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+# System Architecture
+
+This document describes the architecture of the Plesk Agency Portal SaaS platform.
+
+The platform allows agencies to connect multiple Plesk servers and manage hosting subscriptions from a single dashboard.
+
+---
+
+# High Level Components
+
+The system consists of the following components:
+
+Frontend Dashboard  
+Backend API  
+Supabase Database  
+Plesk Servers  
+Future Worker Services  
+Future Billing Integration
+
+At a high level, users interact with the Frontend Dashboard, which sends authenticated requests to the Backend API (Next.js API routes). The backend reads and writes persistent state in Supabase, and executes remote operations against connected Plesk servers via API/CLI integrations. Future worker services will run asynchronous sync and retry workflows, while future billing integration will enforce plan limits and subscription state.
+
+---
+
+# Frontend
+
+The frontend is a Next.js application using the App Router.
+
+Responsibilities include:
+
+User authentication  
+Dashboard interface  
+Displaying domains and subscriptions  
+Triggering sync operations  
+Sending management actions (suspend / unsuspend)
+
+The frontend communicates with backend API routes.
+
+---
+
+# Backend
+
+The backend uses Next.js API routes.
+
+Responsibilities include:
+
+Handling authenticated requests  
+Connecting to Supabase  
+Executing Plesk API and CLI commands  
+Processing sync operations  
+Writing operational logs
+
+Business logic should be implemented in reusable modules inside the lib directory.
+
+---
+
+# Database
+
+Supabase Postgres is used for persistent storage.
+
+Key tables include:
+
+agencies  
+agency_members  
+plesk_instances  
+plesk_subscriptions  
+plesk_domains  
+actions_log  
+billing_accounts
+
+Relationship overview:
+
+- `agencies` is the tenant root.
+- `agency_members` associates users to agencies and roles.
+- `plesk_instances` stores connected remote Plesk endpoints per agency.
+- `plesk_subscriptions` stores subscription-level records linked to instances.
+- `plesk_domains` stores domain-level records linked to subscriptions/instances.
+- `actions_log` stores operational/audit events.
+- `billing_accounts` stores plan/subscription billing state per agency.
+
+The multi-tenant model is enforced using Supabase Row Level Security policies so users can only read/write rows that belong to their agency.
+
+---
+
+# Plesk Integration
+
+The platform communicates with remote Plesk servers.
+
+Integration methods:
+
+Plesk REST API  
+Plesk CLI commands
+
+Typical operations include:
+
+Listing subscriptions  
+Retrieving domain data  
+Suspending subscriptions  
+Unsuspending subscriptions
+
+CLI parsing must be defensive due to formatting differences between Plesk versions.
+
+---
+
+# Sync System
+
+The system supports synchronization of Plesk servers.
+
+Manual sync is triggered from the dashboard.
+
+During sync:
+
+The backend connects to a Plesk instance  
+Subscription and domain data are retrieved  
+Records are updated in Supabase  
+The dashboard displays updated data
+
+Future development will add automated background sync workers.
+
+---
+
+# Logging System
+
+Operational actions are recorded in the actions_log table.
+
+Examples:
+
+instance_connected  
+sync_started  
+sync_completed  
+subscription_suspended  
+subscription_unsuspended
+
+Logging supports debugging and auditability.
+
+---
+
+# Future Worker Architecture
+
+Future versions of the system will include background workers for:
+
+Automatic instance synchronization  
+Retry queues  
+Monitoring tasks
+
+Workers may run as:
+
+Cron jobs  
+Queue processors  
+Dedicated worker services
+
+---
+
+# Billing Architecture (Future)
+
+Stripe will be integrated for SaaS billing.
+
+Billing responsibilities will include:
+
+Agency subscription plans  
+Usage limits  
+Payment handling
+
+Billing enforcement will eventually control:
+
+Number of connected Plesk instances  
+Number of managed domains
+
+---
+
+# Development Philosophy
+
+The system prioritizes:
+
+Stability  
+Security  
+Multi-tenant isolation  
+Safe integrations with external systems
+
+Developers should extend existing systems rather than rewriting them.