python - SQLAlchemy 线程池执行器 "Too many clients"

Question

我用这种逻辑编写了一个脚本，以便在生成许多记录时将它们插入到 PostgreSQL 表中。

#!/usr/bin/env python3
import asyncio
from concurrent.futures import ProcessPoolExecutor as pool
from functools import partial

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base


metadata = sa.MetaData(schema='stackoverflow')
Base = declarative_base(metadata=metadata)


class Example(Base):
    __tablename__ = 'example'
    pk = sa.Column(sa.Integer, primary_key=True)
    text = sa.Column(sa.Text)


sa.event.listen(Base.metadata, 'before_create',
    sa.DDL('CREATE SCHEMA IF NOT EXISTS stackoverflow'))

engine = sa.create_engine(
    'postgresql+psycopg2://postgres:password@localhost:5432/stackoverflow'
)
Base.metadata.create_all(engine)
session = sa.orm.sessionmaker(bind=engine, autocommit=True)()


def task(value):
    engine.dispose()
    with session.begin():
        session.add(Example(text=value))


async def infinite_task(loop):
    spawn_task = partial(loop.run_in_executor, None, task)
    while True:
        await asyncio.wait([spawn_task(value) for value in range(10000)])


def main():
    loop = asyncio.get_event_loop()
    with pool() as executor:
        loop.set_default_executor(executor)
        asyncio.ensure_future(infinite_task(loop))
        loop.run_forever()
        loop.close()


if __name__ == '__main__':
    main()

这段代码工作得很好，创建了一个进程池，进程数与我的 CPU 内核数一样多，并且一直愉快地运行下去。我想看看线程如何与进程进行比较，但我找不到一个有效的例子。以下是我所做的更改:

from concurrent.futures import ThreadPoolExecutor as pool

session_maker = sa.orm.sessionmaker(bind=engine, autocommit=True)
Session = sa.orm.scoped_session(session_maker)


def task(value):
    engine.dispose()
    # create new session per thread
    session = Session()
    with session.begin():
        session.add(Example(text=value))
    # remove session once the work is done
    Session.remove()

此版本运行一段时间后出现大量“客户端过多”异常:

sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) FATAL:  sorry, too many clients already

我错过了什么？

Answer 1

事实证明，问题出在 engine.dispose()，用 Mike Bayer (zzzeek) 的话来说就是“让 PG 连接处于打开状态以供垃圾收集。”

来源:https://groups.google.com/forum/#!topic/sqlalchemy/zhjCBNebnDY

所以更新后的 task 函数如下所示:

def task(value):
    # create new session per thread
    session = Session()
    with session.begin():
        session.add(Example(text=value))
    # remove session object once the work is done
    session.remove()