基于scoped_session实现线程安全

一、基于scoped_session实现线程安全

from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session
from models import Users

engine = create_engine("mysql+pymysql://root:123@127.0.0.1:3306/db_flask", max_overflow=0, pool_size=5)
Session = sessionmaker(bind=engine)

"""
# 线程安全，基于本地线程实现每个线程用同一个session
# 特殊的：scoped_session中有原来方法的Session中的一下方法：

public_methods = (
    '__contains__', '__iter__', 'add', 'add_all', 'begin', 'begin_nested',
    'close', 'commit', 'connection', 'delete', 'execute', 'expire',
    'expire_all', 'expunge', 'expunge_all', 'flush', 'get_bind',
    'is_modified', 'bulk_save_objects', 'bulk_insert_mappings',
    'bulk_update_mappings',
    'merge', 'query', 'refresh', 'rollback',
    'scalar'
)
"""
#scoped_session类并没有继承Session,但是却又它的所有方法
session = scoped_session(Session)
# ############# 执行ORM操作 #############
obj1 = Users(name="randy")
session.add(obj1)

# 提交事务
session.commit()
# 关闭session
session.close()

二、底层实现原理

2.1 scoped_session进入类

from sqlalchemy.orm import scoped_session
session = scoped_session(Session)

for meth in Session.public_methods:
    setattr(scoped_session, meth, instrument(meth))
    
public_methods = (
        "__contains__",
        "__iter__",
        "add",
        "add_all",
        "begin",
        "begin_nested",
        "close",
        "commit",
        "connection",
        "delete",
        "execute",
        "expire",
        "expire_all",
        "expunge",
        "expunge_all",
        "flush",
        "get_bind",
        "is_modified",
        "bulk_save_objects",
        "bulk_insert_mappings",
        "bulk_update_mappings",
        "merge",
        "query",
        "refresh",
        "rollback",
        "scalar",
    )

1. 首先进入点击scoped_session进入scoped_session类查看如下内容

   for meth in Session.public_methods:
       setattr(scoped_session, meth, instrument(meth))
   

   

   为scoped_sesion类设置属性

2. public_methods中内容:

   这是在Session类中的内容

   public_methods = (
           "__contains__",
           "__iter__",
           "add",
           "add_all",
           "begin",
           "begin_nested",
           "close",
           "commit",
           "connection",
           "delete",
           "execute",
           "expire",
           "expire_all",
           "expunge",
           "expunge_all",
           "flush",
           "get_bind",
           "is_modified",
           "bulk_save_objects",
           "bulk_insert_mappings",
           "bulk_update_mappings",
           "merge",
           "query",
           "refresh",
           "rollback",
           "scalar",
       )
   

3. setattr(scoped_session, meth, instrument(meth))这句中的instrument是一个闭包函数

   改闭包函数中实现了对数据库中具体操作

   

4. self.registry()中实现如下,为其实现了一个线程

   class scoped_session(object):
     
       session_factory = None 
   
       def __init__(self, session_factory, scopefunc=None):
         
           self.session_factory = session_factory
   
           if scopefunc:
               self.registry = ScopedRegistry(session_factory, scopefunc)
           else:
               self.registry = ThreadLocalRegistry(session_factory)
   
   

   线程内容如下，最终利用线程中的local()模块中实现线程的安全，也就是操作数据的安全：

   class ThreadLocalRegistry(ScopedRegistry):
   
   
       def __init__(self, createfunc):
           self.createfunc = createfunc
           self.registry = threading.local()
   

总结：

• session = scoped_session(Session)这个session是那一个操作过来就是哪一个操作，只操作自己对数据的操作，从而保证了线程的安全