问题
tornado中使用sqlalchemy遇到一个问题，接口在并发的时候，sqlalchemy会报各种错误。
解决办法
原因是sqlalchemy用sessionmaker直接建立的session本身就不是线程安全的，怎么保证线程安全呢，一般官方推荐用scoped_session有范围的session，来封装
class scoped_session(object):def __init__(self, session_factory, scopefunc=None):self.session_factory = session_factoryif scopefunc:self.registry = ScopedRegistry(session_factory, scopefunc)else:self.registry = ThreadLocalRegistry(session_factory)def __call__(self, **kw):if kw:if self.registry.has():raise sa_exc.InvalidRequestError("Scoped session is already present; ""no new arguments may be specified.")else:sess = self.session_factory(**kw)self.registry.set(sess)return sesselse:return self.registry()def remove(self):if self.registry.has():self.registry().close()self.registry.clear()
scoped_session包裹了原来的sessionmaker，实例化了一个TreadLocalRegistry对象，scoped_session()调用__call__ -> self.registry() -> TreadLocalRegistry.__call__方法,
先从本地线程取出value，如果异常则用sessionmaker创建一个session，并放在这个线程变量里面，同一个线程进来则使用同一个session
class ThreadLocalRegistry(ScopedRegistry):"""A :class:`.ScopedRegistry` that uses a ``threading.local()``variable for storage."""def __init__(self, createfunc):self.createfunc = createfuncself.registry = threading.local()def __call__(self):try:return self.registry.valueexcept AttributeError:val = self.registry.value = self.createfunc()return valdef has(self):return hasattr(self.registry, "value")d