# M3.5.4 — Federation: single query across multiple projects

| Field | Value |
|---|---|
| Phase | M3.5 — Distributed API Layer |
| Size | M — 1–3 days |
| Status | ⬜ Not started |
| Flags | — |
| Spec | inlined below |
| Blocks | M3.5.8 |
| Depends | M3.5.3 (query endpoint exists) |

## Goal

Extend query endpoint to support multi-project search. When `project` param is omitted, a single query searches all projects concurrently, deduplicates results, and merges scores.

## Design

**Single-project query (no change):**
```
GET /memory/query?query=Kong+body&project=poimen
→ results from poimen only
```

**Multi-project query (federation):**
```
GET /memory/query?query=Kong+body
→ results from all projects, merged by rerank_score
```

Response is the same shape; add optional `_federation` metadata:
```json
{
  "query": "Kong body",
  "projects_searched": ["poimen", "agent-rust"],
  "results": [...],
  "latency_ms": 512,
  "notes": "Searched 2 projects in parallel; 3 results after dedup"
}
```

## Behavior

**Deduplication:** Same `sha256` across projects is impossible (sha256 includes project name in provenance), so no dedup needed. If two projects happen to have identical text:
- Treat as separate nodes (different projects, different provenance)
- Return both in results (may both rank high)
- Ensure test coverage catches this edge case

**Concurrency:** Query all projects in parallel using `tokio::join_all()` or `futures::stream`:
```rust
let futures: Vec<_> = projects.iter()
  .map(|proj| query_single_project(query_text, proj, limit))
  .collect();
let results: Vec<_> = futures::future::join_all(futures).await;
```

**Merging:** After all projects return, merge result vectors:
- Collect all results from all projects into one vec
- Re-sort by `rerank_score` descending (global order)
- Take top `limit` (e.g., if poimen returns [a,b,c] and agent-rust returns [d,e], merge gives [a,b,c,d,e] → sorted globally → top 5 might be [b,d,a,c,e])

**Timeout:** Per-project timeout is min(timeout_seconds / projects.len(), 2s). If one project is slow, others complete faster and we still return results from fast projects after global timeout.
- E.g., timeout=10s, 2 projects → 5s per project
- If project-a completes in 3s, project-b in 8s, and global timeout is 10s:
  - Return results from both (8s < 10s)
- If project-a completes in 3s, project-b in 12s, and global timeout is 10s:
  - After 10s, cancel project-b, return results from project-a only
  - Note in response: `"warnings": ["project 'agent-rust' timed out"]`

## Steps

1. Parse `project` param:
   - If provided, single-project path (M3.5.3 unchanged)
   - If omitted, multi-project path

2. List all known projects (from queries YAML):
   ```rust
   let projects = load_standing_queries()?.projects();
   ```

3. Spawn concurrent query tasks:
   ```rust
   let futures: Vec<_> = projects.into_iter()
     .map(|proj| {
       let params = params.clone();
       params.project = Some(proj);
       query_handler_impl(&params, store, llm)
     })
     .collect();
   ```

4. Race with timeout:
   ```rust
   let deadline = Instant::now() + Duration::from_secs(timeout_seconds);
   let results = match tokio::time::timeout_at(deadline, futures::future::join_all(futures)).await {
     Ok(vec) => vec.into_iter().flatten().collect(), // flatten per-project results
     Err(_) => { /* partial results + warning */ }
   };
   ```

5. Merge and sort:
   ```rust
   results.sort_by(|a, b| b.rerank_score.partial_cmp(&a.rerank_score).unwrap());
   results.truncate(limit);
   ```

6. Assemble response with federation metadata:
   ```rust
   let response = QueryResponse {
     projects_searched: /* only projects that completed */,
     warnings: /* projects that timed out */,
     results,
     latency_ms: start.elapsed().as_millis() as u64,
     ..
   };
   ```

## Acceptance

- Single project specified: no federation, same result as M3.5.3
- No project specified: all projects queried
- Results merged and globally sorted by rerank_score
- Partial results returned if one project times out

## Verify

**Harness:** Integration tests with two projects in test pgvector DB.

**Integration test** — `tests/it_query_federation.rs`:
1. `a1_single_project_no_federation` — GET /query?project=poimen returns single-project results only.
2. `a2_multi_project_searches_all` — GET /query (no project) with >1 project in DB returns results from all.
3. `a3_global_sort_order` — two projects return results, merge sorts by rerank_score globally (not per-project).
4. `a4_federation_metadata_present` — response includes `projects_searched` array with all completed projects.
5. `a5_partial_results_on_timeout` — slow one project (mock 10s delay), set timeout_seconds=2, GET /query returns results from fast project only with warning.
6. `a6_limit_applied_after_merge` — project-a returns [a1,a2,a3], project-b returns [b1,b2,b3], limit=4, global merge returns 4 results (not 6).
7. `a7_no_project_filter_in_response` — response.project_filter is null or omitted (unlike single-project which sets it).
8. `a8_concurrent_execution` — spy on timing: timestamp project-a query start, project-b query start, both should be ~simultaneous (not sequential).

**Command:** `cargo test -p mem-cli query_federation`

**False pass:**
- Testing only with one project in DB. Federation always "works" if there is nothing to federate.
- Timeout never exercised. Mock a slow project and assert results are partial.
- Per-project sorting instead of global sort. Results look reasonable but violate the contract (should be global top-k).
- Concurrency not verified. Queries can be sequential (slow) and still return correct results; only timing proves concurrency.

## Traps

- Timeout math: if you do `timeout_per_project = timeout_total / num_projects`, a project that completes in 1s uses the full allocated time before returning. Should be `remaining_time = deadline - now()`.
- Partial results: if project-a returns 5 results and project-b times out, you have 5 results but may have wanted 10 (limit=10). Document whether partial results truncate or stay over-limit.
- Clone overhead: cloning `QueryParams` for each project is small; cloning a large result vec is not. Use references/Arc where possible.
- Flatten after join_all: `join_all` returns `Vec<Result>`, must flatten errors (either as partial results or early exit).

---

Background: [DESIGN.md § Distributed API Layer](../DESIGN.md#distributed-api-layer-homelab-frontend)