quebec 0.3.8

Quebec is a simple background task queue for processing asynchronous tasks.

Quebec

Quebec is a simple background task queue for processing asynchronous tasks. The name is derived from the NATO phonetic alphabet for "Q", representing "Queue".

This project is inspired by Solid Queue.

Warning: This project is in early development stage. Not recommended for production use.

Why Quebec?

• Simplified Architecture: No dependencies on Redis or message queues
• Database-Powered: Leverages RDBMS capabilities for complex task queries and management
• Rust Implementation: High performance and safety with Python compatibility
• Framework Agnostic: Works with asyncio, Trio, threading, SQLAlchemy, Django, FastAPI, etc.

Features

• Scheduled tasks
• Recurring tasks
• Concurrency control
• Per-queue concurrency limits
• Rate limiting
• Exclusive (stop-the-world) jobs
• Multi-process (fork) mode
• Memory-based worker recycling
• Web dashboard
• Automatic retries
• Signal handling & graceful restart
• Lifecycle hooks

Control Plane

Built-in web dashboard for monitoring jobs, queues, and workers in real-time.

Database Support

• SQLite
• PostgreSQL
• MySQL

Quick Start

Module Runner (Recommended)

Define jobs in a package:

# jobs/email_job.py
import quebec

class EmailJob(quebec.BaseClass):
    queue_as = "default"

    def perform(self, to, subject):
        self.logger.info(f"Sending email to {to}: {subject}")

Export them in __init__.py:

# jobs/__init__.py
from .email_job import EmailJob

Run with python -m quebec:

DATABASE_URL=sqlite:///demo.db?mode=rwc python -m quebec jobs

All configuration via QUEBEC_* environment variables — no boilerplate entry script needed.

Script Mode

For more control, use Quebec directly in a script:

import logging
from pathlib import Path
from quebec.logger import setup_logging

setup_logging(level=logging.DEBUG)

import quebec

db_path = Path('demo.db')
qc = quebec.Quebec(f'sqlite://{db_path}?mode=rwc')


@qc.register_job
class FakeJob(quebec.BaseClass):
    def perform(self, *args, **kwargs):
        self.logger.info(f"Processing job {self.id}: args={args}, kwargs={kwargs}")


if __name__ == "__main__":
    # Enqueue a job (qc is inferred from @qc.register_job)
    FakeJob.perform_later(123, foo='bar')

    # Start Quebec (handles signal, spawns workers, runs main loop)
    qc.run(
        create_tables=not db_path.exists(),
        control_plane='127.0.0.1:5006',  # Optional: web dashboard
    )

Or run the quickstart script directly:

curl -O https://raw.githubusercontent.com/ratazzi/quebec/refs/heads/master/quickstart.py
uv run quickstart.py

Auto-Discovering Jobs

If your jobs are organized in a package (e.g. app.jobs.*), call Quebec.discover_jobs() instead of decorating each class with @qc.register_job or calling qc.register_job_class(...) one by one:

# app/jobs/cleanup.py
class CleanupJob(quebec.BaseClass):
    def perform(self, *args, **kwargs): ...

# main.py
qc = quebec.Quebec(dsn)
qc.discover_jobs("app.jobs", "worker.tasks")   # recursively scans each
qc.run()

discover_jobs takes one or more dotted package paths as positional arguments (varargs) — no need to wrap a single package in a list.

discover_jobs(*packages, recursive=True, on_error="raise"):

• Registers every BaseClass subclass whose __module__ falls under one of the given packages. Classes imported from elsewhere (e.g. from some.lib import JobMixin) are ignored.
• Raises ValueError if two discovered classes share the same __qualname__, since Quebec's worker registry is keyed by qualname and the later registration would otherwise silently replace the earlier one.
• on_error="raise" (default) propagates submodule ImportError. Pass on_error="warn" to emit a RuntimeWarning and keep scanning — useful when a package contains optional-integration modules that may fail to import in some environments. The top-level package is always imported strictly.

Multiple Quebec Instances

Quebec is designed for one instance per process. Registering a job class (via @qc.register_job, qc.register_job_class, or qc.discover_jobs) binds it to that Quebec instance, so MyJob.perform_later(...) shorthand routes to the binding. If a process holds more than one Quebec instance and registers the same job class to each, the most recent registration wins — pass the target instance explicitly to disambiguate:

MyJob.perform_later(qc2, arg1)                  # route to qc2
MyJob.set(queue='critical').perform_later(qc2, arg1)

qc.run() Options

Parameter	Type	Default	Description
create_tables	bool	False	Create database tables (requires DDL permissions)
control_plane	str	None	Web dashboard address, e.g. '127.0.0.1:5006'
spawn	list[str]	None	Components to spawn: ['worker', 'dispatcher', 'scheduler']. None = all

Recommended: configure worker thread count in queue.yml via workers.threads. If you need a one-off override, Quebec(..., worker_threads=3) is also supported.

Multi-Process Mode (fork supervisor)

By default qc.run() runs all components as threads in a single process. To scale across CPU cores, set QUEBEC_SUPERVISOR=1 to fork a pool of child processes instead:

QUEBEC_SUPERVISOR=1 python -m quebec your.jobs

# queue.yml (under your environment, e.g. production:)
workers:
  - queues: "*"
    threads: 5
    processes: 4        # fork 4 worker processes
dispatchers:
  - polling_interval: 1
    processes: 1        # fork 1 dispatcher process

The supervisor forks workers[].processes worker children and dispatchers[].processes dispatcher children, each taking its config from the matching yml entry, and reforks any child that dies (matching Solid Queue's process model). Fork mode is opt-in via the env var so an existing config with processes set doesn't silently switch process model on upgrade; spawn is ignored in this mode. Outside supervisor mode the processes keys are ignored and Quebec uses the single-process threaded runtime.

Force Queue Override (multi-branch development)

Set QUEBEC_FORCE_OVERRIDE_QUEUE to pin every enqueue and consumption to one queue — handy when several development branches share a single database:

QUEBEC_FORCE_OVERRIDE_QUEUE=branch_x python -m quebec your.jobs

Every enqueue path rewrites queue_name to this value (ignoring whatever the class, call site, or scheduler specified), and the worker only consumes that queue — so jobs enqueued by one branch are never picked up by another. URL-hostile characters in the name are sanitized to -.

Transactional Enqueue

[!IMPORTANT] Enqueuing is not part of your database transaction — even on the same database. Quebec's enqueue runs through the Rust engine on its own connection pool, completely separate from your Python connection (SQLAlchemy / Django / psycopg). There is no way to atomically commit a business write and a job enqueue together.

This is the deliberate cost of keeping the engine fully decoupled from your ORM and connection — the upside is that Quebec drags no Python database dependencies into your app, but it means the enqueue cannot join your transaction. Two failure windows follow:

• The business transaction commits but the enqueue fails → the job is lost.
• The enqueue commits but the business transaction rolls back → the job runs against missing or stale data.

Recommendations:

• Enqueue after your business transaction commits. This removes the worse direction — a job running for a write that was rolled back.
• Make jobs idempotent and tolerant of data that may not be visible yet; lean on retries.
• If you genuinely need atomicity, use a transactional outbox: write an outbox row inside your own transaction (business + outbox commit atomically), then relay it into a real job (at-least-once delivery).

Delayed Jobs

from datetime import timedelta

# Run after 1 hour
FakeJob.set(wait=3600).perform_later(arg1)

# Run at specific time
FakeJob.set(wait_until=tomorrow_9am).perform_later(arg1)

# Override queue and priority
FakeJob.set(queue='critical', priority=1).perform_later(arg1)

Automatic Retries

from datetime import timedelta

class PaymentJob(quebec.BaseClass):
    retry_on = [
        quebec.RetryStrategy(
            (ConnectionError, TimeoutError),
            wait=timedelta(seconds=30),
            attempts=3,
        ),
        quebec.RetryStrategy(
            (ValueError,),
            wait=timedelta(seconds=5),
            attempts=1,
            # Called once retries are exhausted; receives (job, error).
            handler=lambda job, error: notify_admin(error),
        ),
    ]

    def perform(self, order_id):
        process_payment(order_id)

Multiple RetryStrategy entries can target different exception types with independent wait/attempts. The optional handler fires only when a strategy's attempts are exhausted (mirroring ActiveJob's retry_on ... do |job, error| block) and is called with (job, error). discard_on and rescue_from handlers use the same (job, error) signature.

Concurrency Control

Limit how many jobs with the same key can run simultaneously:

class ReportJob(quebec.BaseClass):
    concurrency_limit = 3          # max 3 concurrent executions per key
    concurrency_duration = 120     # semaphore TTL in seconds

    def concurrency_key(self, account_id, **kwargs):
        return str(account_id)     # final key: "ReportJob/123"

    def perform(self, account_id):
        generate_report(account_id)

The actual concurrency key is "ClassName/key" (e.g. "ReportJob/123"), so different job classes never conflict. When the limit is reached, new jobs are blocked until a slot becomes available. The concurrency_duration acts as a safety TTL — the semaphore is released automatically if a worker crashes.

Rate Limiting (experimental)

Cap how many jobs run within a sliding time window, scoped per key:

from datetime import timedelta

class ApiCallJob(quebec.BaseClass):
    rate_limit_max = 5                          # at most 5 runs...
    rate_limit_duration = timedelta(seconds=2)  # ...per rolling 2-second window
    rate_limit_on_throttle = quebec.RateLimitConflict.Reschedule  # default

    def rate_limit_key(self, region="us", **kwargs):
        return region                           # bucket key: "ApiCallJob/us"

    def perform(self, region="us"):
        call_external_api(region)

Like concurrency control, the bucket is "ClassName/key", and rate_limit_key defaults to the class name when not overridden. rate_limit_duration must be a datetime.timedelta of at least one second. When the window is exhausted, rate_limit_on_throttle decides what happens: Reschedule (the default) pushes the job to a later run, while Discard drops it.

Exclusive Jobs

Let an occasional memory-heavy job own the whole worker process while it runs:

class RebuildSearchIndexJob(quebec.BaseClass):
    exclusive = True

    def perform(self):
        rebuild_index()                         # runs alone on this worker

When an exclusive job is claimed, the worker stops claiming new jobs, waits for any in-flight siblings to finish, then runs the exclusive job by itself before resuming normal claiming. The scope is the current worker process — it does not coordinate across separate worker processes; pair it with concurrency_limit = 1 and a concurrency_key if you also need cluster-wide single-instance execution.

Graceful Restart (quiet-then-exit)

Drain in-flight work and exit on a quiet signal, for zero-downtime rolling restarts:

qc = quebec.Quebec(database_url="...", quiet_then_exit=True)
qc.run()

Sending SIGUSR1 (or SIGTSTP) puts the worker into quiet mode: it stops claiming new jobs but keeps running until every in-flight job finishes, then exits cleanly — with no time limit (unlike the SIGTERM path, which is bounded by shutdown_timeout). The usual flow is: signal the old instance quiet, start a new instance, and the old one exits once drained. Opt-in (default off), and standalone-only — under the fork supervisor a self-exited child would just be reforked, so use a supervisor-level rolling restart there instead. Also settable via QUEBEC_QUIET_THEN_EXIT=1.

Memory-Based Worker Recycling

Long-lived Python workers tend to hold onto RSS the interpreter never returns to the OS. Quebec can recycle a bloated worker by draining it and exiting with a dedicated code, leaving the actual restart to your process supervisor. It is configured by environment variables — there is no in-process restart:

QUEBEC_WORKER_MAX_RSS_MB=512                  # soft limit; unset = disabled
QUEBEC_WORKER_MEMORY_RECYCLE_CONFIRMATIONS=3  # consecutive over-limit samples before recycling (default)
QUEBEC_WORKER_MEMORY_CHECK_INTERVAL=5s        # how often RSS is sampled (default)

When a worker's RSS stays above the limit for that many consecutive samples, it enters quiet mode, stops claiming, drains its in-flight jobs (no time limit), and exits with code 75 — the planned-recycle code. The supervisor then relaunches a fresh process. Under the built-in fork supervisor (QUEBEC_SUPERVISOR=1) this refork is automatic; under systemd, Restart=on-failure relaunches the worker after the non-zero recycle exit:

# /etc/systemd/system/quebec-worker.service
[Service]
ExecStart=/usr/bin/python -m quebec your.jobs
Environment=QUEBEC_DATABASE_URL=postgresql://localhost/myapp
Environment=QUEBEC_WORKER_MAX_RSS_MB=512
Restart=on-failure

[Install]
WantedBy=multi-user.target

Exit code 75 is non-zero, so Restart=on-failure treats the planned recycle as a failure and relaunches the worker. If you'd rather not have planned recycles show up as failures (in systemctl status or the start-limit counter), add SuccessExitStatus=75 together with RestartForceExitStatus=75 — the former keeps 75 out of the failure tally, the latter still forces the restart.

Per-Queue Concurrency (experimental)

Cap how many jobs run concurrently across the cluster for specific queues, independent of per-class concurrency_key:

qc = quebec.Quebec(
    database_url="...",
    experimental_queue_concurrency={"reports": 2, "exports": 1},
)
qc.run()

Each listed queue acquires a queue:<name> semaphore at claim time; queues not present are unlimited. Useful for isolating a misbehaving queue during remediation. Naming and semantics are experimental and may change.

TLS Configuration (PostgreSQL)

Quebec links sqlx against rustls + webpki-roots. Public CAs (AWS RDS, Neon, Google Cloud SQL, Supabase, etc.) are trusted out of the box — no OS trust store is consulted.

Pass libpq-style SSL options as Quebec(...) kwargs, as DSN query params, or via QUEBEC_SSL* environment variables:

qc = quebec.Quebec(
    "postgresql://user:pass@host:5432/db",
    sslmode="verify-full",             # or QUEBEC_SSLMODE
    sslrootcert="/etc/ssl/certs/ca.pem",  # internal CAs only
)

Priority is kwargs > env > DSN query. Passing any ssl* kwarg/env against a non-postgres URL raises ValueError.

sslmode	Transport	Certificate verification	Hostname verification
disable	plaintext	—	—
prefer	TLS if offered, else plaintext	—	—
require	TLS (fails if unsupported)	— (accepts any cert)	—
verify-ca	TLS	CA-signed	—
verify-full	TLS	CA-signed	hostname matches CN/SAN

For public CAs, verify-full works zero-config. Use sslrootcert for internal/self-signed CAs. sslcert + sslkey enable client certificate (mTLS) auth.

sslmode=allow is rejected with a ValueError. Upstream sqlx-postgres 0.8 treats allow identically to disable (plaintext, marked FIXME in the driver); to avoid a silent downgrade, Quebec refuses it. Use prefer for opportunistic TLS, or require/verify-* to enforce it.

Note: some managed Postgres services (e.g. Neon) terminate TLS at a proxy layer. In those cases pg_stat_ssl.ssl may report false because the backend sees plaintext from the proxy — not the client.

Lifecycle Hooks

Quebec provides several lifecycle hooks that you can use to execute code at different stages of the application lifecycle:

• @qc.on_start: Called when Quebec starts
• @qc.on_stop: Called when Quebec stops
• @qc.on_worker_start: Called when a worker starts
• @qc.on_worker_stop: Called when a worker stops
• @qc.on_shutdown: Called during graceful shutdown

These hooks are useful for:

• Initializing resources
• Cleaning up resources
• Logging application state
• Monitoring worker lifecycle
• Graceful shutdown handling