I'm trying to separate the Read and write DB operations via Flask Sqlalchemy. I'm using binds to connect to the mysql databases. I would want to perform the write operation in Master and Reads from slaves. There does not seem to be an in built way to handle this.
I'm new to python and was surprised that a much needed functionality like this is not pre-built into flask-sqlalchemy already. Any help is appreciated. Thanks
One of which is that Flask-SQLAlchemy has its own API. This adds complexity by having its different methods for ORM queries and models separate from the SQLAlchemy API. Another disadvantage is that Flask-SQLAlchemy makes using the database outside of a Flask context difficult.
Every pool implementation in SQLAlchemy is thread safe, including the default QueuePool . This means that 2 threads requesting a connection simultaneously will checkout 2 different connections. By extension, an engine will also be thread-safe.
Flask-SQLAlchemy Query Returns the Database Class and the Data.
Primary keys are marked with primary_key=True . Multiple keys can be marked as primary keys in which case they become a compound primary key. date and time expressed as Python datetime object.
There is no official support, but you can customize Flask-SQLalchemy session to use master slave connects
from functools import partial
from sqlalchemy import orm
from flask import current_app
from flask_sqlalchemy import SQLAlchemy, get_state
class RoutingSession(orm.Session):
def __init__(self, db, autocommit=False, autoflush=True, **options):
self.app = db.get_app()
self.db = db
self._bind_name = None
orm.Session.__init__(
self, autocommit=autocommit, autoflush=autoflush,
bind=db.engine,
binds=db.get_binds(self.app),
**options,
)
def get_bind(self, mapper=None, clause=None):
try:
state = get_state(self.app)
except (AssertionError, AttributeError, TypeError) as err:
current_app.logger.info(
'cant get configuration. default bind. Error:' + err)
return orm.Session.get_bind(self, mapper, clause)
# If there are no binds configured, use default SQLALCHEMY_DATABASE_URI
if not state or not self.app.config['SQLALCHEMY_BINDS']:
return orm.Session.get_bind(self, mapper, clause)
# if want to user exact bind
if self._bind_name:
return state.db.get_engine(self.app, bind=self._bind_name)
else:
# if no bind is used connect to default
return orm.Session.get_bind(self, mapper, clause)
def using_bind(self, name):
bind_session = RoutingSession(self.db)
vars(bind_session).update(vars(self))
bind_session._bind_name = name
return bind_session
class RouteSQLAlchemy(SQLAlchemy):
def __init__(self, *args, **kwargs):
SQLAlchemy.__init__(self, *args, **kwargs)
self.session.using_bind = lambda s: self.session().using_bind(s)
def create_scoped_session(self, options=None):
if options is None:
options = {}
scopefunc = options.pop('scopefunc', None)
return orm.scoped_session(
partial(RoutingSession, self, **options),
scopefunc=scopefunc,
)
Than the default session will be master
, when you want to select from slave you can call it directly, here the examples:
In your app:
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql:///master'
app.config['SQLALCHEMY_BINDS'] = {
'slave': 'postgresql:///slave'
}
db = RouteSQLAlchemy(app)
Select from master
session.query(User).filter_by(id=1).first()
Select from slave
session.using_bind('slave').query(User).filter_by(id=1).first()
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With