MultiforaDB/ROADMAP.md

# Remaining performance work

Five items, in dependency order. Each is sized to be landed and verified on
its own; the ordering constraints between them are the load-bearing part, so
read those before picking one up.

Current numbers and what they mean are in the README; the recorded baseline
is `tests/e2e/results/phase1.txt`, reproduced with
`bash tests/e2e/compare-run.sh 1g 16k`.

## Ground rules

These held for everything already done and should hold here.

- **Measure A/B on one harness. Do not trust the model.** Several predictions
  made while planning this work were wrong in both directions: CRC32 cost
  twice what was estimated, one compaction fix turned out to be numerically
  identical to the code it replaced, and a matcher change that measured 1.3x
  in isolation did not move its benchmark row at all. The cheap way to A/B is
  to flip one line, rebuild, run, flip it back.
- **Mutation-check any test guarding an invariant.** Break the thing the test
  is supposed to catch and confirm it goes red. Several tests here were
  written, looked reasonable, and only proved to have teeth after this.
- **The index invariant is absolute** (`src/index.zig` header): an index only
  generates candidates, and the full filter is re-applied afterwards.
  Over-approximating is slow. Under-approximating is a wrong answer.
- **The database must always open.** A unique index that finds duplicates in
  existing data warns and keeps going; replay never refuses to start over
  recoverable damage.

Verification for every item:

```sh
zig build test                       # unit, ReleaseFast
zig build test -Doptimize=ReleaseSafe # again with safety checks on
zig build                            # NOTE: `zig build test` does not refresh the binary
node tests/e2e/e2e6.js               # 72 checks, self-contained, includes kill -9
```

Anything touching the write path or the log format also needs the crash pair
(`e2e2.js crash-a`, `kill -9`, restart, `e2e2.js crash-b`), and anything
touching indexes needs `e2e3.js` and `e2e4.js`.

---

## 1. B-tree over the encoded keys

**Why.** Index entries live in one sorted array, so inserting an entry
memmoves the tail. Building an index is fine (entries are appended and sorted
once) and removal is fine (regenerated and found by binary search), but
*inserting into a collection that already has an index* is quadratic. It also
blocks item 2.

**What.** Replace `Index.entries` with a B+tree whose nodes live in a flat
`ArrayListUnmanaged(Node)` addressed by `u32`. Slotted 4 KiB pages; keys
longer than a quarter of a node spill to an overflow slab (BSON strings reach
16 MB, so this is not optional). Leaves linked for ordered iteration.

The flat `u32`-indexed node array is a deliberate choice over pointer-linked
nodes: it makes the whole index one contiguous byte range that item 4 can
write into a checkpoint and read back without rebuilding.

**Keep.** `append_doc_entries` + `finish_bulk` become bulk leaf packing
(sort, fill leaves, build interior levels bottom-up). `remove_doc`'s
regenerate-and-search approach carries over unchanged — only the lookup
underneath it changes.

**Watch.** Deletion is where B+trees get subtly wrong. Not rebalancing on
delete (leaving underfull leaves, letting compaction reclaim them) is a
legitimate simplification and much easier to get right; take it unless
there is a reason not to.

**Tests that must keep passing, unchanged:** the randomized `lookup_range`
check against a brute-force filter, and the `remove_doc`-versus-scan
differential. Both already exist and both are mutation-checked.

---

## 2. Ordered `_id` index — **depends on 1**

**Why.** `sort({_id: ...})` still materializes every candidate, and integer
`_id`s still fall back to a full collection scan on every `findOne`,
`updateOne` and `deleteOne`.

**What.** Give `Collection` an index over the encoded `_id`, maintained in
`upsert`, `evict_doc` and `apply_record`. Every document has an `_id` and it
is not sparse, so entry count equals document count and a full index scan
cannot miss a document — which is what the sort planner's full-scan plan
requires.

Then delete `value_fast_path_safe` and friends (`src/index.zig`). They exist
only because `serialize_value` gives `int32 1`, `int64 1` and `double 1.0`
different bytes despite comparing equal. `bson.encode_key` already gives them
identical bytes, so the guard is obsolete.

**Why it depends on item 1.** This index updates on *every* insert. Against a
sorted array that is a tail memmove each time — roughly 51 GB of memmove over
65,536 documents. It only looks acceptable because ObjectIds increase
monotonically and therefore append at the end; random or descending `_id`s
would collapse bulk insert, currently the project's best result. Do not land
this against the array without an explicit, documented "append-ordered `_id`s
only" caveat.

**Watch.** A real `_id_` index may start appearing in `listIndexes` and
writing an `index_create` record to the log. `e2e3.js` asserts on index
listings — check it before assuming this is invisible.

---

## 3. Block-framed compressed log

**Why.** The largest remaining gap: 1.0 GB on disk against MongoDB's 93 MB,
because payloads are stored raw. Breaking the format is fine.

**What.** A file header plus a sequence of blocks; each block holds the
existing record framing, so `Engine.apply_record` does not change. Target
~256 KiB per block, and records never straddle blocks.

**The hash covers the compressed bytes, as they sit on disk.** If it covered
the uncompressed bytes you would have to run the decompressor over
possibly-corrupt input before you could validate it, leaning on the
decompressor's error handling for durability. Hashing the stored bytes means
the decompressor only ever sees input already proven intact.

**Torn tail versus interior corruption must stay distinguishable**, exactly as
today: a short read, an impossible length, or a hash mismatch *in the final
block* means a crash mid-append — truncate and return cleanly. A hash
mismatch anywhere else is `error.InvalidLog`.

**Codec: write an LZ4 block compressor** (~200 lines). Zig 0.16 ships zstd
*decompression only*, so zstd needs vendoring; `std.compress.flate` has a
compressor but deflate level 1 runs slower than the current insert rate, so
it would make writes worse to make the disk smaller. Keep a codec byte in the
header so raw stays legal (needed anyway when compression does not help) and
zstd can be swapped in later.

**Watch.** `Engine.compact` goes through the same `Log` API and so gets
compression for free — but it must still defer syncing and commit once.

---

## 4. Stop giving every document its own arena

**Why.** Two gaps at once. RSS is 2.0 GB against 1.4 GB for a 1.0 GB dataset
because each document carries an `ArenaAllocator` and a second full copy of
its data as a `Pair` tree. And the range-scan gap is *not* the matcher — it is
walking 65,536 documents that each live in a separate allocation, one pointer
chase apiece.

**What.** Store canonical BSON bytes in a per-collection slab; the docs map
holds a reference into it. Match against those bytes directly, walking the
element stream and skipping by length any field the filter does not name — for
the benchmark document that is ~40 bytes touched instead of 16 KB. Add a
borrowed parse (spine only, pointing into the stored bytes) for sort,
projection and update. `emit_docs` with no projection becomes a memcpy.

**Depends on item 1** in spirit: index entry keys no longer alias document
arenas (that was fixed when entries moved to encoded bytes), but `Entry.id`
still aliases the docs map key, and that aliasing needs to be understood
before documents start moving between slabs.

**Watch.** This is the deepest change of the five and touches every test.
Land the byte matcher behind a flag and differential-test it against the
existing matcher on a corpus before switching over. Also replace the
fabricated `bson.Document{ .arena = undefined, ... }` values scattered through
`query.zig` and `commands.zig` with an explicit view type — they work today
only because nothing calls `deinit` on them, and they are exactly what breaks
when `Document` changes meaning.

---

## 5. Decompose the global lock

**Why.** One reader/writer lock covers the entire engine and is held across
fsync, compaction and reply construction, so writes cannot use more than one
core.

**What.** A catalog lock over the database and collection maps, plus a lock
per collection. Ordering is catalog then collection, never the reverse, and
never two collection locks at once — the only cross-collection operations are
the TTL sweep and compaction, and both must take collections one at a time.
Then cross-connection group commit: whichever writer finds no commit in
flight becomes the leader, seals the buffer, compresses, writes and syncs
once for everyone waiting.

**Before touching anything, enumerate what the current code relies on.**
`Entry.id` aliases the docs map key. `Engine.dup_index` is engine-global state
written by one call and read by the next. `Log.scratch` is a single shared
buffer justified by appends being single-writer. Collections are stored *by
value* in a hash map, so any insert can move them — that one needs fixing
first regardless.

**A durability guarantee changes here, and it should be a decision, not a
surprise.** Today the log always describes at least as much as memory —
strictly stronger than MongoDB. Under leader/follower commit a reader can
observe a write before its fsync completes, which is ordinary `w:1, j:true`
semantics. Check what `e2e2.js crash-b` actually asserts before changing it.