Dash Background Callbacks与SQLAlchemy兼容异常求助

问题 #

使用Dash 2.6.1，尝试让Background Callbacks与SQLAlchemy协同工作。在官方教程的第一个示例中添加from sqlalchemy import create_engine语句后，点击网页上的Run Job!按钮触发sqlite3.OperationalError: disk I/O error错误。

代码示例 #

from sqlalchemy import create_engine # This breaks Dash.

import time
import os

import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html

if 'REDIS_URL' in os.environ:
    # Use Redis & Celery if REDIS_URL set as an env variable
    from celery import Celery
    celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
    background_callback_manager = CeleryManager(celery_app)

else:
    # Diskcache for non-production apps when developing locally
    import diskcache
    cache = diskcache.Cache("./cache")
    background_callback_manager = DiskcacheManager(cache)

app = dash.Dash(__name__)

app.layout = html.Div(
    [
        html.Div([html.P(id="paragraph_id", children=["Button not clicked"])])],
        html.Button(id="button_id", children="Run Job!"),
    ]
)

@dash.callback(
    output=Output("paragraph_id", "children"),
    inputs=Input("button_id", "n_clicks"),
    background=True,
    manager=background_callback_manager,
)
def update_clicks(n_clicks):
    time.sleep(2.0)
    return [f"Clicked {n_clicks} times"]


if __name__ == "__main__":
    app.run_server(debug=True)

报错信息 #

Dash is running on http://127.0.0.1:8050/

 * Serving Flask app 'main'
 * Debug mode: on
Process Process-3:
Traceback (most recent call last):
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/multiprocess/process.py", line 315, in _bootstrap
    self.run()
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/multiprocess/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/dash/long_callback/managers/diskcache_manager.py", line 179, in job_fn
    cache.set(result_key, user_callback_output)
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/diskcache/core.py", line 796, in set
    with self._transact(retry, filename) as (sql, cleanup):
  File "/Users/one/.pyenv/versions/3.9.9/lib/python3.9/contextlib.py", line 119, in __enter__
    return next(self.gen)
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/diskcache/core.py", line 710, in _transact
    sql = self._sql
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/diskcache/core.py", line 648, in _sql
    return self._con.execute
  File "/Users/one/.pyenv/versions/live/lib/python3.9/site-packages/diskcache/core.py", line 623, in _con
    con = self._local.con = sqlite3.connect(
sqlite3.OperationalError: disk I/O error

解决方案 #

这不是Dash的Bug，根源是SQLAlchemy导入后会修改全局sqlite3的连接配置，导致diskcache使用的sqlite3连接出现资源冲突。可通过以下方式解决：

• 延迟导入SQLAlchemy：不在全局作用域导入create_engine，而是在实际需要使用它的函数内部导入。比如把SQLAlchemy相关操作放在回调函数里：

  def update_clicks(n_clicks):
      from sqlalchemy import create_engine
      # 这里执行SQLAlchemy相关逻辑
      time.sleep(2.0)
      return [f"Clicked {n_clicks} times"]
  

• 调整diskcache的sqlite连接参数：初始化diskcache时，指定SQLiteDisk并设置超时时间，避免连接阻塞：

  import diskcache
  from diskcache import SQLiteDisk
  cache = diskcache.Cache("./cache", disk=SQLiteDisk, timeout=30)
  

• 切换到Celery+Redis后台：如果有条件部署Redis，直接使用CeleryManager替代DiskcacheManager，彻底避开sqlite3的冲突问题。

内容的提问来源于stack exchange，提问作者Jon