JustAPI is a Python web framework with a Rust core. You write Python logic. Rust handles everything else — networking, TLS, routing, validation, serialization, SSE, database I/O, rate limiting. One pip install, a server that starts in milliseconds, and numbers that hold up under real traffic.

Python writes the logic. Rust does everything else.

Quickstart

pip install justapi

# main.py
from justapi import JustAPIApp

app = JustAPIApp()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

app.run()

$ python main.py
INFO:     JustAPI running on http://127.0.0.1:8000

Interactive OpenAPI docs at http://127.0.0.1:8000/docs — generated from your code, zero configuration.

Async? Native. DB? Native. SSE? Native.

import asyncio
from justapi import JustAPIApp, native_async

app = JustAPIApp()
app.set_database("postgres://user:pass@host/db")
app.sse_native("/events", count=1000, interval_ms=0)   # Rust streams events — 0 Python per event

@app.get("/users/{uid}")
@native_async
async def get_user(request, uid: int):
    row = await app.db.query_async(                     # runs on the DB's tokio runtime
        "SELECT * FROM users WHERE id = ?", [uid])      # GIL released — loop never blocks
    return row

app.run()

Performance

Hello-world, 100 concurrent connections, 30s, same hardware. Full methodology in BENCHMARKS.md.

More numbers, honestly:

• Route lookup: 51 ns average on a 500-route table (matchit radix trie)
• Rust-native CRUD: 181k SELECT req/s — ×125 vs FastAPI on the same DB fixture
• Native async DB awaits (query_async): 53× faster than the blocking path on slow queries (320 vs 6 RPS) — the asyncio loop is never blocked
• Multi-worker prefork (justapi serve --workers N): 1.88× throughput at 4 workers (99.7k RPS), auto-scale included
• Free-threaded CPython (3.14t): CPU-bound Python handlers 12.4× faster — the GIL ceiling is gone

Honest caveats

• Native fast path requires a schema. Routes without a Schema fall back to the Python handler path (~60k req/s). The ~700k numbers apply to routes with native=True + Schema.
• Python handler throughput is GIL-bound. On GIL-locked CPython, the Python dispatch path caps at ~100-120k req/s on this hardware regardless of framework — that is the GIL, not a framework limitation. Free-threaded Python (3.14t) removes this ceiling.
• Light async handlers are loop-bound. Async handlers that only sleep/echo (~4-12k req/s) are capped by the asyncio loop + thread hop. Three experiments (ADR-090/091/092) proved no coroutine driver, bridge, or multi-loop approach beats asyncio's own stepping. We win where it matters: real async workloads (1ms-sleep handlers: 3.9k vs Granian 2.9k RPS) and anything DB/SSE-heavy via native ops.
• LLM-serving claims are NOT part of this release. Inference phases are unverified (no real GPU run, ADR-067) — the tested story is the web framework.

How a request flows

The entire request path runs in Rust. Python executes only your application logic:

Kernel (epoll/io_uring)
  → tokio connection manager
  → TLS (rustls)
  → HTTP parse (hyper)
  → Router (matchit, 51ns)
  → Middleware chain (auth, CORS, rate-limit, compression)
  → Python boundary (zero-copy PyO3, GIL pool)
  → Your async Python handler
  → Rust serializer (serde_json / simd-json)
  → Response write → socket

Routes marked native=True + Schema skip Python entirely — Rust validates the body and writes the response. That is the ~700k req/s path.

Feature highlights

Coming from FastAPI?

JustAPI mirrors FastAPI's decorator API and Pydantic integration, so existing apps migrate with minimal changes — often just the import:

👉 Full migration guide.

Architecture

justapi/
├── crates/
│   ├── justapi-core/       # Networking, routing, middleware, serialization
│   ├── justapi-py/         # PyO3 bindings + Python package (python/justapi)
│   ├── justapi-cli/        # CLI binary (serve, create, check, gen-client)
│   └── justapi-bench/      # Internal benchmark harness
├── assets/                 # README diagrams (animated SVG)
├── benchmarks/             # Workload scripts for baseline comparisons
├── docs_site/              # Full documentation site (141 pages)
├── skills/                 # Reusable engineering knowledge
├── PLAN.md                 # Living roadmap (single source of truth)
├── DECISIONS.md            # Append-only ADR log
└── BENCHMARKS.md           # Append-only performance ledger

Crate dependency graph:

justapi-cli  ──→  justapi-core
justapi-py   ──→  justapi-core
justapi-bench ─→  justapi-core

Development

Prerequisites

• Rust 1.85+ — install via rustup
• Python 3.11+ — download
• maturin — pip install maturin

Build & test

cargo build --workspace --release
cd crates/justapi-py && maturin develop --release   # build the Python package

cargo test --workspace                              # Rust tests
cargo clippy --workspace --tests -- -D warnings     # lint
cargo fmt --check                                   # format
cargo deny check                                    # supply-chain audit
bash scripts/sanitize.sh                            # ASan + Miri memory-safety gate
python -m pytest                                    # Python tests (in the venv)

Benchmark

bash benchmarks/run_baselines.sh                    # FastAPI / Granian / JustAPI A/B

See BENCHMARKS.md for methodology, hardware, and history.

Deployment

# Docker (multi-stage, uv-based — fast reproducible builds)
docker build -t justapi-app .
docker run -p 8000:8000 justapi-app

# Multi-worker + auto-scale (saturate all cores)
justapi serve --workers 4 --scale

• Kubernetes: helm install my-api helm/justapi/ --set image.repository=your-registry/justapi-app
• Cloud guides: EKS · GKE · AKS · Fly.io · Railway
• Observability: OTel tracing + Prometheus metrics + health probes out of the box

Documentation

Contributing

Contributions welcome. Read AGENTS.md for conventions before opening a PR.

git clone [email protected]:swadhinbiswas/JustAPI.git
cd JustAPI
git checkout -b feat/my-feature
# ... make changes ...
cargo test --workspace
cargo clippy --workspace --tests -- -D warnings
cargo fmt --check

Every PR is gated on: tests green, clippy -D warnings, fmt clean, sanitizers clean (core touched), benchmarks appended (perf touched), ADR recorded (architecture touched).

License

MIT © JustAPI contributors