Examples: MySQL named locks and Python context managers

code_with_locking_context.py

"""same logic as in above, but using a locking context manager
"""
from app_services import db_session
from locking_context import named_lock

def single_running_func():
    """this code should never execute in parallel
    """
    with named_lock(db_session, 'my_lock', 5):
        # Do some stuff that should not run in parallel
        pass  # <-- here for syntax correctness

code_with_named_lock.py

from app_services import db_session

def single_running_func():
    """this code should never execute in parallel
    """
    lock = db_session.execute("SELECT GET_LOCK('my_lock', 5)").scalar()
    if not lock:
        raise RuntimeError("Could not obtain lock for 5 seconds")
        
    # Do some stuff that should not run in parallel
    db_session.execute("SELECT RELEASE_LOCK('my_lock')")

locking_context.py

"""locking_context - locking related context managers
"""
import contextlib

@contextlib.contextmanager
def named_lock(db_session, name, timeout):
    """Get a named mysql lock on a DB session
    """
    lock = db_session.execute("SELECT GET_LOCK(:lock_name, :timeout)",
                              {"lock_name": name, "timeout": timeout}).scalar()
    if lock:
        try:
            yield db_session
        finally:
            db_session.execute("SELECT RELEASE_LOCK(:name)", {"name": name})
    else:
        e = "Could not obtain named lock {} within {} seconds".format(
            name, timeout)
        raise RuntimeError(e)