MarsDB

An embeddable property-graph database with an openCypher query subset: single binary, single file, optional in-memory mode. Full manual: knoguchi.github.io/marsdb.

$ marsdb :memory:
MarsDB graph database. Enter Cypher statements terminated by `;`. Ctrl-D to exit.
marsdb> CREATE (a:Person {name: 'Alice'})-[:KNOWS]->(b:Person {name: 'Bob'});
marsdb> MATCH (a:Person)-[:KNOWS]->(b:Person) RETURN a.name, b.name;
a.name | b.name
Alice | Bob

Install

CLI — installs the marsdb binary:

cargo install marsdb-cli

Or on macOS/Linux via Homebrew (tap):

brew install knoguchi/marsdb/marsdb

Rust library:

cargo add marsdb

let db = marsdb::Database::in_memory()?; // or Database::open("path/to.db")
db.execute("CREATE (a:Person {name: 'Alice'})")?;
let result = db.execute("MATCH (n:Person) RETURN n.name")?;

// Bound work from untrusted callers. A CancellationToken can also be
// cloned and cancelled from another thread.
let options = marsdb::ExecutionOptions {
    max_intermediate_rows: Some(100_000),
    max_result_rows: Some(10_000),
    max_relationship_expansions: Some(1_000_000),
    timeout: Some(std::time::Duration::from_secs(5)),
    ..Default::default()
};
let result = db.execute_with_options("MATCH (n) RETURN n", &options)?;

// Group statements into one atomic unit. Reads through `tx` see its earlier
// writes; any statement error aborts and closes the whole transaction.
let mut tx = db.begin_transaction()?;
tx.execute("CREATE (:Person {name: 'Bob'})")?;
tx.execute("CREATE (:Person {name: 'Carol'})")?;
tx.commit()?;

// Operational checks and crash-consistent backup. The backup destination
// must be new, so an existing file is never overwritten.
db.backup_to("path/to-backup.db")?;
let mut db = db;
let report = db.check_integrity()?;
assert_eq!(report.nodes, 3);

// Or run a `;`-separated batch, one transaction per statement, one
// QueryResult per statement back:
let results = db.execute_batch("CREATE (a:Person {name: 'Alice'}); CREATE (b:Person {name: 'Bob'})")?;

ExecutionOptions::observer accepts an ExecutionObserver callback for dependency-free telemetry. Events contain duration, outcome category, read/write classification, result-row count, and relationship expansions; they deliberately exclude query text and error messages. Syntax and missing- parameter rejections are reported too, and observer panics are contained.

More: cargo run -p marsdb --example task_tracker (CRUD + aggregation), --example social_graph (variable-length traversal, MATCH...CREATE), or --example params_and_batch ($parameters, execute_batch) — full source in marsdb/examples/. Each also writes an SVG chart of its query result (via plotters) to the current directory.

Python:

pip install marsdb

import marsdb
db = marsdb.Database.in_memory()  # or .open(path)
db.execute("CREATE (a:Person {name: 'Alice'})-[:KNOWS]->(b:Person {name: 'Bob'})")
db.execute("MATCH (n:Person) RETURN n.name")
# -> [{'n.name': 'Alice'}, {'n.name': 'Bob'}]

Prebuilt wheels cover macOS (arm64, x86_64) and Linux (x86_64, manylinux); other platforms install from the source distribution and need a Rust toolchain. To build from source directly:

cd marsdb-python
python3 -m venv .venv && source .venv/bin/activate
pip install maturin && maturin develop

Go: not published yet — no go get-able module path. Bindings live in marsdb-go (cgo, via a C ABI crate, marsdb-capi); see that README for the two-step build (Rust cdylib, then go build) and a full example.

db, _ := marsdb.InMemory() // or marsdb.Open(path)
db.Execute("CREATE (a:Person {name: 'Alice'})-[:KNOWS]->(b:Person {name: 'Bob'})")
rows, _ := db.Execute("MATCH (n:Person) RETURN n.name AS name")
// rows -> []map[string]any{{"name": "Alice"}, {"name": "Bob"}}

CLI usage

marsdb                                  # in-memory REPL
marsdb mydata.db                        # file-backed REPL
marsdb mydata.db "MATCH (n) RETURN n"   # run one query, exit
marsdb :memory: "..."                   # explicit in-memory, one-shot
marsdb mydata.db "CREATE (a); CREATE (b); MATCH (n) RETURN n"  # ;-separated batch

Natural language -> Cypher

marsdb-nl2cypher translates an English question into Cypher against a database's actual schema (labels/relationship-types/properties in use, introspected automatically), validates its syntax and variable/type binding, and retries once with the exact validation error fed back if the first attempt is invalid. No HTTP/LLM-SDK dependency in the core crate — bring your own LlmClient:

use marsdb::Database;
use marsdb_nl2cypher::{translate_and_run, LlmClient};

let db = Database::in_memory()?;
db.execute("CREATE (:Person {name: 'Alice'})-[:KNOWS]->(:Person {name: 'Bob'})")?;

let (cypher, result) = translate_and_run(&db, &my_llm_client, "who does Alice know?")?;

translate_and_run enforces read-only generated Cypher. Model-generated writes are rejected before execution unless the caller explicitly uses translate_and_run_with_policy(..., ExecutionPolicy::AllowWrites) after performing its own authentication and authorization.

A real, runnable example against a local Ollama instance:

ollama serve &
ollama pull llama3.2
cargo run -p marsdb-nl2cypher --example ollama_demo

MarsDB's narrower Cypher subset (vs. full Neo4j Cypher) is a deliberate fit for this — a smaller grammar means fewer ways an LLM can generate something unparseable. The prompt tells the model what's supported and what to avoid (no bare --> shorthand, MERGE capped at one hop, etc.) — see marsdb-nl2cypher/src/lib.rs's CAPABILITIES constant.

Architecture

marsdb-storage   thin trait boundary over redb (file + in-memory backends)
marsdb-graph     property graph model, CRUD, KV/adjacency encoding
marsdb-query     openCypher subset: ANTLR4 grammar -> AST -> IR -> executor
marsdb           embeddable public Rust API (Database::open/in_memory/execute)
marsdb-cli       the `marsdb` binary (REPL + one-shot mode)
marsdb-python    PyO3 bindings, builds via maturin
marsdb-capi      C ABI (opaque handle + JSON results), basis for non-Rust bindings
marsdb-go        Go bindings, via cgo against marsdb-capi
marsdb-nl2cypher natural-language -> Cypher: schema introspection, prompt building, validate-and-repair

Storage runs on redb, a pure-Rust single-file MVCC embedded KV engine. Query execution compiles Cypher to a small Gremlin-shaped logical IR (AllNodesScan, NodeByLabelScan, Seed, Expand, VarExpand, Filter) so a future Gremlin frontend can target the same executor. Every Cypher statement runs inside one transaction — a read-only MATCH ... RETURN opens a ReadTransaction (a consistent snapshot that runs alongside other concurrent readers or a concurrent writer without contending for redb's single-writer lock), everything else opens a WriteTransaction, committed or aborted as a whole. Database::begin_transaction lets callers explicitly extend that atomic boundary across multiple statements. MarsDB records its own table/record format version in metadata when the file is created or first opened by a version-aware build, and refuses to open a database written by a newer unsupported format.

Numbers: BENCHMARKS.md.

The logical read plan runs as a pull-based row stream through node-ID scans, filters, and relationship expansions. A non-aggregating, non-distinct RETURN ... LIMIT k without ORDER BY stops that pipeline after k rows, so downstream limits avoid unnecessary expansions. Clause boundaries and inherently blocking operations still materialize: WITH, optional-match reconciliation, variable-length traversal results for each input row, aggregation, DISTINCT, mutations, and the public QueryResult. Use ExecutionOptions to put hard ceilings on intermediate rows, result rows, relationship expansions, and elapsed time.

There is not yet a general cost-based optimizer. Two targeted optimizations complement streaming: a direct MATCH (n[:Label]) RETURN ... LIMIT k scan pushes the limit into storage; and every ORDER BY ... LIMIT k site (WITH's own, non-aggregating RETURN's, aggregating RETURN's) uses a top-k partial selection (slice::select_nth_unstable_by + a sort of just the k-sized prefix) instead of a full sort of every row.

Cypher coverage

Full breakdown — every supported clause/expression/temporal-type shape, the error taxonomy, and a real, measured openCypher TCK conformance table by category — lives in CYPHER_COVERAGE.md.

Short version: 3878/3880 TCK scenarios pass (99.9%), 0 wrong-result scenarios. The only 2 non-passing scenarios need dates at year ±999,999,999 — a real storage/library range limitation, not a bug (see CYPHER_COVERAGE.md for the exact explanation). Recent changes: CHANGELOG.md.

Roadmap

• From-scratch storage engine (page format, B-tree, crash recovery) as an alternate marsdb-storage backend, independent of redb
• Gremlin frontend targeting the existing IR
• Composite indexes (single-property indexes already exist) with transactional maintenance
• Real query optimizer: cardinality estimates and cost-based join/traversal ordering (beyond the existing index-seek fusion and top-k ORDER BY ... LIMIT selection)
• Node/relationship-valued $parameters (map-valued already works)

Testing

cargo test --workspace                                             # ~8s
cargo test -p marsdb-graph --test stress -- --ignored --nocapture  # ~15s, large-scale
cargo test -p marsdb-crash-harness -- --ignored --nocapture        # ~7s/30 runs, SIGKILL-and-verify
cargo bench -p marsdb-graph
cargo bench -p marsdb

marsdb-crash-harness is a process-crash durability check, not a full power-loss test (OS stays up, page cache intact — a real power-loss test needs fault injection like dm-flakey, not just SIGKILL): it spawns a child process committing one transaction at a time, SIGKILLs it at an unpredictable point, reopens the file fresh, and asserts every acknowledged commit survived intact with no gaps or duplicates. CRASH_TEST_RUNS=200 (default 30) for more confidence at the cost of runtime.

The Cypher parser (the one part of MarsDB that takes raw, untrusted string input directly) is fuzzed via cargo-fuzz — needs nightly:

cargo install cargo-fuzz
cd marsdb-query && cargo +nightly fuzz run parse -- -max_total_time=120

Only claim: never panics. A parse error (Result::Err) is the expected, correct outcome for most fuzzer-generated input. Keep at least one real Cypher string in fuzz/corpus/parse/ — running against a genuinely empty corpus made libFuzzer's own startup/seed-bootstrapping phase (not anything in MarsDB) take upwards of an hour instead of seconds on this target; a seeded corpus doesn't hit it.

marsdb-tck runs a real subset of the openCypher TCK (pulled in as a git submodule pinned to a fixed commit — see marsdb-tck/VENDOR.md; 220 .feature files, 3880 scenarios) against MarsDB, via a purpose-built Gherkin-subset parser and structural result comparison (not string matching):

git submodule update --init marsdb-tck/openCypher
cargo run --release -p marsdb-tck

Attempts every scenario in the vendored TCK — currently 3878/3880 pass, and the 2 that don't (dates far outside the representable range, see CYPHER_COVERAGE.md) report as a distinct "unexpected" outcome, not lumped in with genuine wrong answers. 0 scenarios currently return the wrong answer — the real, checked-for-real signal this exists to produce, not the flat pass-rate over the whole suite. Side-effect assertions and the TCK's typed error taxonomy aren't checked (see the crate's doc comments for why).

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.