adr/000-ADR-Template.md

@@ -1,6 +1,6 @@
 # Architecture Decision Record: \<Title\>
 
-Name: \<Name\>
+Initial Author: \<Name\>
 
 Initial Date: \<Date\>
 

adr/009-helm-and-kustomize-handling.md

@@ -1,6 +1,6 @@
 # Architecture Decision Record: Helm and Kustomize Handling
 
-Name: Taha Hawa
+Initial Author: Taha Hawa
 
 Initial Date: 2025-04-15
 

adr/010-monitoring-and-alerting.md

@@ -1,7 +1,7 @@
 # Architecture Decision Record: Monitoring and Alerting
 
-Proposed by: Willem Rolleman
+Initial Author : Willem Rolleman
-Date: April 28 2025
+Date : April 28 2025
 
 ## Status
 

adr/011-multi-tenant-cluster.md

@@ -0,0 +1,160 @@
+# 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
+3. **Phase 3**: Integrate Keycloak for centralized identity management
+4. **Phase 4**: 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.