fastapi-stream-lease

Limit simultaneously active SSE, LLM token streams, and WebSocket sessions across FastAPI/Starlette workers with Redis. Set a per-user limit, a global limit, or both. An extra connection receives HTTP 429 before streaming begins.

Request rate limiters answer “how many requests arrived this minute?” This package answers “how many streams are open right now?” It is useful when a connection can remain open for seconds or minutes. For a general-purpose distributed semaphore, consider py-redis-limiters or py-redis-semaphore.

Install

Requires Python 3.10+ and Redis 5.0+.

With Redis 5 and redis-py 8+, construct your client with redis.from_url(url, protocol=2): redis-py 8 defaults to RESP3, which Redis 5 does not support. See the redis-py protocol documentation.

pip install 'fastapi-stream-lease[fastapi]'

The fastapi extra supplies FastAPI for the HTTP helpers; the core package only requires redis.

Try it locally

The runnable SSE example accepts one API key from STREAM_DEMO_TOKEN and uses it as a demo identity. Start Redis, clone this repository, then run:

git clone https://github.com/agustin18/fastapi-stream-lease.git
cd fastapi-stream-lease
pip install -e '.[fastapi]' uvicorn
export STREAM_DEMO_TOKEN=local-secret
export REDIS_URL=redis://localhost:6379/0
uvicorn examples.sse_demo:app

In another terminal, open two streams, then try a third with the same key:

curl -N -H 'X-API-Key: local-secret' http://localhost:8000/stream

The first two requests stream events; the third receives 429 with a Retry-After header until a slot is freed. Replace the demo API key with your application's authenticated user or account ID before production use. Never use an unverified request parameter as the user ID.

FastAPI integration

from fastapi import Depends, FastAPI, Request
from fastapi.responses import StreamingResponse
import redis.asyncio as redis

from fastapi_stream_lease import (
    LeaseConfig,
    StreamLeaseManager,
    StreamLeaseRejected,
    StreamLeaseUnavailable,
)

app = FastAPI()
# Set explicit timeouts so slow Redis calls do not block worker threads
redis_client = redis.from_url(
    "redis://localhost:6379",
    socket_timeout=1.0,
    socket_connect_timeout=1.0,
)
manager = StreamLeaseManager(
    redis_client,
    LeaseConfig(max_per_user=2, max_global=500, lease_seconds=30),
)


async def authenticated_user_id() -> str:
    # Illustrative placeholder: replace with your auth dependency (e.g. API key or JWT sub).
    # For fully runnable code, see examples/sse_demo.py and examples/websocket_demo.py.
    return "user-123"


@app.exception_handler(StreamLeaseRejected)
async def rejected(request: Request, exc: StreamLeaseRejected):
    return exc.as_response()


@app.exception_handler(StreamLeaseUnavailable)
async def unavailable(request: Request, exc: StreamLeaseUnavailable):
    return exc.as_response()


@app.get("/stream")
async def stream(user_id: str = Depends(authenticated_user_id)):
    lease = await manager.acquire(user_id)

    async def events():
        yield "data: first event\n\n"
        # Yield tokens or events here...

    return StreamingResponse(lease.wrap(events()), media_type="text/event-stream")

Close the Redis client in your application's lifespan shutdown handler. Reuse the same StreamLeaseManager and key_prefix across workers that share limits. max_per_user=0 or max_global=0 disables that limit; the global count is unavailable when global tracking is disabled.

For WebSockets, keep the context open for the whole session:

async with manager.lease(user_id) as lease:
    while True:
        message = await websocket.receive_text()
        await websocket.send_text(message)

The manager context and async with lease both renew while open. Handle normal WebSocket disconnects in your route as usual. If renewal fails or the lease expires, StreamLeaseLost interrupts the stream or context. Catch it at the application boundary if you want to record a metric or send an application-specific WebSocket close code. wrap(auto_renew=False) disables automatic renewal; use it only if you renew the lease yourself.

Behavior and limits

• Acquisition, renewal, expiration cleanup, and release use atomic Redis Lua scripts. The keys share a Redis Cluster hash tag, so a user and global limit can be checked in one script.
• Every lease expires after lease_seconds without a successful renewal. wrap() and manager.lease() renew every half interval by default. Transient Redis connection errors trigger fast retries across the remaining lease TTL (Adaptive Grace Period), preventing temporary hiccups from dropping active streams.
• If Redis is unavailable on initial acquisition, StreamLeaseUnavailable (HTTP 503) is raised by default (fail_open=False). Set fail_open=True in LeaseConfig if your application prefers allowing streams during Redis outages (graceful degradation).
• Normal completion or cancellation attempts immediate release. If Redis is unavailable during release, the lease is removed after expiration; cleanup of the key itself uses a longer TTL. An async iterator abandoned without being closed may also hold its slot until expiration. Use contextlib.aclosing() if your own consumer stops iteration early.
• get_active_count(user_id) counts active leases for one identity; get_active_count() counts globally when max_global is enabled. Neither is a historical usage metric.
• All workers sharing limits must use the same key prefix and compatible limit settings. Lease expiration is measured by Redis, avoiding clock differences among application workers.

Production and Operational Guide

• Redis Client Timeouts: Always configure explicit timeouts on your Redis client (e.g. socket_timeout=1.0, socket_connect_timeout=1.0). Without timeouts, an unreachable Redis instance can block asyncio event loop execution indefinitely.
• Fail-Open vs. Fail-Closed Strategy:
  • fail_open=False (Default): Raises StreamLeaseUnavailable (HTTP 503) when Redis is down. Ensures concurrency limits are strictly enforced under all conditions, at the cost of service availability during infrastructure outages.
  • fail_open=True: Automatically grants in-memory fallback leases when Redis encounters network or timeout errors. Keeps streaming endpoints open to users during outages, with the operational cost that limits are not coordinated across workers until Redis recovers. Authentication, authorization, and script syntax errors never fail open.
• Definitive Revocation vs. Network Errors: If Redis explicitly reports that a lease is missing or expired (renew() returning 0), wrap() and lease() cancel the stream immediately to prevent exceeding limits. Transient network disconnects trigger rapid retries until the monotonic lease deadline is reached.
• Recommended Observability Metrics:
  • stream_leases_active (Gauge): Current concurrent streams per worker.
  • stream_lease_rejected_total (Counter): Concurrency rejections (HTTP 429).
  • stream_lease_unavailable_total (Counter): Redis backend outage errors (HTTP 503).
  • stream_lease_lost_total (Counter): Streams terminated mid-flight due to lease expiration or revocation.
• Redis Cluster: All keys use Redis hash tags ({prefix}:user:... and {prefix}:global), guaranteeing user and global sorted sets reside on the same hash slot. As with any multi-key Lua script, test failover behavior in staging under your specific Cluster topology.

Contributing and security

See CONTRIBUTING.md for the local workflow and SECURITY.md for private vulnerability reports. Changes are proposed through pull requests and merged by the maintainer after CI passes. The package is licensed under MIT.

CI checks formatting, lint, types, Redis 5 and 7 behavior, package build, and a minimum of 95% combined line and branch coverage. This is a small beta project; reports from real deployments are especially helpful for documenting operational limits.