# TDD Wave 2: Query Methods Implementation
**Date**: 2026-02-19
**Focus**: Checkpoint retrieval, listing, and restoration
---
## Wave 2 Tests Summary
| 11 | Retrieve checkpoint by ID | ✅ |
| 12 | Get by ID returns None for non-existent | ✅ |
| 13 | List checkpoints by session | ✅ |
| 14 | Session isolation (checkpoints don't leak) | ✅ |
| 15 | List checkpoints by tag | ✅ |
| 16 | Restore checkpoint state | ✅ |
| 17 | Restore fails for invalid checkpoint | ✅ |
| 18 | Checkpoint ordering by timestamp | ✅ |
| 19 | Checkpoint summary has correct info | ✅ |
| 20 | List returns populated results | ✅ |
**Total**: 10 new tests, all passing (20/20 cumulative)
---
## Implementation Details
### New Methods Added to `TemporalCheckpointManager`
```rust
/// Get a checkpoint by ID
pub fn get(&self, id: &CheckpointId) -> Result<Option<TemporalCheckpoint>>
/// List checkpoints for a specific session
pub fn list_by_session(&self, session_id: &SessionId) -> Result<Vec<CheckpointSummary>>
/// List checkpoints with a specific tag
pub fn list_by_tag(&self, tag: &str) -> Result<Vec<CheckpointSummary>>
/// Create a checkpoint with tags
pub fn checkpoint_with_tags(
&self,
message: impl Into<String>,
tags: Vec<String>,
) -> Result<CheckpointId>
/// Restore state from a checkpoint
pub fn restore(&self, checkpoint: &TemporalCheckpoint) -> Result<DebugStateSnapshot>
/// Get a summary of a checkpoint by ID
pub fn get_summary(&self, id: &CheckpointId) -> Result<Option<CheckpointSummary>>
```
### Storage Implementation Update
Added in-memory cache to `SqliteGraphStorage`:
```rust
pub struct SqliteGraphStorage {
graph: RefCell<SqliteGraph>,
cache: RefCell<HashMap<CheckpointId, TemporalCheckpoint>>,
}
```
**Rationale**: SQLiteGraph's query API is limited for complex filtering. The in-memory cache enables:
- O(1) lookup by ID
- Fast filtering by session_id and tags
- Simple implementation for MVP
**Future work**: Implement proper SQLite queries when sqlitegraph API supports it.
---
## Key Design Decisions
### 1. In-Memory Cache Pattern
For MVP, we use a hybrid approach:
- Store checkpoint in SQLiteGraph (persistence)
- Also store in HashMap cache (fast queries)
This gives us durability + query performance without complex SQL.
### 2. Session Isolation
Each `TemporalCheckpointManager` is bound to a session:
- `list()` returns only current session's checkpoints
- `list_by_session()` allows cross-session queries
- Sessions are isolated by `SessionId` (UUID)
### 3. Tag-Based Filtering
Tags are stored as `Vec<String>` on each checkpoint:
- Simple contains-check for filtering
- No index needed for MVP scale
- Can add database index later if needed
### 4. Restore Validation
Restore validates checkpoint state:
```rust
if checkpoint.state.working_dir.is_none() {
return Err(ReasoningError::InvalidState(...));
}
```
This prevents restoring from corrupted/incomplete checkpoints.
---
## Code Metrics
| `src/checkpoint.rs` | ~320 | +60 |
| `src/storage_sqlitegraph.rs` | ~165 | +70 |
| `tests/checkpoint_tests.rs` | ~350 | +220 |
---
## Next Wave Preview
**Wave 3: Persistence & Durability**
- File-based storage (not just in-memory)
- Checkpoint compaction/rotation
- Import/export functionality
- Error recovery
**Potential Wave 4: Thread Safety**
- Replace `Rc<RefCell>` with `Arc<Mutex>`
- Concurrent checkpoint operations
- Background persistence
---
## Running the Tests
```bash
cd /home/feanor/Projects/forge/forge-reasoning
cargo test
# Just Wave 2 tests
cargo test test_get_checkpoint
cargo test test_list
cargo test test_restore
```
---
**Wave 2 Complete** ✅