Vercel APScheduler integration

Run APScheduler 3.x schedules through delayed Vercel Queue messages, with Redis as the durable job store and lifecycle coordinator.

Configure the scheduler

Use APScheduler's standard Redis job store:

from os import environ

from apscheduler.jobstores.redis import RedisJobStore
from apscheduler.schedulers.blocking import BlockingScheduler
from redis import ConnectionPool

scheduler = BlockingScheduler(
    timezone="UTC",
    jobstores={
        "default": RedisJobStore(
            connection_pool=ConnectionPool.from_url(
                environ["REDIS_URL"],
                socket_connect_timeout=5,
                socket_timeout=5,
            ),
        )
    },
)


@scheduler.scheduled_job(
    "cron",
    hour=4,
    id="cleanup",
    replace_existing=True,
)
def cleanup() -> None: ...

Declare that object as a Python subscriber:

[[tool.vercel.subscribers]]
entrypoint = "scheduler:scheduler"

The Vercel Python builder detects APScheduler before importing the subscriber, installs the integration, and extracts its internal Queue subscriptions from the same registry used by Celery and Dramatiq. Topic names, consumer groups, scheduler IDs, and installation hooks are not application configuration.

No Vercel-specific job store is required. v1 requires exactly one job store, named default, and it must be APScheduler's Redis-backed RedisJobStore. The integration uses that store's configured Redis client for its internal lifecycle coordination. A missing REDIS_URL fails the import with a KeyError, which is intended: there is no implicit localhost fallback.

Set explicit socket timeouts on the connection pool, as shown above. The runtime performs its automatic-activation Redis work around request handling, bounded by a fixed wait; without socket timeouts an unreachable Redis holds that entire bound instead of failing fast.

Automatic activation

Production deployments activate automatically on their first real request. The integration is registered while the application imports, but the Redis transition and first Queue send are deferred until the runtime has installed that request's OIDC credentials. Builds never enqueue messages.

Preview deployments are inactive by default. Opt a project into request-driven preview scheduling with:

[tool.vercel.apscheduler.previews]
enabled = true
idle_timeout = "30m"

Each active Function runtime renews the preview's durable Redis activity deadline on incoming requests, throttled to at most once every five minutes (or one third of a shorter timeout). This is not a background timer and it does not emit periodic Queue messages. If no request renews the deadline:

• a queued start or wake becomes stale before it can run;
• an in-flight wake may finish its current work but cannot publish a successor; and
• the next request creates one new generation and skips occurrences from the inactive interval.

An explicit pause() remains paused across later requests; automatic activation never overrides it. Production scheduling has no idle timeout. In either environment, a deployment that has never received a request cannot start automatically because it has not received request-scoped OIDC credentials.

Start, pause, and resume

On Vercel, the normal APScheduler lifecycle methods operate the durable Queue driver for the deployment executing the call:

scheduler.start()  # idempotently start
scheduler.pause()  # idempotently pause
scheduler.resume()  # idempotently resume

Use these methods when explicit operational control is needed. Call them from an authenticated runtime route or another trusted runtime entrypoint.

from fastapi import FastAPI

from scheduler import scheduler

app = FastAPI()


@app.post("/scheduler/start", status_code=202)
def start_scheduler() -> dict[str, str]:
    scheduler.start()
    return {"state": "running"}


@app.post("/scheduler/pause")
def pause_scheduler() -> dict[str, str]:
    scheduler.pause()
    return {"state": "paused"}


@app.post("/scheduler/resume", status_code=202)
def resume_scheduler() -> dict[str, str]:
    scheduler.resume()
    return {"state": "running"}

There is no separate control object, start token, or public message-publishing API. Each deployment controls only its own scheduler.

Off Vercel, these methods retain their normal APScheduler behavior.

Runtime job changes

The chain sleeps until the next persisted job is due. It emits no idle heartbeat. add_job(), modify_job(), reschedule_job(), pause_job(), resume_job(), and removals update Redis and rearm the one current wake as needed.

Automatic activation establishes the runtime-mutation boundary before the user application handles a production request (or an opted-in preview request). In environments without automatic activation, call scheduler.start() first in each Function instance that changes jobs; before that boundary, add_job() calls are treated as module-level declarations. The call is idempotent:

@app.post("/jobs")
def add_job() -> dict[str, str]:
    scheduler.start()
    scheduler.add_job(
        send_report,
        "date",
        run_date="2026-08-01 09:00:00+00:00",
        id="report-2026-08-01",
    )
    return {"state": "scheduled"}

Job writes and wake rearming happen in one Redis transaction. Raw writes to the APScheduler Redis keys bypass that transaction and are unsupported.

Guarantees

Redis atomically stores one lifecycle generation and one current wake token per scheduler, scoped by environment in production and by deployment in previews. This gives the driver the following guarantees:

• Concurrent or repeated start() calls converge on one start message.
• Only the current wake token can run and reserve one successor.
• pause() durably fences the current generation.
• resume() creates one new generation, even under concurrent calls.
• Rapid pause()/resume() cannot overlap a new generation with an in-flight handler from the old generation.
• Runtime job changes cannot create a second chain.
• Concurrent job execution and mutation cannot overwrite or resurrect a stale job value.
• A crash between reserving and publishing a successor is repaired by a retry.
• Occurrences during a pause are skipped on resume instead of replayed in a catch-up burst.
• Production schedules, dynamically added jobs, and the wake chain survive promotions: one deployment owns the chain at a time, taking ownership syncs the store to that code's declarations (a job deleted from code never runs again, a changed trigger takes effect), and a demoted deployment's touches are inert.
• A wake whose queue message died is presumed lost once it is well past due with no live owner, and republished by the owner.
• Concurrent first requests converge on one automatic generation and one start identity.
• Preview idle expiry fences both claims and successor publication.
• A later preview request creates one new generation; concurrent requests converge on that generation.

The scheduler's durable identity derives from its RedisJobStore jobs_key, so renaming variables or moving modules never orphans state. Two schedulers need distinct jobs_key values; the scheduler_id option pins an identity explicitly.

start() and job mutation calls are durable after they return successfully. If a process dies before returning, an idempotent start() repairs any pending publication, and repeating an interrupted mutation republishes the pending wake even when the retry itself fails on a conflicting job id. With no idle heartbeat, an ambiguous failure while publishing the first wake for a dormant scheduler is repaired by a later start() or mutation call, not by a periodic timer. Unless a job chooses its own misfire_grace_time, occurrences run when their wake arrives, however late; set a finite misfire_grace_time on jobs that must not run late.

These are chain guarantees, not exactly-once job execution. Vercel Queues is at-least-once, so a delivery interrupted after a job's side effect may run that job again. Scheduled work must still be idempotent. A job already running when pause() commits may finish, but it cannot extend the paused chain.

Redis lifecycle state has no TTL. Losing or evicting it would violate reliable pause semantics, so use a durable Redis service rather than an ephemeral cache. Redis failures fail closed: lifecycle calls raise and Queue deliveries retry without running unfenced work.

Backends

Configuring a RedisJobStore selects the Redis backend above. Without one, the integration runs on the Vercel Runtime Cache instead (explicitly: VERCEL_APSCHEDULER_BACKEND=redis|cache). The two differ in what they can guarantee:

Under vercel dev the cache client falls back to per-process memory, which makes the cache backend the zero-infrastructure development mode: the queue-serving process drives the schedule, and chain progress travels in the messages themselves rather than shared state.

v1 restrictions

• APScheduler 3.x only.
• Exactly one Redis-backed job store named default.
• The default inline executor only; custom thread/process executors are rejected.
• Jobs declared in code need explicit stable IDs.
• When the same ID already exists in Redis, declare it with replace_existing=True. scheduled_job() already enables replacement.
• Runtime mutation APIs require prior activation in that Function instance, either automatically on the request or through scheduler.start().
• Job execution is at-least-once.

See SCHEDULER.md for the state machine and failure model. A deployable example is in examples/cleanup.