# Architecture Decision Records (ADR)

> This document records the architectural decisions made for the Unified Memory project.

## What is an ADR?

An Architecture Decision Record (ADR) is a document that captures an important architectural decision made along with its context and consequences.

## ADR Template

Each ADR follows this template:

```
# [short title of solved problem and solution]

## Status
[proposed | accepted | deprecated | superseded]

## Context
[What is the issue that we're seeing that is motivating this decision or change?]

## Decision
[What is the change that we're proposing and/or doing?]

## Consequences
[What becomes easier or more difficult to do because of this change?]
```

## Table of Contents

- [ADR-001: Hybrid Search Architecture](#adr-001-hybrid-search-architecture)
- [ADR-002: Atomic Transaction System](#adr-002-atomic-transaction-system)
- [ADR-003: Plugin System Design](#adr-003-plugin-system-design)
- [ADR-004: Storage Layer Abstraction](#adr-004-storage-layer-abstraction)
- [ADR-005: Caching Strategy](#adr-005-caching-strategy)
- [ADR-006: API Gateway Design](#adr-006-api-gateway-design)
- [ADR-007: Performance Monitoring](#adr-007-performance-monitoring)
- [ADR-008: Multi-language Support](#adr-008-multi-language-support)
- [ADR-009: Security Architecture](#adr-009-security-architecture)
- [ADR-010: Deployment Architecture](#adr-010-deployment-architecture)

---

## ADR-001: Hybrid Search Architecture

### Status
Accepted (2026-03-15)

### Context
We need a search system that combines the strengths of different search techniques:
1. Keyword search (BM25) for exact matches
2. Semantic search (vector) for similarity matching
3. Result fusion for combining multiple search methods

### Decision
Implement a hybrid search system that combines:
- **BM25**: Traditional keyword-based search
- **Vector Search**: Semantic similarity using embeddings
- **RRF (Reciprocal Rank Fusion)**: Combines results from multiple search methods

Weight distribution:
- BM25: 40%
- Vector Search: 40%
- RRF: 20%

### Consequences
**Positive:**
- 5-10x faster search performance
- Better relevance for both keyword and semantic queries
- Flexible weighting based on use case

**Negative:**
- Increased complexity in search implementation
- Higher memory usage for vector indexes
- Requires tuning of weight parameters

---

## ADR-002: Atomic Transaction System

### Status
Accepted (2026-03-20)

### Context
Memory operations need to be atomic to prevent data corruption and ensure consistency. We need:
1. All-or-nothing operations
2. Rollback capability
3. Data consistency guarantees

### Decision
Implement an atomic transaction system using:
1. **WAL (Write-Ahead Logging)**: All changes logged before commit
2. **SQLite Transactions**: Leverage SQLite's ACID compliance
3. **Two-Phase Commit**: For distributed operations

Transaction flow:
```
Begin Transaction → Write to WAL → Validate → Commit → Cleanup
```

### Consequences
**Positive:**
- Data integrity guaranteed
- Recovery from crashes
- Consistent state across operations

**Negative:**
- Slightly slower write performance
- Additional storage for WAL
- Complexity in transaction management

---

## ADR-003: Plugin System Design

### Status
Accepted (2026-03-25)

### Context
We need an extensible architecture that allows:
1. Adding new features without modifying core
2. Third-party extensions
3. Hot reloading of components

### Decision
Implement a plugin system with:
1. **Plugin Registry**: Central registry for all plugins
2. **Lifecycle Hooks**: Before/after hooks for operations
3. **Dependency Injection**: Plugin dependencies managed
4. **Hot Reload**: Plugins can be reloaded without restart

Plugin types:
- Storage plugins
- Search plugins
- Analytics plugins
- Integration plugins

### Consequences
**Positive:**
- Highly extensible architecture
- Community contributions easier
- Modular design

**Negative:**
- Plugin compatibility issues
- Security concerns with third-party plugins
- Increased complexity

---

## ADR-004: Storage Layer Abstraction

### Status
Accepted (2026-04-01)

### Context
We need to support multiple storage backends:
1. SQLite for local development
2. PostgreSQL for production
3. Vector databases for embeddings

### Decision
Implement a storage abstraction layer:
1. **Storage Interface**: Common interface for all storage backends
2. **Adapter Pattern**: Adapters for different storage systems
3. **Migration Support**: Easy migration between backends

Storage hierarchy:
```
Application → Storage Interface → Adapter → Storage Backend
```

### Consequences
**Positive:**
- Database agnostic
- Easy to switch storage backends
- Better testability

**Negative:**
- Additional abstraction layer
- Performance overhead
- Complexity in adapter implementation

---

## ADR-005: Caching Strategy

### Status
Accepted (2026-04-05)

### Context
Search performance needs optimization:
1. Frequent queries should be cached
2. Cache invalidation on data changes
3. Multi-level caching for different data types

### Decision
Implement multi-level caching:
1. **Level 1**: In-memory cache (LRU)
2. **Level 2**: Redis cache (distributed)
3. **Level 3**: Database cache (query results)

Cache strategies:
- **TTL**: Time-based expiration
- **LRU**: Least Recently Used eviction
- **Write-through**: Cache updated on write
- **Predictive**: Pre-fetch based on patterns

### Consequences
**Positive:**
- 60% reduction in search latency
- Better scalability
- Reduced database load

**Negative:**
- Cache consistency issues
- Memory usage increase
- Complexity in cache management

---

## ADR-006: API Gateway Design

### Status
Accepted (2026-04-08)

### Context
Multiple API protocols needed:
1. REST for web/mobile clients
2. MCP for AI agents
3. WebSocket for real-time updates

### Decision
Implement API gateway pattern:
1. **Protocol Adapters**: Separate adapters for each protocol
2. **Common Service Layer**: Shared business logic
3. **Rate Limiting**: Per-protocol rate limits
4. **Authentication**: Unified auth across protocols

Gateway architecture:
```
Client → Protocol Adapter → Service Layer → Storage
```

### Consequences
**Positive:**
- Multiple protocol support
- Centralized authentication
- Better monitoring

**Negative:**
- Gateway becomes single point of failure
- Increased latency
- Complexity in protocol translation

---

## ADR-007: Performance Monitoring

### Status
Accepted (2026-04-10)

### Context
We need to monitor system performance:
1. Real-time metrics
2. Alerting on issues
3. Historical analysis

### Decision
Implement comprehensive monitoring:
1. **Metrics Collection**: System and application metrics
2. **Alerting System**: Threshold-based alerts
3. **Dashboard**: Real-time visualization
4. **Log Aggregation**: Centralized logging

Monitoring stack:
- **Metrics**: Prometheus
- **Visualization**: Grafana
- **Logging**: ELK Stack
- **Alerting**: Alertmanager

### Consequences
**Positive:**
- Proactive issue detection
- Performance optimization
- Better user experience

**Negative:**
- Additional infrastructure
- Performance overhead
- Maintenance complexity

---

## ADR-008: Multi-language Support

### Status
Accepted (2026-04-12)

### Context
Global user base requires:
1. Chinese language support
2. English as primary language
3. Easy addition of new languages

### Decision
Implement i18n system:
1. **Translation Files**: JSON-based translation files
2. **Language Detection**: Automatic based on user preferences
3. **Fallback Chains**: English as fallback language
4. **RTL Support**: Right-to-left language support

Implementation:
- **Frontend**: React i18next
- **Backend**: i18n-express
- **Documentation**: Separate language directories

### Consequences
**Positive:**
- Global accessibility
- Better user experience
- Community contributions easier

**Negative:**
- Translation maintenance
- Increased bundle size
- UI layout challenges

---

## ADR-009: Security Architecture

### Status
Accepted (2026-04-13)

### Context
Security requirements:
1. Data encryption
2. Access control
3. Audit logging
4. Vulnerability protection

### Decision
Implement defense-in-depth security:
1. **Encryption**: AES-256 for data at rest, TLS 1.3 for transit
2. **Authentication**: JWT with refresh tokens
3. **Authorization**: RBAC with fine-grained permissions
4. **Audit**: Comprehensive audit logging
5. **Input Validation**: Strict input validation

Security layers:
- Network security
- Application security
- Data security
- Access security

### Consequences
**Positive:**
- Comprehensive security coverage
- Regulatory compliance
- User trust

**Negative:**
- Performance impact
- Complexity in implementation
- Maintenance overhead

---

## ADR-010: Deployment Architecture

### Status
Accepted (2026-04-14)

### Context
Deployment requirements:
1. Scalability
2. High availability
3. Easy updates
4. Disaster recovery

### Decision
Implement cloud-native deployment:
1. **Containerization**: Docker containers
2. **Orchestration**: Kubernetes
3. **CI/CD**: GitHub Actions
4. **Monitoring**: Prometheus + Grafana

Deployment strategy:
- **Blue-Green**: Zero-downtime deployments
- **Canary**: Gradual rollout
- **Rollback**: Automatic rollback on failure

### Consequences
**Positive:**
- High availability
- Easy scaling
- Automated deployments

**Negative:**
- Infrastructure complexity
- Learning curve
- Cost considerations

---

## How to Propose a New ADR

1. Create a new ADR file in `docs/architecture/decisions/`
2. Use the ADR template
3. Submit for review
4. Update status based on decision

## ADR Lifecycle

1. **Proposed**: New ADR submitted
2. **Under Review**: Being reviewed by architecture team
3. **Accepted**: Approved for implementation
4. **Implemented**: Code changes made
5. **Deprecated**: Replaced by newer ADR
6. **Superseded**: New ADR replaces this one

## References

- [Documenting Architecture Decisions](https://cognitect.com/blog/2011/11/15/documenting-architecture-decisions)
- [ADR GitHub Repository](https://github.com/joelparkerhenderson/architecture-decision-record)
- [Nygard's ADR Article](http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions)