python endpoint starting a thread with locking

Question

I'm using FASTApi and trying to implement an endpoint, which starts a job. Once the job is started, the endpoint shall be "locked" until the previous job finished. So far its implemented like this:

myapp.lock = threading.Lock()

@myapp.get("/jobs")
def start_job(some_args):
    if myapp.lock.acquire(False):
        th = threading.Thread(target=job,kwargs=some_args)
        th.start()
        return "Job started"
    else:
        raise HTTPException(status_code=400,detail="Job already running.")

So, when the job gets started, a thread will be created using the method job:

def job(some_args):
    try:
        #doing some stuff, creating objects and writing to files
    finally:
        myapp.lock.release()

So far so good, the endpoint is working, starts a job and locks as long as the job is running. But my problem is that the thread is still alive although the job "finished" and released the lock. I was hoping that the thread would close itself after execution. Maybe the problem is that myapp is keeping it alive? How can I stop it?

Answer 1

I figured out this kind of solution:

myapp.lock = False

@myapp.get("/jobs")
async def start_job(some_args, background_tasks: BackgroundTasks):
    if not myapp.lock:
        background_tasks.add_task(job, some_args)
        return "Job started"
    else:
        raise HTTPException(status_code=400,detail="Job already running.")

def job(some_args):
    try:
        myapp.lock = True
        #doing some stuff, creating objects and writing to files
    finally:
        myapp.lock = False

Answer 2

I have faced the similar issue. I would like to block the request that want to change the state of a specific object (an object is a simulation in my case). I use PostgreSQL and have created the following

1. Table

    create table simulation_lock (simulation_id INTEGER PRIMARY KEY, is_locked BOOLEAN DEFAULT False);

2. Lock context:

from starlette.exceptions import HTTPException

from fmr.database import SessionContext


class SimulationLock:

    def __init__(self, simulation_id: int):
        self._simulation_id = simulation_id
        self._was_acquired = False

    def __enter__(self):
        with SessionContext() as session:
            query = f"update simulation_lock set is_locked = true where " \
                    f"simulation_id = {self._simulation_id} " \
                    f"and is_locked = false returning is_locked;"
            result = session.execute(query).first()
            session.commit()

            if result is not None and result[0] is True:
                self._was_acquired = True
            else:
                raise HTTPException(
                    status_code=500, detail="Already some operations on the "
                                            "simulation are performed.")

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self._was_acquired:
            with SessionContext() as session:
                query = f"update simulation_lock set is_locked = false " \
                        f"where simulation_id = {self._simulation_id};"
                session.execute(query)
                session.commit()

3. Endpoint:

@scenario_router.post(
    "/run"
)
async def run(
        run_scenario_request: RunScenarioRequest,
):
    with SimulationLock(run_scenario_request.scenario_id):
        run_scenario(run_scenario_request)