# Architecture Decision Record: Multi-Tenancy Strategy for Harmony Managed Clusters

Initial Author: Jean-Gabriel Gill-Couture

Initial Date: 2025-05-26

## Status

Proposed

## Context

Harmony manages production OKD/Kubernetes clusters that serve multiple clients with varying trust levels and operational requirements. We need a multi-tenancy strategy that provides:

1. **Strong isolation** between client workloads while maintaining operational simplicity
2. **Controlled API access** allowing clients self-service capabilities within defined boundaries
3. **Security-first approach** protecting both the cluster infrastructure and tenant data
4. **Harmony-native implementation** using our Score/Interpret pattern for automated tenant provisioning
5. **Scalable management** supporting both small trusted clients and larger enterprise customers

The official Kubernetes multi-tenancy documentation identifies two primary models: namespace-based isolation and virtual control planes per tenant. Given Harmony's focus on operational simplicity, provider-agnostic abstractions (ADR-003), and hexagonal architecture (ADR-002), we must choose an approach that balances security, usability, and maintainability.

Our clients represent a hybrid tenancy model:
- **Customer multi-tenancy**: Each client operates independently with no cross-tenant trust
- **Team multi-tenancy**: Individual clients may have multiple team members requiring coordinated access
- **API access requirement**: Unlike pure SaaS scenarios, clients need controlled Kubernetes API access for self-service operations

The official kubernetes documentation on multi tenancy heavily inspired this ADR : https://kubernetes.io/docs/concepts/security/multi-tenancy/

## Decision

Implement **namespace-based multi-tenancy** with the following architecture:

### 1. Network Security Model
- **Private cluster access**: Kubernetes API and OpenShift console accessible only via WireGuard VPN
- **No public exposure**: Control plane endpoints remain internal to prevent unauthorized access attempts
- **VPN-based authentication**: Initial access control through WireGuard client certificates

### 2. Tenant Isolation Strategy
- **Dedicated namespace per tenant**: Each client receives an isolated namespace with access limited only to the required resources and operations
- **Complete network isolation**: NetworkPolicies prevent cross-namespace communication while allowing full egress to public internet
- **Resource governance**: ResourceQuotas and LimitRanges enforce CPU, memory, and storage consumption limits
- **Storage access control**: Clients can create PersistentVolumeClaims but cannot directly manipulate PersistentVolumes or access other tenants' storage

### 3. Access Control Framework
- **Principle of Least Privilege**: RBAC grants only necessary permissions within tenant namespace scope
- **Namespace-scoped**: Clients can create/modify/delete resources within their namespace
- **Cluster-level restrictions**: No access to cluster-wide resources, other namespaces, or sensitive cluster operations
- **Whitelisted operations**: Controlled self-service capabilities for ingress, secrets, configmaps, and workload management

### 4. Identity Management Evolution
- **Phase 1**: Manual provisioning of VPN access and Kubernetes ServiceAccounts/Users
- **Phase 2**: Migration to Keycloak-based identity management (aligning with ADR-006) for centralized authentication and lifecycle management

### 5. Harmony Integration
- **TenantScore implementation**: Declarative tenant provisioning using Harmony's Score/Interpret pattern
- **Topology abstraction**: Tenant configuration abstracted from underlying Kubernetes implementation details
- **Automated deployment**: Complete tenant setup automated through Harmony's orchestration capabilities

## Rationale

### Network Security Through VPN Access
- **Defense in depth**: VPN requirement adds critical security layer preventing unauthorized cluster access
- **Simplified firewall rules**: No need for complex public endpoint protections or rate limiting
- **Audit capability**: VPN access provides clear audit trail of cluster connections
- **Aligns with enterprise practices**: Most enterprise customers already use VPN infrastructure

### Namespace Isolation vs Virtual Control Planes
Following Kubernetes official guidance, namespace isolation provides:
- **Lower resource overhead**: Virtual control planes require dedicated etcd, API server, and controller manager per tenant
- **Operational simplicity**: Single control plane to maintain, upgrade, and monitor
- **Cross-tenant service integration**: Enables future controlled cross-tenant communication if required
- **Proven stability**: Namespace-based isolation is well-tested and widely deployed
- **Cost efficiency**: Significantly lower infrastructure costs compared to dedicated control planes

### Hybrid Tenancy Model Suitability
Our approach addresses both customer and team multi-tenancy requirements:
- **Customer isolation**: Strong network and RBAC boundaries prevent cross-tenant interference
- **Team collaboration**: Multiple team members can share namespace access through group-based RBAC
- **Self-service balance**: Controlled API access enables client autonomy without compromising security

### Harmony Architecture Alignment
- **Provider agnostic**: TenantScore abstracts multi-tenancy concepts, enabling future support for other Kubernetes distributions
- **Hexagonal architecture**: Tenant management becomes an infrastructure capability accessed through well-defined ports
- **Declarative automation**: Tenant lifecycle fully managed through Harmony's Score execution model

## Consequences

### Positive Consequences
- **Strong security posture**: VPN + namespace isolation provides robust tenant separation
- **Operational efficiency**: Single cluster management with automated tenant provisioning
- **Client autonomy**: Self-service capabilities reduce operational support burden
- **Scalable architecture**: Can support hundreds of tenants per cluster without architectural changes
- **Future flexibility**: Foundation supports evolution to more sophisticated multi-tenancy models
- **Cost optimization**: Shared infrastructure maximizes resource utilization

### Negative Consequences
- **VPN operational overhead**: Requires VPN infrastructure management
- **Manual provisioning complexity**: Phase 1 manual user management creates administrative burden
- **Network policy dependency**: Requires CNI with NetworkPolicy support (OVN-Kubernetes provides this and is the OKD/Openshift default)
- **Cluster-wide resource limitations**: Some advanced Kubernetes features require cluster-wide access
- **Single point of failure**: Cluster outage affects all tenants simultaneously

### Migration Challenges
- **Legacy client integration**: Existing clients may need VPN client setup and credential migration
- **Monitoring complexity**: Per-tenant observability requires careful metric and log segmentation
- **Backup considerations**: Tenant data backup must respect isolation boundaries

## Alternatives Considered

### Alternative 1: Virtual Control Plane Per Tenant
**Pros**: Complete control plane isolation, full Kubernetes API access per tenant
**Cons**: 3-5x higher resource usage, complex cross-tenant networking, operational complexity scales linearly with tenants

**Rejected**: Resource overhead incompatible with cost-effective multi-tenancy goals

### Alternative 2: Dedicated Clusters Per Tenant
**Pros**: Maximum isolation, independent upgrade cycles, simplified security model
**Cons**: Exponential operational complexity, prohibitive costs, resource waste

**Rejected**: Operational overhead makes this approach unsustainable for multiple clients

### Alternative 3: Public API with Advanced Authentication
**Pros**: No VPN requirement, potentially simpler client access
**Cons**: Larger attack surface, complex rate limiting and DDoS protection, increased security monitoring requirements

**Rejected**: Risk/benefit analysis favors VPN-based access control

### Alternative 4: Service Mesh Based Isolation
**Pros**: Fine-grained traffic control, encryption, advanced observability
**Cons**: Significant operational complexity, performance overhead, steep learning curve

**Rejected**: Complexity overhead outweighs benefits for current requirements; remains option for future enhancement

## Additional Notes

### Implementation Roadmap
1. **Phase 1**: Implement VPN access and manual tenant provisioning
2. **Phase 2**: Deploy TenantScore automation for namespace, RBAC, and NetworkPolicy management
4. **Phase 3**: Work on privilege escalation from pods, audit for weaknesses, enforce security policies on pod runtimes
3. **Phase 4**: Integrate Keycloak for centralized identity management
4. **Phase 5**: Add advanced monitoring and per-tenant observability

### TenantScore Structure Preview
```rust
pub struct TenantScore {
    pub tenant_config: TenantConfig,
    pub resource_quotas: ResourceQuotaConfig,
    pub network_isolation: NetworkIsolationPolicy,
    pub storage_access: StorageAccessConfig,
    pub rbac_config: RBACConfig,
}
```

### Future Enhancements
- **Cross-tenant service mesh**: For approved inter-tenant communication
- **Advanced monitoring**: Per-tenant Prometheus/Grafana instances
- **Backup automation**: Tenant-scoped backup policies
- **Cost allocation**: Detailed per-tenant resource usage tracking

This ADR establishes the foundation for secure, scalable multi-tenancy in Harmony-managed clusters while maintaining operational simplicity and cost effectiveness. A follow-up ADR will detail the Tenant abstraction and user management mechanisms within the Harmony framework.
