# Phase 3 Implementation Summary: Session Isolation

## Overview
Phase 3 of multi-instance support ensures that user sessions are properly isolated between Jellyfin instances in a multi-instance deployment. Each instance now tracks which sessions belong to it and only manages its own sessions.

## Completed Tasks

### 1. ✅ Database Schema Updates
- **Added `InstanceId` column to `Device` entity** (`src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/Security/Device.cs`)
  - Nullable `Guid?` for backward compatibility
  - Tracks which instance last authenticated the device
  
- **Added `InstanceId` property to `SessionInfo` class** (`MediaBrowser.Controller/Session/SessionInfo.cs`)
  - Non-nullable `Guid` property
  - Set automatically when sessions are created
  - Used for filtering sessions by instance

### 2. ✅ Instance Registry Service
- **Created `IInstanceRegistry` interface** (`Jellyfin.Server.Implementations/Clustering/IInstanceRegistry.cs`)
  - Methods: RegisterInstanceAsync, UpdateHeartbeatAsync, ShutdownInstanceAsync
  - GetActiveInstancesAsync, GetInstanceAsync, CleanupStaleInstancesAsync
  - IsPrimaryInstanceAsync for primary election
  
- **Implemented `InstanceRegistry` service** (`Jellyfin.Server.Implementations/Clustering/InstanceRegistry.cs`)
  - Generates unique InstanceId per instance (GUID)
  - Automatic heartbeat mechanism (every 30 seconds)
  - Registers instance on startup with hostname, ports, version
  - Marks instance as shutdown on disposal
  
### 3. ✅ SessionManager Integration
- **Updated `SessionManager` constructor** (`Emby.Server.Implementations/Session/SessionManager.cs`)
  - Added `IInstanceRegistry` parameter (optional for backward compatibility)
  - Stores reference to instance registry
  
- **Modified session creation** (`CreateSessionInfo` method)
  - Sets `InstanceId` on new `SessionInfo` objects
  - Uses `_instanceRegistry.CurrentInstanceId` if available
  - Falls back to `Guid.Empty` for single-instance deployments
  
- **Updated `Sessions` property**
  - In multi-instance mode: filters sessions by CurrentInstanceId
  - In single-instance mode: returns all sessions (backward compatible)
  - Ensures each instance only sees its own sessions

### 4. ✅ Dependency Injection Registration
- **Registered services in `ApplicationHost`** (`Emby.Server.Implementations/ApplicationHost.cs`)
  - Added `IInstanceRegistry` → `InstanceRegistry` (singleton)
  - Added `IDistributedLockManager` → `DistributedLockManager` (singleton)
  - Registered before SessionManager to ensure availability

### 5. ✅ Startup Integration
- **Modified `RunStartupTasksAsync`** (`Emby.Server.Implementations/ApplicationHost.cs`)
  - Changed from synchronous to async (`Task` instead of `Task.CompletedTask`)
  - Calls `RegisterInstanceAsync` after core initialization
  
- **Created `RegisterInstanceAsync` method**
  - Retrieves network configuration for ports
  - Registers instance in database with hostname, process ID, ports, version
  - Logs successful registration with InstanceId
  - Handles registration failures gracefully

### 6. ✅ Database Migration Updates
- **Updated EF Core migration** (`20260305000000_AddMultiInstanceSupport.cs`)
  - Added `InstanceId` column to `Devices` table
  - Created index on `Devices.InstanceId`
  - Updated Down migration to drop column
  
- **Updated SQL migration script** (`sql/add_multi_instance_support.sql`)
  - Added `ALTER TABLE Devices ADD COLUMN InstanceId UUID`
  - Created `idx_devices_instance` index
  - Added verification query for Devices table

## Architecture Benefits

### Session Isolation
- **Resource Separation**: Each instance manages only its own transcoding processes, network connections, and WebSocket sessions
- **No Interference**: One instance cannot accidentally terminate or modify another instance's active sessions
- **Clean Shutdown**: When an instance shuts down, only its sessions are affected

### Backward Compatibility
- **Optional Registry**: `IInstanceRegistry` parameter is optional, allowing single-instance deployments to continue working
- **Nullable Device.InstanceId**: Existing devices without InstanceId can still function
- **Fallback Logic**: SessionManager gracefully handles missing instance registry

### Scalability
- **Independent Scaling**: Add more instances without session conflicts
- **Load Balancer Support**: Users can connect to any instance, but their session is managed by one instance
- **Sticky Sessions**: Future enhancement - route users to the instance managing their session

## Usage

### Single Instance (Backward Compatible)
No configuration changes needed. The instance registry will register the instance, but session filtering is transparent.

### Multi-Instance Deployment
1. Apply database migration:
   ```bash
   psql -U jellyfin -d jellyfin -f sql/add_multi_instance_support.sql
   ```

2. Start multiple Jellyfin instances:
   - Each gets a unique InstanceId automatically
   - Each registers itself in the database
   - Each only manages its own sessions

3. Monitor instances:
   ```sql
   SELECT * FROM library."Instances" WHERE "Status" = 'Active';
   ```

4. View sessions by instance:
   ```sql
   SELECT s."Id", s."DeviceName", i."Hostname", i."HttpPort"
   FROM library."Devices" s
   LEFT JOIN library."Instances" i ON s."InstanceId" = i."InstanceId"
   WHERE i."Status" = 'Active';
   ```

## Testing Checklist

- [ ] Start single instance - verify it registers
- [ ] Create a session - verify InstanceId is set
- [ ] View sessions API - verify sessions are returned
- [ ] Shutdown instance - verify status updated to 'Shutdown'
- [ ] Start two instances - verify both register with different InstanceIds
- [ ] Create session on Instance A - verify Instance B doesn't see it
- [ ] Create session on Instance B - verify Instance A doesn't see it
- [ ] Stop Instance A - verify Instance B continues working
- [ ] Check heartbeat mechanism - verify LastHeartbeat updates every 30s
- [ ] Test stale instance detection - stop instance without graceful shutdown, verify marked as 'Failed' after 2 minutes

## Next Steps

**Phase 4: Cache Coordination**
- Implement PostgreSQL LISTEN/NOTIFY for cache invalidation
- Create CacheCoordinator service
- Hook into item update events
- Enable multi-instance cache consistency

**Phase 5: Primary Instance Election**
- Implement primary election algorithm
- Coordinate scheduled tasks to run only on primary
- Coordinate database migrations
- Handle primary failover

**Phase 6: File System Monitor Coordination**
- Create FileSystemChanges table
- Update LibraryMonitor to write to database
- Reduce duplicate file scanning across instances

## Files Modified

### New Files Created
- `Jellyfin.Server.Implementations/Clustering/IInstanceRegistry.cs`
- `Jellyfin.Server.Implementations/Clustering/InstanceRegistry.cs`

### Files Modified
- `src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/Security/Device.cs` - Added InstanceId property
- `MediaBrowser.Controller/Session/SessionInfo.cs` - Added InstanceId property
- `Emby.Server.Implementations/Session/SessionManager.cs` - Integrated InstanceRegistry, session filtering
- `Emby.Server.Implementations/ApplicationHost.cs` - Service registration, instance registration on startup
- `src/Jellyfin.Database/Jellyfin.Database.Providers.Postgres/Migrations/20260305000000_AddMultiInstanceSupport.cs` - Added Devices.InstanceId
- `sql/add_multi_instance_support.sql` - Added Devices.InstanceId

## Build Status
✅ Build successful (only IDE warnings, no compilation errors)

## Compatibility
- ✅ Backward compatible with single-instance deployments
- ✅ No breaking changes to existing APIs
- ✅ Nullable columns for incremental adoption
- ✅ Optional DI parameters for gradual integration