MultiforaDB/docs/M2_DESIGN_REVIEW.md

# M2 design review — aggregation expansion

PLAN §3 gives M2 one line of scope ("pipeline stages and expression engine,
tiered scope defined at M2 design review") and one line of gate ("aggregate
spec suite green"), and §6 lists the three questions it deferred: the
stage/expression tiers, which spec files are the gate, and whether
`$lookup`/`$unwind`/`$facet` make the first cut.

This document answers them, and reports one finding that has to be settled
before any of the rest is worth discussing.

Everything below is measured against the M1 tip (`a748a3d`) unless it says
otherwise. Nothing here is recalled.

---

## 1. The gate named in the plan does not exist

`mongodb/specifications` has no aggregation suite. The full list of `source/`
directories at the pinned commit is: `auth`, `bson-*`, `causal-consistency`,
`change-streams`, `client-side-encryption`, `collation`,
`collection-management`, `command-logging-and-monitoring`, `compression`,
`connection-*`, `crud`, `gridfs`, `index-management`, `load-balancers`,
`logging`, `read-write-concern`, `retryable-*`, `run-command`, `sessions`,
`server-discovery-and-monitoring`, `server-selection`, `transactions`,
`unified-test-format`, `versioned-api`, and a handful of others. There is no
`aggregation`.

The thirteen `aggregate-*.json` files we do run live inside `crud`, and they
test the aggregate **command surface** — cursor and `batchSize`, `readConcern`
and `readPreference` routing, the write semantics of `$out`/`$merge`,
`collation`, `let`, `allowDiskUse`, `rawData`. They barely touch stages, and
they touch expressions not at all: `$lookup`, `$unwind`, `$facet`,
`$addFields`, `$replaceRoot` do not appear anywhere in the pinned corpus.

So "aggregate spec suite green" is not a gate for the milestone the scope line
describes. It is a gate for a much smaller milestone that happens to share the
command name. **Whichever option §4 picks, PLAN's M2 gate line has to be
rewritten**, and that is the first decision this review asks for.

Where the current corpus stands, for reference:

```
aggregate-*.json overall     9 pass   13 fail   19 skip
```

| failure | cases |
|---|---|
| `$merge` unimplemented | 5 |
| `$out` unimplemented | 3 |
| `db.aggregate()` — `{aggregate: 1}`, no collection name | 3 |
| `collation` in aggregate | 1 |
| `let` | 1 |

All 19 skips are environmental -- 10 distinct requirements, some covering a
whole file at once (`needs topology replicaset`, `needs server >= 5.0`, `needs
server >= 8.2.0`, `needs server <= 4.2.99`). Not one can turn green on a
standalone, so a literal reading of "green" is unreachable for that reason
alone, and the gate has to be stated as *0 fail* rather than *all pass*.

---

## 2. Where the implementation actually is

**Seven stages**, all in one `if/else if` chain in `cmd_aggregate`:
`$match`, `$sort`, `$skip`, `$limit`, `$project`, `$group`, `$count`.
Anything else answers `Location40324`, with the code verified against a real
mongod.

(`$set` and `$unset` exist in the tree as **update operators** in
`src/update.zig`. They are not pipeline stages. An earlier count of nine
stages in this project's notes was wrong.)

**There is no expression engine.** `$field` path resolution is open-coded in
two places inside `run_group`, and nowhere else. `$sum` is the only
accumulator.

That last sentence is the important one, because of *how* the missing pieces
fail. Probed against a live server on three documents
(`{g:'a',x:10}, {g:'a',x:20}, {g:'b',x:30}`):

| pipeline | answer | correct answer |
|---|---|---|
| `$group: {_id:'$g', s:{$sum:'$x'}}` | `[{a,30},{b,30}]` | ✅ |
| `$group: {_id:'$g', a:{$avg:'$x'}}` | `[{a,0},{b,0}]` | `[{a,15},{b,30}]` |
| `$group: {_id:'$g', m:{$max:'$x'}}` | `[{a,0},{b,0}]` | `[{a,20},{b,30}]` |
| `$group: {_id:'$g', p:{$push:'$x'}}` | `[{a,0},{b,0}]` | arrays |
| `$group: {_id:{g:'$g'}, n:{$sum:1}}` | one group, `_id` = `{"g":"$g"}` | two groups |
| `$project:{g:1}` then `$match:{x:10}` | matches | matches nothing |
| `$group: {_id:{$literal:1}, ...}` | `_id` = `{"$literal":1}` | `_id` = `1` |
| `$addFields` | `Location40324` | (unimplemented) |

**Six of those eight are silent wrong answers with `ok: 1`.** Only
`$addFields` fails honestly. That is the shape of the risk in this area and it
should drive the milestone: an unrecognised stage is a bug report, an `$avg`
that returns `0` is a corrupted report that nobody files. It is also precisely
the class the M1 spec-runner work was about — and the corpus cannot see any of
it, because the corpus has no stage tests.

**The pipeline model will not carry the milestone as it stands.** The stream is
a materialized window `[start, end)` over either slab offsets or generated
documents, and each stage moves the bounds. That works for the filtering and
reordering stages and nothing else:

- `$project` is not a stage transform at all. It sets a variable that is
  applied once, at emit — so only the last `$project` in a pipeline has any
  effect, and a `$match` after a `$project` sees the unprojected document.
- `$unwind` is 1→N. There is no way to express that in a window over the input.
- `$lookup` needs a second collection under a second lock, inside a stage.
- `$facet` needs the same stream fanned into several independent sub-pipelines.
- `allowDiskUse` is accepted and ignored. `$group` and `$sort` are unbounded in
  memory; the three passing `allowdiskuse` cases pass because the option is
  parsed, not because anything spills. `src/spill.zig` exists and is used by
  index builds, not here.

So M2 is not "add stages to the chain". It is: build an expression evaluator,
change the stream model to a per-stage document iterator, and then stages
become cheap. Anything that adds stages before those two exist adds more
silent wrong answers.

---

## 3. What that implies for tiers

The dependency order is forced, not chosen:

**Tier 0 — stop lying (no new features).** Refuse what is not implemented
instead of answering `0`: unknown accumulators, non-path expressions where a
path is expected, `$project` used anywhere but last. Every one of these becomes
an error with a measured mongod code. This is small, and it converts six silent
wrong answers into six honest ones. It is also the only tier that makes the
later ones measurable, for the same reason Stage 1 preceded Stage 3 in M1.

**Tier 1 — the expression engine.** A real evaluator over `bson.Value`:
field paths, `$literal`, comparison and boolean operators, arithmetic,
`$cond`/`$ifNull`/`$switch`, string and date operators. Everything above
depends on it, including `$project`'s computed fields and `$group`'s `_id`.

**Tier 2 — the stage iterator.** Replace the window with a per-stage pull
model, so a stage can emit more or fewer documents than it consumed. `$project`
becomes a real transform. Carries `$addFields`/`$set`, `$unset`,
`$replaceRoot`, `$unwind`, `$sortByCount`.

**Tier 3 — the accumulators.** `$avg`, `$min`, `$max`, `$first`, `$last`,
`$push`, `$addToSet`, `$count`, `$mergeObjects`. Cheap once Tier 1 exists.

**Tier 4 — the expensive stages.** `$lookup` (second collection, lock order),
`$facet` (sub-pipelines), `$unionWith`, `$graphLookup`. Each is a design
question of its own.

**Tier 5 — the write stages.** `$out`, `$merge`. These are not aggregation
work at all — they are a write path that happens to be spelled as a stage, with
their own durability and atomicity questions. They are also 8 of the 13 current
`aggregate-*` failures.

**Orthogonal, and cheap:** `db.aggregate()` (`{aggregate: 1}`), 3 failures.
**Orthogonal, and not cheap:** `allowDiskUse` meaning something, which is a
memory-bound question for `$group`/`$sort` and belongs with whichever tier
first makes those able to hold a lot.

---

## 4. Gate options

Each option is stated with what it would actually prove and what it costs.

### Option A — "the aggregate command surface"

Gate: the 13 `aggregate-*.json` files in the crud corpus, minus the 13
environmental skips. Scope: Tier 5 + `db.aggregate()` + collation + `let`.

- **Proves:** the aggregate *command* behaves — cursors, read concern, write
  stages, database-level form.
- **Does not prove:** any stage or expression is correct. All six silent wrong
  answers survive this gate untouched.
- **Cost:** small. Two write stages and three small fixes.
- **Honest name:** this is not "aggregation expansion". If chosen, M2 should be
  renamed and the aggregation engine deferred to its own milestone.

### Option B — a hand-built stage corpus, modelled on mongod's own

Gate: a new `tests/spec/aggregate/` corpus in the unified format the runner
already reads, with cases ported from `mongod`'s `jstests/aggregation` and
every expected result **measured against mongod 8.3.7**, the way the cursor and
`lsid` error codes were.

- **Proves:** exactly what the milestone claims to build, at whatever tier
  depth the corpus is written to.
- **Cost:** the corpus is the milestone's second deliverable and a real one —
  but the runner, the entity model, the matchers and `expectEvents` all exist
  already, so the marginal cost is writing cases, not machinery.
- **Risk, stated plainly:** a corpus we author is a corpus we can accidentally
  write to match our own bugs. The mitigation is the one this project already
  uses: every expectation measured against a real mongod before it is
  committed, never recalled.

### Option C — differential against mongod

Gate: a harness that runs the same pipelines against MultiforaDB and mongod
8.3.7 and diffs the results, over generated pipelines and a fixed corpus.

- **Proves:** the most, by a distance, and it finds the silent-wrong-answer
  class by construction rather than by someone thinking to test for it.
- **Cost:** mongod becomes a test dependency (it already is one for
  measurement, but not for the gate). Result ordering and floating-point
  formatting need normalising, and `$group` output order is genuinely
  unspecified.
- **Note:** this is a *harness*, not a corpus, so it composes with B rather
  than competing with it — B gives regression cases with committed
  expectations, C finds new ones.

### Option D — keep the plan's line and reinterpret "green"

Gate: `aggregate-*.json` at 22 pass / 0 fail / 13 skip, and declare the skips
out of scope.

- **Proves:** the same as A.
- **Why it is listed:** only to be rejected explicitly. It reads as if the
  aggregation milestone were gated on aggregation tests, and it is not. If A is
  the decision, say A.

---

## 5. Decision

**Taken: the command surface first (Option A as M2), the engine second (M2.5),
with Tier 0 folded into M2.** Recorded as PLAN amendment A6; the §3 rows are
rewritten and §6's deferred entry now points here.

The cost of that order is on the record: the engine is what gives wrong answers
today, and this leaves them alive for a further milestone. Tier 0 is what makes
the trade acceptable rather than merely convenient — every unimplemented
construct stops answering `0` and starts answering an error, so the wrong
answers become visible bug reports instead of quiet ones while the command
surface is built. It is not optional under this sequencing.

The recommendation the review arrived at independently is kept below, since a
decision is easier to revisit when the argument it overrode is still legible.

## 5.1 Recommendation as written before the decision

**Tier 0 first, alone, before any scoping decision is final** — the same
argument that put `expectEvents` before the free list in M1. Six silent wrong
answers are six results nobody can measure past, and turning them into errors
is a day's work that changes what every later number means. It will move the
scorecard *down*, and that is the point.

**Then B as the gate, with C as the harness that feeds it.** Option A is a real
piece of work and should happen — the `$merge`/`$out` failures are 8 of 13 —
but it should be named for what it is and probably belongs in M6 (admin/ops) or
its own small milestone, not as the gate for an engine.

**Tiers 1→3 as M2 proper**, with Tier 4 explicitly out of the first cut. That
answers §6's third question: **`$unwind` yes** (it falls out of Tier 2 for
free), **`$lookup` and `$facet` no** — `$lookup` because a stage that takes a
second collection lock while the pipeline holds one is a lock-order design
question this milestone should not be carrying, and `$facet` because it needs
sub-pipeline execution that Tier 2's iterator should be allowed to settle
first.

**One thing to decide that this review cannot decide alone:** whether
`allowDiskUse` must mean something by the end of M2. It is currently accepted
and ignored, which is the same category of lie as `$avg` returning `0` — a
client that passes it believes it is protected. If the answer is yes, the
memory bound on `$group`/`$sort` joins Tier 2 rather than trailing it.

---

## 6. What this review does not cover

- `$out`/`$merge` durability semantics (atomic swap? logged? crash behaviour?).
  Needed before Tier 5 is scoped.
- Whether the expression engine is shared with `update`'s pipeline-update form,
  which M3 will need. Probably yes; it changes where the code lives.
- Index use inside a pipeline beyond the existing leading-`$match` pushdown.