Server closed the connection unexpectedly

RylanSchaeffer · June 22, 2022, 8:08pm

I’m running a Flask backend service, Postgres database and a React frontend service. Locally, these three work together just fine. When I try deploying to Render, I get the following backend logs, which suggest something with the database is going wrong. However, I don’t know what. Can someone help me understand what’s going wrong and why?

Jun 22 12:59:52 PM  Traceback (most recent call last):
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1820, in _execute_context
Jun 22 12:59:52 PM      cursor, statement, parameters, context
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 732, in do_execute
Jun 22 12:59:52 PM      cursor.execute(statement, parameters)
Jun 22 12:59:52 PM  psycopg2.OperationalError: server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.
Jun 22 12:59:52 PM  server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.
Jun 22 12:59:52 PM  server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.
Jun 22 12:59:52 PM  
Jun 22 12:59:52 PM  
Jun 22 12:59:52 PM  The above exception was the direct cause of the following exception:
Jun 22 12:59:52 PM  
Jun 22 12:59:52 PM  Traceback (most recent call last):
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/flask/app.py", line 2077, in wsgi_app
Jun 22 12:59:52 PM      response = self.full_dispatch_request()
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/flask/app.py", line 1525, in full_dispatch_request
Jun 22 12:59:52 PM      rv = self.handle_user_exception(e)
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/flask_cors/extension.py", line 165, in wrapped_function
Jun 22 12:59:52 PM      return cors_after_request(app.make_response(f(*args, **kwargs)))
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/flask/app.py", line 1523, in full_dispatch_request
Jun 22 12:59:52 PM      rv = self.dispatch_request()
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/flask/app.py", line 1509, in dispatch_request
Jun 22 12:59:52 PM      return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
Jun 22 12:59:52 PM    File "/opt/render/project/src/app.py", line 344, in signup
Jun 22 12:59:52 PM      user = User.lookup(username=username)
Jun 22 12:59:52 PM    File "/opt/render/project/src/src/db/models.py", line 98, in lookup
Jun 22 12:59:52 PM      return cls.query.filter_by(username=username).one_or_none()
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 2845, in one_or_none
Jun 22 12:59:52 PM      return self._iter().one_or_none()
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 2906, in _iter
Jun 22 12:59:52 PM      execution_options={"_sa_orm_load_options": self.load_options},
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1712, in execute
Jun 22 12:59:52 PM      result = conn._execute_20(statement, params or {}, execution_options)
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1631, in _execute_20
Jun 22 12:59:52 PM      return meth(self, args_10style, kwargs_10style, execution_options)
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/sql/elements.py", line 326, in _execute_on_connection
Jun 22 12:59:52 PM      self, multiparams, params, execution_options
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1508, in _execute_clauseelement
Jun 22 12:59:52 PM      cache_hit=cache_hit,
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1863, in _execute_context
Jun 22 12:59:52 PM      e, statement, parameters, cursor, context
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2044, in _handle_dbapi_exception
Jun 22 12:59:52 PM      sqlalchemy_exception, with_traceback=exc_info[2], from_=e
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 208, in raise_
Jun 22 12:59:52 PM      raise exception
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1820, in _execute_context
Jun 22 12:59:52 PM      cursor, statement, parameters, context
Jun 22 12:59:52 PM    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 732, in do_execute
Jun 22 12:59:52 PM      cursor.execute(statement, parameters)
Jun 22 12:59:52 PM  sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.
Jun 22 12:59:52 PM  server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.
Jun 22 12:59:52 PM  server closed the connection unexpectedly
Jun 22 12:59:52 PM      This probably means the server terminated abnormally
Jun 22 12:59:52 PM      before or while processing the request.

RylanSchaeffer · June 22, 2022, 9:01pm

I can find related problems , but I don’t know whether they’re relevant to Render. How can I find out whether this error is caused by Render or something else?

al_ps · June 23, 2022, 8:35pm

Hi Rylan,

Thanks for reaching out.

When we’ve seen similar SQLAlchemy issues in the past, adding an env var of SQLALCHEMY_ENABLE_POOL_PRE_PING with a value of true seems to resolve it. Could you give that a try and see if it helps?

Thanks

Alan

al_ps · June 23, 2022, 8:40pm

Hey Rylan,

Apologies! That env var is used by a specific package (Redash).

I think in your case you’d need to add the pool_pre_ping=True option to the SQLAlchemy connection setup in your code.

Kind regards

Alan

RylanSchaeffer · June 23, 2022, 8:46pm

Yep! That did it. I’m not sure why this error isn’t more descriptive, but thank you for confirming that the fix I found is correct

system · July 23, 2022, 8:47pm

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.

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