  Low Density
   Noisy Neighbor Risk                                           No Interference

The Three Models

Pool Model (Shared Everything)

┌─────────────────────────────────────────────────────┐
│                   Application Layer                  │
│    All tenants share compute, storage, database      │
├─────────────────────────────────────────────────────┤
│                     Database                         │
│  ┌─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┐  │
│  │ T1  │ T2  │ T3  │ T4  │ T5  │ T6  │ T7  │ ... │  │
│  └─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┘  │
│           (Row-Level Security separates data)        │
└─────────────────────────────────────────────────────┘

Characteristics:
- Single database, shared schema
- Row-level security for isolation
- All tenants share compute resources
- Lowest cost per tenant
- Highest tenant density (millions possible)

Best For:

• B2C SaaS with many small tenants
• Startup MVP (minimize infrastructure)
• Cost-sensitive markets
• Low compliance requirements

Challenges:

• Noisy neighbor problems
• Single point of failure
• Complex schema migrations
• Limited customization per tenant

Silo Model (Fully Isolated)

┌──────────────────┐  ┌──────────────────┐  ┌──────────────────┐
│    Tenant A      │  │    Tenant B      │  │    Tenant C      │
│  ┌────────────┐  │  │  ┌────────────┐  │  │  ┌────────────┐  │
│  │    App     │  │  │  │    App     │  │  │  │    App     │  │
│  ├────────────┤  │  │  ├────────────┤  │  │  ├────────────┤  │
│  │  Database  │  │  │  │  Database  │  │  │  │  Database  │  │
│  └────────────┘  │  │  └────────────┘  │  │  └────────────┘  │
└──────────────────┘  └──────────────────┘  └──────────────────┘

Characteristics:
- Dedicated resources per tenant
- Complete data isolation
- Independent scaling
- Independent maintenance windows
- Highest cost per tenant

Best For:

• Enterprise B2B with strict compliance
• Regulated industries (healthcare, finance)
• Tenants requiring custom SLAs
• Data sovereignty requirements
• High-value customers justifying cost

Challenges:

• High infrastructure cost
• Operational complexity at scale
• Tenant onboarding latency
• Resource underutilization

Bridge Model (Hybrid)

┌─────────────────────────────────────────────────────────────────┐
│                        Shared Services                          │
│     (Authentication, Billing, Monitoring, Configuration)        │
└─────────────────────────────────────────────────────────────────┘
                    │                       │
        ┌───────────┴───────────┐   ┌───────┴───────┐
        │   Pooled Tier         │   │  Premium Tier │
        │  ┌────┬────┬────┐     │   │  ┌─────────┐  │
        │  │ T1 │ T2 │ T3 │     │   │  │ Tenant  │  │
        │  └────┴────┴────┘     │   │  │  (Silo) │  │
        │  (Shared Database)    │   │  └─────────┘  │
        └───────────────────────┘   └───────────────┘

Characteristics:
- Shared control plane
- Mix of pool and silo at data plane
- Tenant tiering (Standard = Pool, Premium = Silo)
- Gradual migration path from pool to silo

Best For:

• Growth-stage SaaS with diverse customer base
• Companies offering standard and enterprise tiers
• Migration from pool to silo
• Balancing cost with enterprise requirements

Decision Framework

Quick Decision Tree

                    Start
                      │
          ┌───────────┴───────────┐
          │  Strict compliance    │
          │  requirements?        │
          └───────────┬───────────┘
                Yes ──┤──── No
                  │         │
             ┌────▼────┐    │
             │  SILO   │    │
             └─────────┘    │
                      ┌─────▼─────┐
                      │ >10,000   │
                      │ tenants?  │
                      └─────┬─────┘
                    Yes ────┤──── No
                      │           │
                 ┌────▼────┐ ┌────▼────┐
                 │  POOL   │ │ BRIDGE  │
                 └─────────┘ └─────────┘

Weighted Decision Matrix

Model Comparison

Implementation Patterns

Pool Model Implementation

Key Components:
1. Tenant Resolver Middleware
   - Extract tenant from subdomain, header, or JWT claim
   - Set tenant context for request

2. Row-Level Security
   - Database-level isolation (SQL Server RLS, PostgreSQL RLS)
   - All queries filtered by tenant_id

3. Shared Connection Pool
   - Single connection string
   - Tenant context passed per-request

4. Global Query Filters (EF Core)
   - Automatic tenant filtering on all entities

See database-isolation skill for implementation details.

Silo Model Implementation

Key Components:
1. Tenant Routing
   - DNS-based (tenant.app.com)
   - API Gateway routing by tenant identifier

2. Dedicated Infrastructure
   - Database per tenant
   - Optionally dedicated compute

3. Tenant Catalog
   - Central registry of tenant configurations
   - Connection strings, feature flags, customizations

4. Infrastructure as Code
   - Automated provisioning (Terraform, Bicep)
   - Template-based tenant environments

See tenant-provisioning skill for implementation details.

Bridge Model Implementation

Key Components:
1. Tiered Tenant Classification
   - Standard tier → Pool
   - Premium tier → Silo

2. Unified Control Plane
   - Single admin portal
   - Central configuration management
   - Aggregated monitoring

3. Data Plane Routing
   - Route to pool or silo based on tier
   - Seamless upgrade path (pool → silo)

4. Migration Support
   - Data export from pool
   - Import to dedicated resources
   - Zero-downtime migration

Cloud Provider Considerations

Azure

AWS

Cosmos DB Specific

Evolution Path

Startup to Enterprise

Phase 1: MVP (Pool)
└── Single database, RLS
└── Minimize costs
└── Validate product-market fit

Phase 2: Growth (Bridge)
└── Introduce premium tier (Silo)
└── Keep standard tier (Pool)
└── First enterprise customers

Phase 3: Scale (Optimized Bridge)
└── Cell-based architecture
└── Regional deployment
└── Per-tenant SLA options

Migration Triggers

Move from Pool → Bridge when:

• First enterprise customer requires isolation
• Compliance requirements emerge
• Noisy neighbor problems occur

Move from Bridge → More Silo when:

• Premium revenue justifies infrastructure
• Enterprise segment grows
• Per-tenant customization demand increases

Anti-Patterns to Avoid

Best Practices

1. Tenant Context is Sacred
   - Validate on every request
   - Never assume tenant from client input
   - Log tenant ID with every operation

2. Defense in Depth
   - Application-level filtering
   - Database-level RLS
   - Network-level isolation (for silo)

3. Observability per Tenant
   - Metrics tagged by tenant
   - Per-tenant dashboards
   - Alerting thresholds per tier

4. Cost Attribution
   - Track resource usage per tenant
   - Enable chargebacks for enterprise
   - Monitor for cost anomalies

5. Test Isolation
   - Automated cross-tenant access tests
   - Penetration testing for isolation
   - Chaos testing for noisy neighbor

Related Skills

• database-isolation - Implementation patterns for data isolation
• tenant-context-propagation - Context resolution in microservices
• noisy-neighbor-prevention - Resource isolation techniques
• tenant-provisioning - Automated tenant onboarding

References

• Load references/pool-model.md for detailed Pool implementation
• Load references/silo-model.md for detailed Silo implementation
• Load references/bridge-model.md for detailed Bridge implementation
• AWS Well-Architected SaaS Lens: https://docs.aws.amazon.com/wellarchitected/latest/saas-lens/
• Azure Multi-tenant patterns: https://learn.microsoft.com/en-us/azure/architecture/guide/multitenant/

Last Updated: 2025-12-26