feat: migrate system metadata from 'index' to '_system' directory with backward compatibility
BREAKING CHANGE: System metadata location changed from 'index/' to '_system/' directory
- Rename INDEX_DIR to SYSTEM_DIR following database conventions
- Implement dual-read/write strategy for zero-downtime migration
- Add automatic migration from old to new location on first access
- Support mixed service versions sharing S3/cloud storage
- Add 30-day grace period for gradual rollout (configurable)
- Store distributed config alongside statistics in _system folder
- Add comprehensive migration guide and documentation
Migration features:
- Read from both locations (new first, fallback to old)
- Write to both during migration period
- Automatic data migration when found only in old location
- Services can update independently without coordination
- Full backward compatibility for production deployments
The change improves clarity ('_system' better represents system metadata than 'index')
and follows standard database conventions (MongoDB's _system, PostgreSQL's pg_*).
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docs/STORAGE_MIGRATION_GUIDE.md
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docs/STORAGE_MIGRATION_GUIDE.md
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# Storage Migration Guide: `index` → `_system`
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## Overview
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Brainy is migrating its system metadata storage from the `index/` directory to `_system/` directory to better reflect its purpose and follow database conventions. This migration is designed to be **zero-downtime** and **backward compatible**.
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## Migration Strategy
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### Dual Read/Write Approach
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The migration uses a **dual-read, migrate-on-write** strategy to ensure compatibility between services running different versions:
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1. **Read Priority**: Try new location first (`_system/`), fallback to old (`index/`)
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2. **Dual Write**: During migration, write to both locations
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3. **Automatic Migration**: When data is found only in old location, it's automatically copied to new
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4. **Gradual Rollout**: Services can be updated independently without coordination
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## Timeline
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### Phase 1: Dual Mode (Current)
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- **Duration**: 30 days (configurable via `BRAINY_MIGRATION_GRACE_DAYS`)
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- Services write to both `_system/` and `index/`
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- Services read from both locations (new first, then old)
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- Automatic migration on first read from old location
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### Phase 2: New Primary (After Grace Period)
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- Services primarily use `_system/`
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- Legacy `index/` kept for emergency rollback
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- Monitoring for any services still using old location
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### Phase 3: Cleanup (After Verification)
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- Remove dual-write code
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- Archive or delete `index/` directory
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- Update documentation
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## Deployment Guide
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### For S3/Cloud Storage (Most Critical)
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**Step 1: Rolling Update**
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```bash
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# Update services one by one - no coordination needed
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kubectl rollout restart deployment/brainy-service-1
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# Wait for health checks
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kubectl rollout restart deployment/brainy-service-2
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# Continue for all services
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```
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**Step 2: Monitor Migration**
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```bash
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# Check for migration events in logs
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kubectl logs -l app=brainy --since=1h | grep "Storage Migration"
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# Verify both directories have data
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aws s3 ls s3://your-bucket/_system/
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aws s3 ls s3://your-bucket/index/
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```
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**Step 3: Verify Consistency**
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```javascript
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// Check that statistics match between locations
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const oldStats = await storage.getMetadata('statistics'); // from index/
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const newStats = await storage.getStatistics(); // from _system/
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console.assert(oldStats.nounCount === newStats.nounCount);
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```
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### For Local/FileSystem Storage
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The migration happens automatically on first run:
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```bash
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# Before update
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brainy-data/
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├── index/ # Old location
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│ └── statistics.json
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└── ...
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# After update (automatic)
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brainy-data/
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├── index/ # Kept for compatibility
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│ └── statistics.json
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├── _system/ # New location
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│ └── statistics.json
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└── ...
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```
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## Configuration Options
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### Environment Variables
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```bash
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# Control migration grace period (default: 30 days)
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export BRAINY_MIGRATION_GRACE_DAYS=30
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# Force single-write mode (after migration confirmed)
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export BRAINY_DISABLE_DUAL_WRITE=true
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# Enable verbose migration logging
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export BRAINY_MIGRATION_DEBUG=true
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```
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### Per-Instance Configuration
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```javascript
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const storage = new FileSystemStorage({
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rootDirectory: './data',
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useDualWrite: true // Set to false after migration
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});
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```
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## Monitoring
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### Key Metrics to Watch
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1. **Storage Operations**
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- Successful reads from `_system/`
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- Fallback reads from `index/`
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- Dual write operations
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- Migration events
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2. **Performance Impact**
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- Minimal: ~5-10ms additional latency during dual-write
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- No impact on read performance (cache used)
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3. **Log Messages**
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```
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[Brainy Storage Migration] Migrating statistics from legacy location
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[Brainy Storage Migration] Failed to write to legacy location (non-critical)
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[Brainy Storage Migration] Migration completed successfully
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```
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## Rollback Plan
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If issues occur, rollback is simple:
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1. **Immediate Rollback**: Revert service to previous version
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- Old versions continue using `index/`
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- New versions read from both locations
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2. **Data Recovery**: If data corruption occurs
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```bash
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# Copy data back from index to _system
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aws s3 sync s3://bucket/index/ s3://bucket/_system/
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```
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## FAQ
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### Q: What happens if services are on different versions?
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**A:** The dual-read/write strategy ensures compatibility. Newer services write to both locations and read from both, while older services continue using only the old location.
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### Q: Is there data duplication?
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**A:** Yes, temporarily. During the migration period, data exists in both locations. This ensures zero-downtime migration and provides a safety net.
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### Q: What about distributed configurations?
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**A:** The distributed configuration (previously in metadata) now lives with statistics in `_system/`, making it easier to find and manage.
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### Q: Can I disable dual-write immediately?
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**A:** Not recommended. Keep dual-write enabled for at least 7 days to ensure all services are updated and caches are refreshed.
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### Q: What if a service can't write to the new location?
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**A:** The service will log a warning but continue operating using the fallback location. This ensures service availability over migration perfection.
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## Testing the Migration
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### Unit Test Example
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```javascript
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describe('Storage Migration', () => {
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it('should read from both locations', async () => {
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// Write to old location
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await fs.writeFile('data/index/statistics.json', oldData);
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// Initialize storage (triggers migration)
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const storage = new FileSystemStorage({ rootDir: 'data' });
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// Should read and migrate
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const stats = await storage.getStatistics();
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expect(stats).toBeDefined();
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// Should now exist in both locations
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expect(fs.existsSync('data/_system/statistics.json')).toBe(true);
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expect(fs.existsSync('data/index/statistics.json')).toBe(true);
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});
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});
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```
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### Integration Test
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```bash
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# Start with old version
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docker run -v data:/data brainy:old-version
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# Create some data
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curl -X POST localhost:3000/api/data
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# Upgrade to new version
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docker run -v data:/data brainy:new-version
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# Verify data is accessible and migrated
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curl localhost:3000/api/stats | jq '.migrationMetadata'
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```
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## Support
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For issues or questions about the migration:
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1. Check logs for migration-related messages
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2. Verify both directories exist and are accessible
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3. Ensure proper permissions for creating `_system/` directory
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4. Contact support with migration logs if issues persist
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## Summary
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This migration is designed to be:
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- **Safe**: Dual-read/write prevents data loss
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- **Gradual**: No big-bang migration required
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- **Automatic**: Minimal manual intervention
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- **Reversible**: Easy rollback if needed
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- **Transparent**: Services continue operating normally
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The key is patience - let the migration happen gradually across your fleet rather than forcing immediate updates.
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