如何在Flask Sqlalchemy中分离主从机(DB读/写)

Question

我正在尝试通过Flask Sqlalchemy来分离读写DB操作。我正在使用绑定来连接mysql数据库。我想在Master中执行写操作，从slaves中读取。似乎没有一种内置的方法来处理这个问题。

我是python的新手，令我惊讶的是，像这样一个非常需要的功能还没有预先构建到flask-sqlalchemy中。任何帮助都是非常感谢的。谢谢

Answer 1

没有官方支持，但您可以自定义Flask-SQLalchemy会话以使用主从连接

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,
        )

那么默认的会话将是master，当你想从slave中选择时，你可以直接调用它，这里有示例：

在你的应用中：

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)

从母版中选择

session.query(User).filter_by(id=1).first() 

从从设备中选择

session.using_bind('slave').query(User).filter_by(id=1).first() 

Answer 2

这是文档http://packages.python.org/Flask-SQLAlchemy/binds.html