# Data Model: Core Distributed Architecture (Phase 1)

## Entities

### 1. Key-Value Pair (Raw)
- **Key**: `Vec<u8>` (Arbitrary bytes)
- **Value**: `Vec<u8>` (Arbitrary bytes)
- **Scope**: `rdb-storage` (Raw Put)

### 2. Key-Value Pair (Versioned / CAS)
- **Key**: `Vec<u8>`
- **Value**: `Vec<u8>` (Metadata + Payload)
- **Version**: `u64` (Monotonic sequence)
- **Scope**: `rdb-storage` (CAS)

### 3. TSO Timestamp
- **Physical**: `u64` (48 bits, milliseconds)
- **Logical**: `u64` (16 bits, counter)
- **Combined**: `u64` (Physical << 16 | Logical)
- **Scope**: `rdb-pd`

## State Transitions (CAS)

1. **Empty -> Created**:
   - Current Version: 0 (or None)
   - Expected Version: 0
   - New Version: TSO / Sequence > 0
   - Result: Success

2. **Updated -> Updated**:
   - Current Version: N
   - Expected Version: N
   - New Version: M (M > N)
   - Result: Success

3. **Conflict**:
   - Current Version: N
   - Expected Version: M (M != N)
   - Result: Failure (Returns N)

## Storage Schema (RocksDB Column Families)

1. **default** (`CF_DEFAULT`):
   - Stores data for Raw Puts.
   - Key: `Key`
   - Value: `Value`

2. **cas** (`CF_CAS` - *Proposed name for CAS data separation*):
   - Stores versioned data.
   - Key: `Key`
   - Value: `[Version: 8 bytes][Data...]`
   - *Note: Storing version in value simplifies atomic update via Read-Modify-Write or MergeOperator.*