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Low-memory graphdb with Bolt+tls support, at-rest encryption & vectors designed for local replica graph use cases.

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# Slater

[![CI](https://github.com/Hikari-Systems/slater/actions/workflows/ci.yml/badge.svg)](https://github.com/Hikari-Systems/slater/actions/workflows/ci.yml)
[![Release](https://img.shields.io/github/v/release/Hikari-Systems/slater?sort=semver)](https://github.com/Hikari-Systems/slater/releases/latest)

**Current version: `v0.25.2`** — [all releases](https://github.com/Hikari-Systems/slater/releases).

> **In one line:** Slater serves **graphs that don't fit in memory** — hundreds of millions of nodes and billions of edges in low hundreds of MB of RAM — over standard **Bolt**, so any neo4j driver just works, with disk-native vector search sitting next to the graph, and it takes **live, durable writes** without giving that up. Resident memory is set by a cache budget you choose, **not** by the size of the graph.

---

**Shortcuts**

## Why Slater exists

A **graph database** stores data as *things* (nodes) and the *relationships between them* (edges), with the relationships as first-class citizens. That's what you want when your questions are about connections rather than rows — "who's within three hops of this account?", "what's the full dependency chain behind this build?", "which accounts share a device, an address, and a card?" — the queries that become a swamp of recursive joins in SQL but fall out naturally in a graph.

**The most common complaint about graph databases is that they don't scale past what you can hold in RAM.** Many of them (eg neo4j, Memgraph, FalkorDB, etc) keep the whole graph resident: a 40&nbsp;GB graph wants 40&nbsp;GB of memory — *per instance*. Want a replica per region, per tenant, or per pod? Multiply the bill. And past a certain size they simply won't load: eg the 90 million node / 1.5B-edge Wikidata graph needs ~64–128&nbsp;GiB resident, so the in-memory engines can't open it at all.

Slater is the rebuttal. Rather than loading the graph into memory, it **compiles it once, offline**: `slater-build` turns your data into a content-addressed, immutable on-disk image, and any number of Slater servers then serve that image over **Bolt** (so your existing neo4j drivers just work), paging blocks in on demand and holding only a fixed cache budget resident. That is how the same 90M-node graph serves from **a few hundred MB of RAM** — graph size and memory bill are decoupled. **A 4&nbsp;GB graph and a 400&nbsp;GB graph cost the same RAM to serve**, so you fan out cheap, stateless read replicas and let the store, not the heap, hold the graph.

That makes it a natural fit for knowledge graphs behind RAG, recommendation and identity graphs, dependency graphs — anything large and connected you want to query cheaply and often. Disk-native vector search lives right next to the graph, so the same engine is the retrieval layer for embeddings too.

Compiled once does not mean frozen, though. That image is a **base**, not a final state: an opt-in write layer sits above it, so a live graph can be corrected and extended without rebuilding anything.

### Reads and writes

The core is immutable; the graph is not. Turn the writable layer on (`delta.enabled`) and you write over Bolt — correct one property, add a node, retract an edge — and the change lands durably, with no rebuild of the image. What keeps it cheap on the read side is *where* the writes live.

Writes accumulate in a **log-structured-merge (LSM) layer over the immutable core**: a write-ahead log and an in-memory table, spilling to immutable delta segments, folded back into a fresh core by a periodic **consolidation**. What that buys you:

- **Reads over an unwritten graph cost exactly what they did before.** An empty delta is a single predictable branch, not a merge — the read path is byte-identical whether or not the writable layer is on.
- **The read cost of a write scales with the size of the delta, not the size of the graph.** Whole-graph answers — `count(*)`, the label and relationship-type marginals — stay *metadata reads* even with writes outstanding: the delta keeps its own counters, so a `count(*)` over a 91.6M-node core with half a million pending writes still answers in tens of milliseconds without touching a single block.
- **Acknowledged means durable.** A single writer drains the queue and returns `SUCCESS` only after the `fsync` that covers the write. Group your writes and they're cheap — a write-`UNWIND` commits one `fsync` per batch rather than per row.
- **Business-key writes, in either dialect.** `MERGE` / `MATCH … SET` / `DELETE` (and `CREATE` / `REMOVE`, detach delete, relationship writes) keyed on a node's identity property — or the equivalent ISO GQL data-modifying statements (`INSERT` / `SET` / `REMOVE` / `DELETE`), which lower onto the same path. Correct, insert, upsert and retract, over nodes and edges, addressed the way your data already is.

With the layer off — the default — Slater serves the pure immutable core and refuses writes. See [The writable layer](#the-writable-layer) for the full model.

> **On the name.** Slater is named after the CIA agent in *Archer* (a great show)
> who insists on going by a single name — "Just… Slater" — and one of my favourite
> characters in it. See the
> [character wiki page](https://archer.fandom.com/wiki/Slater).

### What you get

- **RAM set by your cache budget, not your graph size** — fan out as many read replicas as you like; the graph never has to fit in memory.
- **A drop-in for the graph** — speaks Bolt, so any standard neo4j driver (JS, Python, Go…) works unchanged. It's Cypher (plus a slice of ISO GQL, reads and writes); nothing new to learn.
- **Live, durable writes** — an opt-in LSM layer over the immutable core: business-key `MERGE` / `SET` / `DELETE` over nodes and edges, group-committed and `fsync`-durable, folded back into a fresh core by consolidation. Reads don't pay for it.
- **Deployment by file swap** — build a new content-hashed *generation* offline, atomically flip the `current` pointer, and servers pick it up. Every block is checksummed, so a half-copied image is refused rather than served.
- **Vector search built in** — disk-native approximate-nearest-neighbour (cosine, L2, or dot KNN) sits right next to your graph, for when this is the retrieval layer behind a RAG pipeline, and embeddings are writable in place — no offline rebuild to add or change a vector.
- **Locked down by design** — read and write grants are independent, plus optional at-rest encryption, TLS Bolt, argon2id-hashed ACLs, and a read-only container rootfs for read replicas. Configure a master key and the on-disk image is *authenticated* as well as encrypted — its manifest carries a keyed MAC, so an attacker with write access to the data directory but no key cannot forge a manifest the server will accept. Without a key you still get the content hash, which catches a half-copied or corrupted image — but not a deliberate one. [Which configuration buys what](THREAT_MODEL.md#what-integrity-means-in-each-configuration).

## Features

| Feature | What it means for you |
|---|---|
| **Bounded, predictable memory** | Resident memory tracks three cache budgets *you* set, to within bounded per-entry and allocator overhead — it does **not** grow with graph size; you tune the performance/RAM trade-off instead of provisioning for the whole graph. A jemalloc allocator with background purge returns freed memory to the OS after heavy query bursts, so resident size falls back toward its idle floor rather than staying pinned at the post-burst high-water mark. |
| **Multi-tenant out of the box** | One server hosts many graphs with per-user read grants — multi-database isolation that most graph DBs reserve for a paid/enterprise tier. |
| **Encryption at rest & in transit** | Per-block XChaCha20-Poly1305 sealing (the key is never written to disk) plus optional TLS (`bolt+s://`). GDPR-friendly by construction. Encryption is also what buys **authenticated** integrity: the builder seals the manifest with a keyed MAC, and a server holding the key verifies it and refuses to serve a generation whose manifest is forged, altered, or has had its MAC stripped. An unkeyed (plaintext) image is guarded by the unkeyed content hash only — completeness and corruption, not tampering. See [What integrity means in each configuration](THREAT_MODEL.md#what-integrity-means-in-each-configuration). |
| **Tiny install** | A small stripped binary on a distroless glibc base (no shell/apt) — the multi-arch (amd64/arm64) image pulls at ~22 MB, or ~12 MB for the server-only `slater:latest-lite` tag; pure-Rust TLS, no OpenSSL. Pull and run. |
| **Built for periodic publish** | Build a graph offline, serve it immutable, then atomically swap in a new version with zero downtime — ideal for data-warehouse / scheduled-refresh workloads. |
| **Rugged under load** | The server and offline builder both compile with `#![forbid(unsafe_code)]` — the engine's only `unsafe` lives in the audited jemalloc allocator crate. The core is immutable, so reads take no locks and never wait on a writer; a single writer serialises mutations behind the write path alone. No GC pauses, no data races. One bad query can't take the server down. |
| **Works with your neo4j tools** | Speaks Bolt 5.4 / 4.4 / 4.1 — use the standard neo4j drivers (JS, Python, Go, Java…), `cypher-shell`, or graph browsers unchanged. |
| **Rich Cypher query surface** | A broad read surface: `MATCH`/`WHERE`/`WITH`/`UNION`, `CALL {…}` subqueries, 70+ functions & aggregations, temporal & geospatial values, and regex. |
| **Live, durable writes** | An opt-in single-writer LSM layer over the immutable core (`delta.enabled`): business-key `MERGE` / `SET` / `DELETE` / `CREATE` / `REMOVE` over nodes and relationships, batched write-`UNWIND` (one `fsync` per batch), and `CALL slater.consolidate()` — group-committed, `fsync`-durable, and folded back into a fresh core by consolidation. The read path is byte-identical when the delta is empty. |
| **ISO GQL, read and write** | Speaks a subset of **ISO GQL** (ISO/IEC 39075) over the same Bolt connection — quantified paths, path restrictors, shortest-path selectors, label/type boolean expressions, `FOR`, `CAST`, an optional `GQL`/`CYPHER` dialect prefix — and, with the writable layer on, GQL's data-modifying statements (`INSERT` / `SET` / `REMOVE` / `[DETACH] DELETE`) lower onto the same durable write path. Cypher and GQL, reads and writes, in one engine. |
| **Vectors + graph in one engine** | Disk-native ANN vector search (Vamana + PQ; cosine / L2 / dot) for embeddings/RAG, plus graph algorithms (PageRank, BFS, betweenness, WCC…) — bounded memory even with millions of vectors. Embeddings are **writable** (a FreshDiskANN-style write ladder): insert / update / delete a vector, KNN-visible at once, folded into the base without a rebuild. |
| **Safe on network storage** | Every file is BLAKE3 content-hashed and verified on open; torn or half-copied images are refused, not served. Designed for NFS/remote volumes (no mmap surprises). |
| **Pluggable storage backends** | Serve the same generation format from a local filesystem, an S3 (S3-compatible) bucket, **or** a Google Cloud Storage bucket — publish once, fan out to stateless replicas — with an optional local-SSD cache tier in front of the object store. See [Storage backends](#storage-backends-filesystem--s3--gcs). |

Two binaries make up the workspace:

| Binary | Role |
| --- | --- |
| `slater` | The online Bolt server (the container ENTRYPOINT): serves reads and, with `delta.enabled`, the single-writer durable write path. |
| `slater-build` | The offline compiler: turns a primitive-Cypher dump into an immutable, content-hashed generation directory. |

Slater splits *bulk building* from *serving*: `slater-build` does the heavy lifting
offline — ingesting your data and compiling it into an immutable generation — so a
cold graph is never assembled on the serving hot path. Within the server, the read
surface answers a broad Cypher slice — pattern matching, `WITH`/`UNION`/`CALL {…}`
subqueries, 70+ scalar & aggregate functions, temporal & geospatial values, graph
algorithms (`algo.*`), and disk-native vector KNN (`db.idx.vector.queryNodes`) —
while the writable layer's delta overlay sits *below* that surface and is zero-cost
when empty, so reads never carry the write-side machinery. You can update a graph
two ways: write to it live over Bolt (see [The writable layer](#the-writable-layer)),
or build a new generation offline and atomically swap the `current` pointer, which
the running server picks up via its generation guard (see
[Generation guard](#generation-guard)).

## Documentation

The complete **user manual** lives in **[`docs/manual/`](docs/manual/README.md)** —
a feature-by-feature guide that explains, for every capability, what it is, why it
exists, and how to use it, with worked examples you can run against a bundled
sample graph. Start there for anything beyond this overview.

- **New here?** [Quickstart](docs/manual/02-quickstart.md) builds and serves a
  graph in five steps.
- **Writing queries?** [Querying](docs/manual/07-querying.md),
  [Functions & expressions](docs/manual/08-functions-and-expressions.md),
  [Procedures & algorithms](docs/manual/09-procedures-and-algorithms.md),
  [Vector search](docs/manual/10-vector-search.md),
  [Writing data](docs/manual/11-writing-data.md).
- **Building graphs?** [Building graphs](docs/manual/05-building-graphs.md) and the
  [Build CLI reference](docs/manual/06-build-cli-reference.md).
- **Operating Slater?** [Deployment](docs/manual/13-deployment.md),
  [Storage](docs/manual/12-storage.md),
  [Configuration reference](docs/manual/14-configuration-reference.md),
  [Security](docs/manual/15-security.md),
  [Performance tuning](docs/manual/16-performance-tuning.md).

## Using Slater as a Graphiti memory store

[**graphiti-slater**](https://github.com/Hikari-Systems/graphiti-slater) is an adapter
that lets [Graphiti](https://github.com/getzep/graphiti) store its temporal knowledge
graph in Slater, with a runnable
[`docker-example/`](https://github.com/Hikari-Systems/graphiti-slater/tree/develop/docker-example)
— including exposing it to Claude Code as an MCP server. See that repository for how it
works and how to run it.

## Running with Docker

Slater is designed to be run as a **Docker deployment** — that's the expected way
to use it. Prebuilt multi-arch images (`linux/amd64` + `linux/arm64`) are
published to **Docker Hub** at
[**`hikarisystems/slater`**](https://hub.docker.com/r/hikarisystems/slater),
tagged `:latest` and `:vX.Y.Z` on every release:

```sh
docker pull hikarisystems/slater:latest
```

A Docker-command-only usage, configuration, and operations guide lives in
[`DOCKERHUB.md`](DOCKERHUB.md) (and is mirrored to the Docker Hub overview page) —
**start there if you're deploying.** In short:

```sh
# Build a graph generation with the offline writer:
docker run --rm -v slater-data:/data -v "$PWD/dumps:/dumps:ro" \
  --entrypoint /app/slater-build hikarisystems/slater:latest \
  --input /dumps/people.cypher --graph people --data-dir /data

# Serve it over Bolt on 7687 (read-only unless `delta.enabled`):
docker run -d --name slater -p 7687:7687 \
  -v slater-data:/data:ro -v "$PWD/acl.json:/config/acl.json:ro" \
  hikarisystems/slater:latest
```

To build the image locally instead (e.g. for development):

```sh
# Build the image (both binaries).
docker compose build

# Serve (expects generations under the slater-data volume / your /data mount).
docker compose up slater

# Build a generation with the offline writer (profile `build`):
docker compose run --rm builder \
  --input /dumps/people.cypher --graph people --data-dir /data
```

The builder stage installs `cmake`, `clang` and `libclang-dev` for the rustls
`aws-lc-rs` backend; `git` (already in the base image) is required for the
`hs-utils` git+tag dependency, which `.cargo/config.toml` fetches via the git CLI.

The sections below cover the on-disk format, configuration, ACLs, and a
local (non-Docker) worked example.

## How it works

```
            slater-build                         slater (Bolt server)
   dump.cypher ──────────▶ /data/<graph>/<uuid>/ ──────────▶ neo4j driver
   (offline, atomic)        MANIFEST.json, *.blk,            (bolt / bolt+s)
                            range/*.isam, vector/*.{vamana,pq},
                            current → <uuid>
```

* A **generation** is one immutable directory: a `MANIFEST.json` (symbol tables,
  index descriptors, an optional encryption header), columnar block files
  (`node_props.blk`, `node_labels.blk`, `edge_props.blk`, `topology.csr.blk`,
  `vectors.f32.blk`), range indexes (`range/<name>.isam`), above-threshold ANN
  indexes (`vector/<label>.<prop>.{vamana,pq}`), and a `current` text pointer.
* Every block is zstd-compressed and BLAKE3-checksummed; with `--encrypt` each
  block is additionally sealed with XChaCha20-Poly1305 (AEAD at rest).
* The server opens a generation by **re-hashing every file** against the manifest,
  so a half-copied / truncated image — a torn copy onto the data dir, which may be
  remote/network storage — is refused rather than served.
* Reads flow through **three bounded cache pools** — a decompressed-block LRU, a
  vector-index pool (resident PQ codes + a Vamana-block LRU), and a result LRU —
  each with its own byte budget. Each pool weighs what it holds and evicts to stay
  under its budget, so RSS tracks the budgets to within bounded per-entry and
  allocator overhead instead of growing with the graph.

### The writable layer

With `delta.enabled`, the immutable generation becomes the **fully-compacted bottom
level (the "core")** of a small log-structured-merge tree, and live writes ride on
top of it:

```
   write (Bolt)                                    read (Bolt)
        │                                               │
        ▼                                               ▼
   ┌──────────────┐  flush   ┌──────────────┐    ┌──────────────────────┐
   │ WAL + active │ ───────▶ │  L0 delta    │    │  a query pins one    │
   │  memtable    │          │  segments    │    │  (core, delta) view  │
   └──────────────┘          └──────┬───────┘    │  and reads the merge │
        (fsync = ack)               │            └──────────────────────┘
                          consolidation │  (folds core + delta → fresh core)
                                        ▼
                                 ┌─────────────┐
                                 │  new core   │  (atomic `current` swap)
                                 └─────────────┘
```

* **Durability floor — the WAL.** Each mutation is serialised behind a single
  writer per graph, appended to a per-graph write-ahead log, and `fsync`ed before
  the Bolt `SUCCESS` is returned — so *acknowledged ⇒ durable*, and a torn tail is
  dropped on replay. A batched write-`UNWIND` appends its rows and commits **one**
  `fsync` for the whole batch. The WAL is **local disk only** (it is not routed
  through the storage backend), which makes a *writer* node stateful: it needs a
  durable local volume at `delta.walDir`. Read replicas stay stateless.
* **Memtable → L0 → consolidation.** Writes accumulate in an in-RAM memtable
  (bounded by `delta.memtableBytes`); when it fills it flushes to an immutable L0
  delta segment. A **consolidation** folds `{core + delta}` into a fresh core by
  serialising the merged view back through `slater-build` and swapping `current`
  atomically — the same content-hash guard as any published generation. Trigger it
  by hand with `CALL slater.consolidate()`, automatically at `delta.deltaCorePercent`
  of the core's size (optionally gated to an off-peak `delta.consolidateWindow`), or
  let the `delta.deltaHardBytes` throttle backstop runaway growth.
* **The overlay sits below the read surface.** The executor reads through a
  `ReadView` that is either the bare core (delta always empty) or a merged
  `(core, delta)` view; the engine is monomorphised over it, so an empty delta
  compiles to a single predictable branch and the read-only path is byte-identical.
  Whole-graph counters (`count(*)`, label/reltype marginals) are served from the
  delta's own live counters, so they stay metadata reads even with writes pending.
* **A query sees a stable snapshot.** It pins one `(core, delta)` tuple for its
  whole life. There are no multi-statement transactions and no rollback — a write is
  a durable, business-key-addressed correction, not an OLTP transaction.

The exact write grammar and knobs are in the [Configuration](#environment--configuration)
table (`delta.*`) and the [Worked example](#worked-example) below.

### Range indexes (ISAM)

A range index (`range/<name>.isam`, one per indexed `(label, property)`) lets a
`MATCH (n:Label {prop: v})` or `WHERE n.prop <op> v` resolve to the matching node
ids **without scanning the label**. It is an
**[ISAM](https://en.wikipedia.org/wiki/ISAM)** (Indexed Sequential Access Method)
structure — the classic *static, sorted, block-structured* index, which is exactly
the right shape for an immutable generation: there are no inserts to rebalance, so
the simplicity of ISAM buys what a B-tree's mutation machinery would only
complicate.

* Entries `(value, entity_id)` are sorted by value and packed into the same
  zstd-compressed 256 KiB blocks as everything else.
* A small **resident top-level** holds the first key of each block (a sparse
  index). A lookup binary-searches that in-memory top level to find the *one* block
  a key can be in, reads + decompresses that block, and scans it — so an equality
  lookup is **one block read**, and a range scan walks the contiguous run of blocks
  it spans. (This is why a `meshUi`-indexed lookup is single-digit milliseconds
  while the same match on an unindexed property scans the whole label.)
* The planner picks it via `NodeScan::RangeEq` / `RangeRange`; an unindexed
  predicate falls back to a label sweep or full scan, with the executor re-checking
  every predicate either way.

### Vector search (Vamana + PQ) — cosine, L2 and dot, read *and* write

Vector KNN (`db.idx.vector.queryNodes`) runs over **cosine, L2, or dot-product (MIPS)**
indexes. The base index is built offline with two execution paths, chosen per index by
the `--ann-threshold` (default 50 000 vectors):

* **Below the threshold — brute force.** The full `f32` vectors live in
  `vectors.f32.blk`; a query scans the index's group and computes the exact distance in
  the index's metric. Simple and exact; fine when the vector set is small.
* **At or above the threshold — Vamana + PQ**, the disk-native ANN path that keeps
  resident memory bounded regardless of how many vectors there are:
  * **[Vamana](https://arxiv.org/pdf/2401.11324)** is the graph index from the
    DiskANN line of work: a single proximity graph whose edges are pruned (the
    `--vamana-r` out-degree and `--vamana-alpha` long-edge factor) so a *greedy
    beam search* — start at the medoid, repeatedly hop toward the query, keeping a
    candidate list of width `vectorQuery.beamWidth` — reaches a node's true
    neighbours in few hops, i.e. **few random block reads per query**. The graph
    blocks (`vector/<label>.<prop>.vamana`) are paged in through the vector cache,
    not held wholesale.
  * **[Product quantisation (PQ)](https://medium.com/aiguys/product-quantization-k-nn-for-big-datasets-12431d764c4e)**
    compresses each vector into a short code (`--pq-subspaces` × `--pq-bits`): the
    dimensions are split into subspaces, each independently k-means-clustered, and
    the vector is stored as the tuple of nearest-centroid ids. These codes
    (`vector/<label>.<prop>.pq`) are small enough to keep **resident**, so the
    beam search scores candidates from RAM and only the chosen few full vectors are
    read from disk. That resident PQ set is what the `cache.vectorCacheBytes` pool
    pins.

**Writable embeddings — the vector write ladder ([FreshDiskANN](https://arxiv.org/abs/2105.09613)-style).**
An indexed embedding is a first-class writable value. `SET n.embedding = vecf32([…])` (and `REMOVE`) lands in the
write delta and is **immediately KNN-visible with exact rank**, then survives a segment
flush, a merge, and a consolidation. A query merges up to three levels — the sealed base
index, a sealed per-segment index, and an in-memory **RW-index** (a live mutable Vamana
over the write delta) — so latency stays flat as writes accumulate rather than growing
with the count of pending writes. A delete leaves a *hole*: the node stops being returned
but stays a navigational waypoint until a background **delete-consolidation** splices it
out of the graph, so deletes stop costing query IO. And because the on-disk graph
addresses its neighbours by layout position rather than node id, `CALL slater.consolidate()`
carries the Vamana **by reference** — hard-linked, byte-identical — and rewrites only a
small id column, folding vector writes into the base **without** the O(N·R·L) graph
rebuild. Measured numbers, with caveats, are in the [performance report](docs/PERF-REPORT.md).

## Storage backends (filesystem / S3 / GCS)

Every generation file is opened through an **`ObjectStore`** abstraction rather
than `std::fs` directly, so the *same* on-disk byte format — blocks, indexes,
manifest, `current` pointer — is served unchanged from any backend; only *where
the bytes come from* differs, never the readers, the query engine, or the
integrity checks. The hot path is positional reads (`read_exact_at`), which map
onto a `pread` on a local file and an HTTP byte-range request on an object store
— Slater never mmaps, so the explicit, bounded-read model is identical
everywhere.

**Three first-class backends**, selected by `dataBackend.kind`. The filesystem is
the simple default; **Amazon S3 and Google Cloud Storage are equal, fully
supported object-store backends** — the published image ships with both compiled
in, so each is configuration-only, and a generation built once can be served from
any of them (even migrated `fs` → S3 → GCS) without a rebuild.

| `dataBackend.kind` | Positional read | Integrity at open | Credentials |
| --- | --- | --- | --- |
| `fs` *(default)* | `pread` | full BLAKE3 re-hash of each file | — |
| `s3` | HTTP `Range` GET | server **SHA-256** via `HEAD` (→ BLAKE3 body re-hash if absent) | config keys, AWS chain, or IAM role |
| `gcs` | HTTP range read | server **CRC32C** via `get_object` (→ BLAKE3 body re-hash if absent) | ADC / Workload Identity, or service-account JSON |

Both object stores verify integrity from the **checksum the store already
computes and keeps**, fetched as object metadata: `slater-build` sends the
checksum on upload (the store validates the bytes against it and stores it), and
the server reads it back at open and compares it to the manifest — one metadata
request per file, no body download. It is content-grade and identical in spirit
across S3 (SHA-256) and GCS (CRC32C). When an object carries **no** server-stored
checksum (copied in out-of-band, or uploaded with a different default), the server
**re-hashes the object body against the manifest BLAKE3** rather than trusting its
byte length — a requested integrity check is never silently downgraded to a size
comparison. Slater-published generations always carry the checksum, so they stay on
the cheap metadata path.

What this column checks, on every backend, is that the files **match the manifest**.
Whether the manifest itself can be trusted is a separate question, and it is the
master key that answers it: with a key configured the manifest carries a keyed MAC
that the server verifies before it trusts any field (including these hashes), so a
manifest rewritten to describe tampered files is refused; without a key the comparison is unkeyed throughout, and
someone who can write to the data directory can rewrite a file and the manifest
together. See
[What integrity means in each configuration](THREAT_MODEL.md#what-integrity-means-in-each-configuration).
The check itself can be turned off with `dataBackend.verifyIntegrity: false`, which
trades it for a faster open.

### Filesystem (`fs`)

The default, rooted at `dataBackend.fs.dir`. The right choice for most
deployments: a generation on a local SSD (or an NFS/EBS mount) served read-only.
Integrity is a full BLAKE3 re-hash of every file at open.

### Amazon S3 (`s3`)

An S3 or S3-compatible bucket (AWS, MinIO, localstack). Credentials come **first**
from config (`dataBackend.s3.awsAccessKey` / `awsSecretKey`, plus
`awsSessionToken` for temporary STS credentials) and fall back to the standard AWS
chain (`AWS_ACCESS_KEY_ID` / `AWS_SECRET_ACCESS_KEY` env, shared profile, or
instance/IRSA role) when left empty.

```sh
# serve from S3 (env-var form; see the config table for every key)
dataBackend__kind=s3
dataBackend__s3__bucket=slater
dataBackend__s3__region=eu-west-2
dataBackend__s3__awsAccessKey=…        # omit to use the AWS chain / instance role
dataBackend__s3__awsSecretKey=…
# S3-compatible (e.g. MinIO): also set
dataBackend__s3__endpoint=http://minio:9000
dataBackend__s3__pathStyle=true        # required by most S3-compatible servers
```
```sh
# publish a generation into the bucket (remote `current` pointer written last)
slater-build --input people.cypher --graph people --data-dir /data \
  --publish-s3-bucket slater --publish-s3-region eu-west-2 --publish-s3-prefix prod
#   MinIO: add  --publish-s3-endpoint http://localhost:9000 --publish-s3-path-style
```

### Google Cloud Storage (`gcs`)

A GCS bucket, reached over the JSON API. Authorization is GCP-native: by default
it resolves **Application Default Credentials** — GKE Workload Identity, the GCE
metadata server, or a `gcloud` / `GOOGLE_APPLICATION_CREDENTIALS` key. Set
`dataBackend.gcs.credentialsPath` (a service-account JSON key file) or inline
`credentialsJson` for an explicit key. `dataBackend.gcs.endpoint` points at a
`fake-gcs-server` emulator, and `dataBackend.gcs.anonymous=true` enables
unauthenticated access **for that emulator only** — never against real GCS.

```sh
# serve from GCS (env-var form; see the config table for every key)
dataBackend__kind=gcs
dataBackend__gcs__bucket=slater
dataBackend__gcs__prefix=prod
dataBackend__gcs__credentialsPath=/secrets/sa.json   # omit for ADC / Workload Identity
```
```sh
# publish a generation into the bucket (remote `current` pointer written last)
slater-build --input people.cypher --graph people --data-dir /data \
  --publish-gcs-bucket slater --publish-gcs-prefix prod
#   explicit key: add  --publish-gcs-credentials /secrets/sa.json
```

In all cases `slater-build` writes the finished generation to `--data-dir` first
(its local staging area) and **additionally** uploads it to the bucket; the remote
`current` pointer is written last, so a serving node never sees a half-published
generation.

### When to use an object store (S3 or GCS)

Reach for `s3` or `gcs` when you want generations in durable, central object
storage rather than on a node's disk — typically: publish once and fan out to many
stateless, disk-less server replicas that all read the same bucket; decouple the
build host from the serve hosts; or lean on the store's
durability/versioning/lifecycle instead of managing volumes. The trade-off is
latency: a cold block is a network round-trip (~10–50 ms) instead of a local read
(~0.1 ms). Slater hides most of it with the in-memory block cache, concurrent
read-ahead, **and** the optional disk cache below. If your generations already sit
on fast local storage and you don't need the central-bucket model, `fs` is simpler
and faster.

### Local-disk block cache (object-store second tier)

The in-memory `BlockCache` is deliberately small (bounded RSS is the headline
guarantee), so on a working set larger than RAM the same blocks would be
re-fetched from the object store on every spill. An **optional local-SSD second
cache tier** fixes that: a block evicted from RAM is served from local disk
(~0.1 ms) instead of a fresh object GET, surviving in-memory eviction and cutting
object-store request count/cost — bringing an object-store-backed node close to
local-filesystem performance once warm. It is **opt-in** for both `s3` and `gcs`,
enabled by setting `dataBackend.<s3|gcs>.diskCacheBytes > 0` and a writable
`diskCacheDir`.

* It caches the **sealed** bytes exactly as fetched — already compressed, and
  (for `--encrypt` generations) still AEAD-sealed — *below* decrypt/decompress.
  The cache layer never holds the encryption key and never re-encrypts, so
  at-rest status is preserved for free: an encrypted generation lands on disk
  still sealed.
* Writes are **write-behind**: a miss returns the fetched bytes to the query
  immediately, then a background thread does the disk write and LRU trim, so the
  query path never blocks on disk I/O. Eviction keeps the cache within its byte
  budget; a per-file checksum verified on every read self-heals a corrupt cache
  file to a miss (→ refetch from the object store).
* `diskCacheDir` **must point at a real writable volume — never `tmpfs`** (tmpfs
  is RAM and would defeat the bounded-RSS guarantee). The in-memory index that
  tracks it costs a little RAM (~tens of bytes per cached block), which counts
  against your RSS ceiling — size the directory ≫ the in-memory block cache.
* The tier's other RAM cost is the **write-behind queue**, which stages blocks on
  their way to disk. It is bounded at `blockCacheBytes / 8` (floored by
  `diskCacheBytes`) — 8 MiB at the default — and sheds rather than grows, so a
  cold scan cannot inflate it; a shed block simply refetches on its next miss.
  It needs no configuration: it scales with `blockCacheBytes`, so the disk tier
  adds no new number to the RSS budget beyond its index.

## Mounts

A **read replica** runs with a **read-only root filesystem** and a non-root user
(`appuser:1000`) — everything it needs is mounted read-only. A **writer**
(`delta.enabled`) additionally needs one durable, writable volume for its WAL.

| Path | Purpose | Notes |
| --- | --- | --- |
| `/data` | The graph generations (`<graph>/<uuid>/…` + `current`). | **Read-only** for replicas; produced by `slater-build`. May live on remote/network storage (e.g. NFS), so reads are not assumed to be fast local-SSD latencies. |
| `/sandbox` | Per-environment config overlay + secrets. | `/sandbox/config.json` is deep-merged over the baked-in `config.json`; also holds `acl.json`, TLS PEM material, the at-rest key file. |
| `/tmp`, `/run` | Scratch (`tmpfs`). | A read replica never writes to disk by default. |
| _(writer)_ `delta.walDir` | The write-ahead log + L0 delta segments, when `delta.enabled`. | **Writable**, and a **durable, real volume — never `tmpfs`** (it is the durability floor). A relative path resolves under the data dir; give a writer its own persistent volume here. |
| _(optional)_ disk cache | The local-disk block cache, when `dataBackend.s3.diskCacheBytes` / `dataBackend.gcs.diskCacheBytes > 0`. | **Writable**, and a **real volume — not `tmpfs`**. Used by the `s3` and `gcs` backends; see [Storage backends](#storage-backends-filesystem--s3--gcs). |

## Environment / configuration

Config is loaded by the house-standard layered loader: the baked-in `config.json`,
then `/sandbox/config.json` deep-merged over it, then `KEY__sub` environment
overrides (double underscore for nesting; keys match the camelCase config).

Every configuration knob — its camelCase key, the `KEY__sub` environment override, its default, and what it does — is tabulated in the **[Configuration reference](docs/manual/14-configuration-reference.md)**. The most-tuned knobs are the cache budgets (`cache.*`), the query guards (`query.*`), the connection caps (`server.*`), the storage backend (`dataBackend.*`), and the writable layer (`delta.*`).

**Resident memory** tracks
`blockCacheBytes + vectorCacheBytes + resultCacheBytes` **to within bounded
per-entry and allocator overhead** — each pool weighs its own contents (strings and
containers by allocated capacity) and evicts to stay under budget, but the
per-entry bookkeeping and the allocator's size-class rounding sit on top of the
number you set — plus a small fixed overhead (and up to `degreeColumnBytes` for the
`lazy` degree column, once the degree-sum `count(endpoint)` fast path is
exercised). It is **independent of graph size** — that is the headline guarantee,
exercised by the `rss_stays_bounded_under_sustained_knn_load` integration test,
which holds peak-vs-warm RSS growth well inside the summed budgets. Per-connection
buffers live *outside* the cache budgets, so the guarantee holds under adversarial
load only because `server.maxConnections` bounds how many can exist at once.

### Network posture

Slater is a read replica handle; the **primary** connection-security control is the
network, not the binary. Bind it to a private interface, restrict source ranges at
the network layer (security groups / NetworkPolicy), and — if it faces anything but
trusted clients — front it with a connection-limiting L4 proxy (HAProxy `maxconn` +
a per-source `stick-table`, or nftables `connlimit` + `hashlimit`). That sits before
the file descriptor is ever handed to the process, so it is the most robust limit.

The in-binary limits above (`maxConnections`, `maxPreAuthConnections`,
`maxConnectionsPerIp`, the differential byte caps, and `loginTimeoutMs`) are
**defence-in-depth**: they default on and generous so they are invisible to a
legitimate client population, but they make the bounded-RSS guarantee hold even when
the proxy is forgotten. See **[`docs/HARDENING.md`](docs/HARDENING.md)** for the full
defensive posture, and `THREAT_MODEL.md` / `SECURITY_WORKLIST.md` for the canonical
detail.

### Generation guard

Slater polls each graph's `current` pointer every `generationPollMs`
(**poll, not inotify** — the data dir may be remote/network storage like NFS,
where filesystem change events are unreliable). When it changes:

* `reloadStrategy=exit` (default): the server logs fatal and exits non-zero so the
  orchestrator restarts it cleanly against the new generation.
* `reloadStrategy=swap`: the server opens **and validates** the new generation
  (same content-hash guard as boot), atomically swaps it in, and lets in-flight
  queries finish on the old one. A corrupt/incomplete new image is refused and the
  old generation keeps serving.

## ACL

`acl.json` maps users to argon2id password hashes and per-graph **`read`** / **`write`**
grants. Mint a hash (never store cleartext) with:

```sh
slater hash-password 's3cret'        # prints a $argon2id$… string for acl.json
```

A starter `acl.json` ships at the repo root; its shape is:

```json
{
  "users": {
    "reporting": {
      "passwordArgon2id": "$argon2id$v=19$m=19456,t=2,p=1$<salt>$<hash>",
      "grants": {
        "people": ["read"],
        "products": ["read", "write"]
      }
    }
  }
}
```

* **`users`** — one entry per login, keyed by username.
* **`passwordArgon2id`** — the `$argon2id$…` string from `slater hash-password`
  (never cleartext; the file itself is plain JSON and lives on shared storage).
* **`grants`** — per-graph capability lists. Two permissions are meaningful:
  * **`read`** — query the graph. A graph absent from a user's grants is invisible to them.
  * **`write`** — mutate the graph through the writable layer (`delta.enabled`): the
    `MERGE` / `SET` / `DELETE` statements and `CALL slater.consolidate()`.

They are **independent: a `read` grant confers no write access.** Turning the writable
  layer on therefore cannot promote your existing readers into writers. A writer needs
  both — `["read", "write"]` — because resolving a business key to write it is a read.
  Unrecognised permission strings are ignored (they grant nothing).

Mount it read-only at the path named by `aclPath` (default `/config/acl.json`).
The server reloads it on each generation hot-swap, and the at-rest ACL stamp is
re-checked on every reload (see [`requireAclStamp`](#environment--configuration)).

## Health check

The `slater` binary doubles as its own liveness probe: `slater healthcheck [host]
[port]` performs a **Bolt handshake** (not an HTTP request) against the server and
exits `0` if it negotiates a protocol version, `1` otherwise — defaulting to
`localhost` and the configured Bolt port. This is what the container
`HEALTHCHECK` runs, so orchestrators see a truly Bolt-ready server, not just an
open socket:

```sh
slater healthcheck localhost 7687    # exit 0 = healthy
docker exec slater /app/slater healthcheck   # inside the container
```

## One-shot query

For scripting, CI checks, and quick lookups, `slater query` mounts a graph's
current generation, runs a single read-only Cypher query in-process, prints the
result as a JSON object, and exits — no server, no Bolt connection. It honours
the same config as the server (storage backend, encryption key, query budgets):

```sh
# GRAPH defaults to `defaultGraph`. Without -q, normal datestamped logging
# (config, "opened generation", …) is written to stdout alongside the result.
slater query mygraph 'MATCH (n) RETURN count(n) AS c'

# -q/--quiet ⇒ logging suppressed, so stdout is *only* the compact result JSON
slater query mygraph -q 'MATCH (c:Company) RETURN c.ticker AS t LIMIT 3' | jq
# {"columns":["t"],"rows":[["AUPH"],["KYMR"],["MREO"]]}
```

Nodes and relationships expand to their labels/type and properties. Use `-q`
when you want machine-parseable output (the result JSON is the only thing on
stdout); omit it for an operator-facing run with logs. Without `-q` a
metrics-only summary is logged after each run — e.g.

```text
INFO query executed cost=2389 resultCount=10 execMs=441 limitRowCount=10
```

carrying the query `cost` (elements charged), `resultCount`, `execMs`, and
`limitRowCount` (only when the query specifies a `LIMIT`) — never the query text
or any result value. Exit status is `0` on success, `1` on a parse/open/execute
error (message on stderr).

## Export a graph (`slater dump`)

`slater dump` exports a graph from a **running** server as business-key `MERGE`
Cypher — the same dialect `slater-build` ingests — so a graph round-trips
(dump → `slater-build` → new generation) for migration or text backup. Unlike
`slater query`, it connects over **Bolt**, authenticates, and honours per-graph
ACLs, so it needs no disk access to the server. The password is read from
`SLATER_DUMP_PASSWORD` or stdin (never a flag, keeping it out of `ps`/history).

```sh
# List the graphs the authenticated user may read.
SLATER_DUMP_PASSWORD=pw slater dump --list -u reporting

# Dump a graph to a file (identity keys inferred from range indexes).
SLATER_DUMP_PASSWORD=pw slater dump people -u reporting -o people.cypher

# Rebuild it into a fresh generation.
slater-build --input people.cypher --graph people --data-dir ./data
```

Each label's identity key is the property carried by its range index; override
with `--key Label=prop` (repeatable) or a global `--pk <field>`. `CREATE INDEX`
DDL is emitted first so the rebuild recreates the indexes. A multi-label node
keeps **every** label — it is emitted as `MERGE (n:Ident:Other {key: v})`, with
the identity label (the one supplying the business key) first and the rest
sorted; the merge is keyed on the identity label alone, so the trailing labels
are written onto the node without creating another one. Labels, relationship
types and property keys containing special characters are backtick-quoted on
emission, so unusual names round-trip faithfully and cannot inject Cypher into the
rebuild. Vectors (and other
values with no Cypher-literal spelling) cannot ride a `MERGE` dump and are dropped
with a warning on stderr. Exit status is `0` on success, `1` on error.

## Worked example

A complete, runnable walkthrough — build a graph, serve it, connect with the neo4j **JavaScript** and **Python** drivers, and write to it — is in the manual's **[Quickstart](docs/manual/02-quickstart.md)** and **[Writing data](docs/manual/11-writing-data.md)** pages, using the bundled sample graph in [`docs/manual/examples/`](docs/manual/examples/).

## Development

```sh
export PATH="$HOME/.cargo/bin:$PATH"
cargo build
cargo test            # unit + the bounded-RSS headline integration test
cargo clippy --all-targets -- -D warnings
cargo fmt --all -- --check
```

### Object-store backends are opt-in cargo features

A plain `cargo build` produces a **filesystem-only** binary — the `s3` and `gcs`
backends are gated behind cargo features so the default build stays small (no AWS
or Google SDK, no async runtime). Enable whichever you need on **both** `slater`
(serve) and `slater-build` (publish):

```sh
# S3 only / GCS only / both
cargo build -p slater -p slater-build --features s3
cargo build -p slater -p slater-build --features gcs
cargo build -p slater -p slater-build --features s3,gcs
```

Each crate exposes matching `s3` / `gcs` features that forward to
`graph-format/{s3,gcs}`. Requesting a backend at runtime
(`dataBackend.kind=s3|gcs`, or `slater-build --publish-{s3,gcs}-*`) without its
feature compiled in fails fast with a clear "built without the … feature" error.
The **published Docker image enables both** (Dockerfile `CARGO_FEATURES`), so
prebuilt images need no extra flags — this only matters when building from source.
The integration tests are likewise gated: `--features s3 --test s3_minio`,
`--features gcs --test gcs_emulator` (a `fake-gcs-server`), and `--features gcs
--test gcs_real` (real GCS via ADC); each skips unless its `SLATER_*` env vars are
set.

See `docs/PLAN.md`, `docs/PROGRESS.md` and `docs/DECISIONS.md` for the design,
the milestone ledger, and the decision log.

## Performance

Up to six engines, one single-client suite, graphs from a 62k-node toy to **Wikidata
91.6M nodes / 1.5B edges**. Each engine is **measured in isolation** (every other container
stopped — RSS and latency are its own footprint). The **latency tables** below were
**re-measured on Slater 0.21.0** (the writeable build): the small/medium graphs (MeSH, EU-AI-Act)
freshly, and the 91.6M graph as a fresh **same-box, shared-anchor** slater-vs-Neo4j pass (see
that table). The **resident-memory** figures carry forward from the earlier pass (measured via
container cgroup; the read path is byte-identical with the writable layer idle). The other
engines' numbers are the established cross-engine run (their versions/performance are unchanged).
All figures are medians (ms) or peak resident memory (MiB). **Lower is better everywhere; bold =
best in row.** slater was run on its **local-filesystem (`fs`) backend**; the S3 and GCS backends trade local-read
latency for object-store round-trips (mitigated by the in-memory caches and the optional local-disk
cache tier), so these figures characterise the engine, not a network-storage deployment.

| engine | class | memory bound |
|---|---|---|
| **slater** | disk-backed, paged | `query.maxIntermediate` caps the working set automatically |
| Neo4j 5 | disk-backed, JVM | ~2 GiB heap + off-heap, committed regardless of query |
| Memgraph · FalkorDB | in-memory | whole graph resident in RAM |
| ArcadeDB | in-memory, JVM | whole graph resident; heaviest |
| LadybugDB | embedded, columnar | manual buffer pool that must exceed the query |

The three engines that **page from disk** — slater, Neo4j 5, and LadybugDB — load all five
graphs. The **in-memory trio** (Memgraph · FalkorDB · ArcadeDB) cannot hold the 1.5B edge graph at
all (it needs ~64–128 GiB resident), and ArcadeDB's importer can't finish it either.

### Resident memory (MiB) — bounded as the graph grows ~1,500×

Each figure is **committed working memory** — what the OS cannot reclaim. Every engine *except*
slater holds its graph in committed anonymous memory (own heap, Neo4j's off-heap page cache, or
a buffer pool), so its peak RSS *is* its committed footprint. slater alone serves from the
**reclaimable OS page cache** of its on-disk store, so its figure is the anon working set; the
store's page cache (evictable under pressure — slater keeps serving) is excluded, and shown as
*total* in parentheses for the 91.6M graph. **Bold = lowest.**

| graph (nodes / edges) | slater | Neo4j 5 | Memgraph | FalkorDB | ArcadeDB | LadybugDB |
|---|--:|--:|--:|--:|--:|--:|
| pole — 62k / 106k | **11** | 746 | 114 | 140 | 1,556 | 198 |
| MeSH — 341k / 469k | **63** | 1,083 | 358 | 455 | 1,631 | 121 |
| EU-AI-Act — 21k / 45k (+55 MiB vec) | **99** | 729 | 229 | 312 | 1,948 | 286 |
| Wikidata — 91.6M / 1.5B | **584** *(4,595 total)* | ~2,900 | cannot-load | cannot-load | cannot-load | ~652 † |

slater is the **lowest at every scale** and grows ~50× while the graph grows ~1,500× — its
footprint tracks the *query working set*, not the graph (idle ~16–71 MiB throughout). The
in-memory trio grows ~linearly and can't load the 1.5B graph; Neo4j commits a ~2 GiB heap
regardless of query. († LadybugDB on the bounded shapes only — its hub / var-length /
shortestPath traversals at 1.5B edges need its read pool raised to ≥2 GiB, vs slater's automatic
`maxIntermediate` cap.) The build-time value→count histograms add negligible resident memory —
a few KB for a low-cardinality indexed column, and *zero* for unique-key graphs like Wikidata
(`wikidata_id` exceeds the histogram cardinality cap, so none is stored) — so these figures are
unchanged by that feature.

### Latency (median ms) — graph fits in RAM (MeSH, 341k / 469k)

| shape | slater | Neo4j 5 | Memgraph | FalkorDB | ArcadeDB | LadybugDB |
|---|--:|--:|--:|--:|--:|--:|
| count(*) all nodes | **0.41** | 15.0 | 23.8 | 16.4 | 82.0 | 2.2 |
| label count | **0.42** | 4.2 | 20.7 | 1.1 | 4.4 | 4.3 |
| indexed point lookup | **0.43** | 3.9 | 0.48 | 0.48 | 0.65 | 8.8 |
| idx-eq count | **0.42** | 4.9 | 5.0 | 2.0 | 381 | 2.5 |
| 1-hop (indexed anchor) | 1.28 | 5.8 | **1.21** | 4.1 | 390 | 4.9 |
| 2-hop (unanchored) | **1.40** | 5.6 | 8.5 | 16.7 | 444 | 6.4 |
| group-by / count(DISTINCT) | **0.45** | 47–51 | 63–64 | 31–39 | 411 | 5.3 |
| full-scan `CONTAINS` | **0.43** | 5.4 | 24.1 | 1.7 | 16.3 | 4.1 |

slater owns the **metadata / index / scan** shapes (count, label, idx-eq, scan — ~0.4 ms, 10–200× the
service engines), the **indexed point lookup** (0.43 ms, now edging the in-memory pair's 0.48 ms),
the **unanchored multi-hop** (2-hop 1.40 ms via the relationship-type scan, fastest in the field),
and — via a build-time value→count histogram on the indexed grouping key — the **whole-label
group-by / count(DISTINCT)** (0.45 ms, ahead of LadybugDB's columnar 5.3 ms). The in-memory servers
keep only **raw 1-hop** (Memgraph 1.21 ms vs slater's 1.28 ms). (pole 62k/106k looks the same:
slater sole-fastest on count/scan ~0.4 ms, ~1.3–2.6 ms on hops.)

### Latency (median ms) — vectors (EU-AI-Act kNN, 15k × 1024-dim)

| shape | slater | Neo4j 5 | Memgraph | FalkorDB | LadybugDB |
|---|--:|--:|--:|--:|--:|
| kNN top-10 Concept | 2.9 | 8.6 | 1.9 | **1.2** | 2.8 |
| kNN top-10 Chunk | 2.4 | 5.7 | 1.9 | **1.5** | 3.2 |

slater answers kNN with an **exact brute-force** scan (these sets are below its 50k-vector
ANN threshold) where the others use an approximate resident HNSW — so slater's results are
exact (recall 1.0). A SIMD distance kernel + a resident, pre-normalised vector matrix
took Concept from ~23 → ~2.9 ms and Chunk from ~10 → ~2.4 ms, so slater now beats
Neo4j and LadybugDB and is within ~1.4× of Memgraph, trailing only FalkorDB — while exact.

### Vector write ladder — insert / update / delete without a rebuild

The tables above are cross-engine *read* comparisons. The vector **write** path (the
[FreshDiskANN](https://arxiv.org/abs/2105.09613)-style write ladder over the static Vamana
base) has no cross-engine counterpart — no other engine here does disk-native, *writable*
ANN — so the numbers below are single-engine **component** benchmarks over a synthetic,
embedding-like fixture (a low-rank manifold, dim 768, unequal norms), committed under
[`crates/slater/benches/`](crates/slater/benches) and written up in full — with the
methodology and every caveat — in [`docs/PERF-REPORT.md`](docs/PERF-REPORT.md). Recall is
always measured against an **exact brute force over the live set**, never one index against
another. Scale here is representative and extrapolated only where the metric is size-linear.

| property | measured | why it matters |
|---|---|---|
| **KNN latency vs pending writes** | RW-index **~1.5–2 ms, flat** to 50k pending; the pre-index brute-forced overlay **1.9 → 115 ms** (linear in delta) — **61× at 50k** | query latency does **not** degrade as writes pile up between consolidations |
| **Embedding insert** | **~1.5–2 ms** per vector into the live index | a write is KNN-visible at once; the delta-rebuild budget is ≈ 2 ms × the delta cap |
| **Delete IO at iso-recall** | **2.9× fewer** node-fetches per query at 67 % deleted, **5.2×** at 80 % (recall ≥ 0.90) | a consolidated graph pays no read tax for deleted vectors |
| **Consolidation, pure permutation** | **O(1)** — the `.vamana` is hard-linked byte-identical, only the id column is rewritten | folding vector writes into the base skips the O(N·R·L) rebuild |
| **Recall across the ladder** | consolidated ≥ base for cosine, L2 and dot | the write ladder **preserves** recall at every rung |

The one figure that wants the dedicated perf box is the *slow-path* consolidation rewrite
throughput — when a consolidation carries deletes or new vectors rather than a pure
permutation, it is a sequential recompress bound by single-thread zstd and local disk, so
the absolute MiB/s is environment-specific (the report shows the shape and explains the
environmental range).

### Latency (median ms) — graph ≫ RAM (Wikidata 91.6M / 1.5B)

The in-memory engines (Memgraph / FalkorDB / ArcadeDB) **cannot load** this graph at all
(~64–128 GiB resident). Only slater and Neo4j 5 do. This is a **fresh same-box, same-day pass
against a shared, fixed anchor set** — every query hits the *identical* nodes on both engines,
so the head-to-head is apples-to-apples (a common `wikidata_id` pool of moderate-degree anchors;
see the note below on why that matters). slater is shown at both fanouts (`query.maxFanout` 1 =
throughput default, 8 = the latency dial that overlaps cold block reads). **Bold = best in row.**

| shape | slater (fan 1) | slater (fan 8) | Neo4j 5 |
|---|--:|--:|--:|
| count(*) all nodes | **0.41** | 0.41 | 3606 |
| point lookup (indexed) | 0.72 | **0.49** | 6.3 |
| degree (1-hop count) | **0.43** | 0.44 | 6.0 |
| 1-hop neighbours | 9.8 | **4.5** | 10.1 |
| 2-hop | 37 | **23** | 34.5 |
| 3-hop | 32 | **25** | 74 |
| var-length `*1..2` distinct | 985 | 1056 | **47** |

The honest picture: slater **dominates the metadata / index shapes** — `count(*)` is
metadata-served (**0.41 ms vs Neo4j's 3.6 s** disk scan, ~8800×), and point-lookup / degree /
3-hop run ~2–10× faster — is **even with Neo4j on 1–2-hop** (fanout 8 pulls ahead on cold reads),
but **loses `var-length *1..2 distinct` decisively (≈1 s vs Neo4j's 47 ms)**: slater's
variable-length distinct expansion is materially slower here, a real weakness worth its own
investigation. All of that at a few hundred MB of RSS versus Neo4j's committed ~2 GiB heap.

> **On the anchors.** These traversal numbers depend heavily on *which* nodes you start from —
> a node one link from a Wikidata mega-hub ("human", "country") has a millions-strong 2-hop
> neighbourhood, so var-length/hop cost swings by orders of magnitude with anchor choice. The
> earlier edition of this table sampled each engine's *own* "first N by scan," which is neither
> stable nor comparable; this pass fixes a single shared, degree-bounded anchor set for both
> engines. (shortestPath is omitted from this pass — between two arbitrary anchors it is
> path-existence-dependent and too high-variance to median meaningfully.)

### Multi-hop `count(*)` — memory decoupled from result size

Uncapped multi-hop `RETURN count(*)` counts *during* expansion instead of materialising
the matched rows. Same hub anchors on the 91.6M graph, `maxIntermediate=20M`:

| 3-hop count(*) @ 91.6M | fanout=1 | fanout=8 |
|---|--:|--:|
| latency / peak working set | **554 ms / 0.66 GiB** | **298 ms / 1.9 GiB** |

The count holds O(1) rows. Charging is unchanged, so a mega-hub count still trips
`maxIntermediate` on *compute* (adjacency reads), bounded as before.

### Per-query parallelism (`maxFanout`)

Raising `query.maxFanout` overlaps a query's **cold, I/O-bound** block reads across cores —
it helps large-cold-working-set disk-bound shapes and is flat on warm shapes. On the 1.5B
graph: shortestPath ≤6 **918 → 608 ms** (1.5×, largest search 6,269 → 2,350 ms, 2.7×);
3-hop count **547 → 298 ms**. `maxFanout=1` is the default (throughput-oriented); `8` is the
latency dial, at more transient worker memory.

### Where slater wins / trails

| dimension | slater | best of the field | verdict |
|---|---|---|---|
| resident memory, any scale | 11–584 MiB (62k → 91.6M) | in-memory 1.5–2.7 GiB; can't load 1.5B | **slater** |
| count / metadata / scan | ~0.4 ms | service engines 5–80 ms | **slater** (10–200×) |
| indexed point lookup | **0.43 ms** (MeSH) | Memgraph · FalkorDB 0.48 ms | **slater** (edges the in-memory pair) |
| unanchored multi-hop (rows) | **1.40 ms** (MeSH 2-hop) | Neo4j 5.6 ms | **slater** (relationship-type scan) |
| aggregation (group-by / DISTINCT) | **0.45 ms** | LadybugDB 5 ms (columnar) | **slater** (build-time histogram) |
| kNN | 2.4–2.9 ms (exact) | FalkorDB **1.2 ms** (HNSW) | beats Neo4j/Ladybug; ~1.4× off Memgraph; exact |
| 91.6M metadata / point / degree / 3-hop | 0.4–32 ms | Neo4j 6–3,600 ms | **slater** (2–8800×) |
| 91.6M 1–2-hop | 4.5–23 ms (fan 8) | Neo4j 10–35 ms | ~even |
| 91.6M var-length `*1..2` distinct | ~1 s | Neo4j **47 ms** | **Neo4j** (a real slater weak spot) |
| multi-hop `count(*)` at scale | 0.3–0.6 GiB | in-memory engines materialise the row set | **slater**, bounded |

Full per-engine tables (pole, MeSH, EU-AI-Act + the `blockCacheBytes` RAM↔latency dial,
Wikidata 1M & 91.6M) are in
[`perf/cross-engine-hs/README.md`](perf/cross-engine-hs/README.md); the fresh slater-only pass
(both fanouts, every dataset) is in [`perf/PERF_CURRENT_STATUS.md`](perf/PERF_CURRENT_STATUS.md).

### Concurrency & brown-out (load testing)

The benchmarks above are single-client. The complementary axis — **behaviour under many
concurrent clients** — has its own harness, [`perf/loadtest/`](perf/loadtest/): a Locust
driver over Bolt plus a coordinator that ramps load, reads `CALL slater.diagnostics()`,
finds the capacity knee, and names the limiter (full method in
[`docs/LOAD-TESTING.md`](docs/LOAD-TESTING.md)). Headlines from a 256 MiB-cache run on the
Wikidata-1M graph (one 16-core box):

| result | measurement |
|---|---|
| Holds to **1000 concurrent clients, zero failures** | throughput peaks ~2.5k rps; the latency knee sets in around 750 clients (p99 51 → 750 ms) — queueing under core contention, not a hard cap (single-run, WSL2) |
| Block cache **bounded and effective** | 100% hit rate, 0 evictions, 50 MB resident for a cache-fitting working set |
| RSS held under sustained load | the jemalloc allocator holds RSS to ~0.6 GB across a 100→500-client `wiki_cache_churn` ramp — cache-bound and stable, with **no** `MALLOC_*` tuning (the former `MALLOC_ARENA_MAX=2` + trim threshold is retired); its background purge also returns the post-burst high-water instead of leaving it pinned |
| Aggregate memory bounded | server-wide **`query.maxIntermediateGlobal`** + adjacency-charged expansion hold the `wiki_budget` 2-hop flood at 1000 clients without OOM (RSS ~0.6 GB; the guard sheds ~60% of hub queries as retryable budget errors) |

Both memory issues the load test surfaced are now closed; all tracked in the load-testing doc.

## License

Licensed under the Apache License, Version 2.0. See [`LICENSE`](LICENSE) for the
full text and [`NOTICE`](NOTICE) for attribution. Unless you explicitly state
otherwise, any contribution intentionally submitted for inclusion in this work,
as defined in the Apache 2.0 license, shall be licensed as above, without any
additional terms or conditions.

SPDX-License-Identifier: Apache-2.0

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