Flask SqlAlchemy read lock the a row - flask-sqlalchemy

Question

I would like to have a code to be able to lock the row and restart the transaction. I only need this code for a specific call.
class Mapper(db.Model):
__tablename__ = 'dns_mappers'
id = db.Column(db.Integer, primary_key=True)
services_id = db.Column(db.Integer, db.ForeignKey('services.id'), nullable=False)
eni = db.Column(db.String(80), unique=False, index=True, nullable=True, default="Primary")
interface_type = db.Column(db.Integer, nullable=True, default=InterfaceType.secondary.value)
name = db.Column(db.String(80), unique=True, nullable=True)
type = db.Column(db.Integer, nullable=True, default=DnsType.route53.value, index=True)
enabled = db.Column(db.Integer, nullable=True, default=0)
instance_id = db.Column(db.String(80), nullable=True, index=True)
created_at = db.Column(db.DateTime(timezone=True), server_default=func.now())
updated_at = db.Column(db.DateTime(timezone=True), onupdate=func.now())
last_assignment_completed_at = db.Column(db.DateTime(timezone=True), onupdate=func.now())
last_released_at = db.Column(db.DateTime(timezone=True), nullable=True)
subnet_id = db.Column(db.String(80), nullable=True, index=True)
zone = db.Column(db.String(80), nullable=True, index=True)
dns_group_id = db.Column(db.Integer, nullable=True, default=None, index=True)
#---
def available_by_service_id_dnstype_zone_subnet_group_id(user):
return Mapper.query.filter_by(services_id=service_id, enabled=EnableStatus.enabled.value, type=type,
zone=zone,
subnet_id=subnet_id,
interface_type=interface_type,
dns_group_id=dns_group_id) \
.order_by(DnsMapper.created_at.asc(), DnsMapper.updated_at.asc()).limit(1).with_for_update().one()
However, I am getting dead-lock expectations. Not sure how to avoid this lock.
I am getting a sql-alchemy deadlock exception.
EDIT-2
Documenting the exception received:
(pymysql.err.OperationalError) (1213, 'Deadlock found when trying to get lock; try restarting transaction')

Answer 1

Finally, I think I have found a basic solution. Based on https://dev.mysql.com/doc/refman/8.0/en/innodb-deadlocks.html and suggestion from Dan Grossman on https://stackoverflow.com/questions/6925769/restarting-transaction-in-mysql-after-deadlock. I think I should write a wrapper; however, this is the only place in my code where I need this behavior.
MAXIMUM_RETRY_ON_DEADLOCK=5
def available_by_service_id_dnstype_zone_subnet_group_id(user):
attempt_count = 0
lock_messages_error = ['Deadlock found', 'Lock wait timeout exceeded']
while attempt_count < MAXIMUM_RETRY_ON_DEADLOCK:
try:
return Mapper.query.filter_by(services_id=service_id, enabled=EnableStatus.enabled.value, type=type,
zone=zone,
subnet_id=subnet_id,
interface_type=interface_type,
dns_group_id=dns_group_id) \
.order_by(DnsMapper.created_at.asc(), DnsMapper.updated_at.asc()).limit(1).with_for_update().one()
except OperationalError as e:
if any(msg in str(e) for msg in lock_messages_error) \
and attempt_count <= MAXIMUM_RETRY_ON_DEADLOCK:
print('Deadlock detected. Trying sql transaction once more. Attempts count: %s'
% (attempt_count + 1))
logger.error('Deadlock detected. Trying sql transaction once more. Attempts count: %s'
% (attempt_count + 1))
time.sleep(1)
db.session.rollback()
db.session.close()
else:
raise
attempt_count += 1
if attempt_count >= MAXIMUM_RETRY_ON_DEADLOCK:
raise Exception("Deadlock issue retried :{}".format(MAXIMUM_RETRY_ON_DEADLOCK))