MAX_CONNS not being respected

Hello, I’m make some experimentations running django 3 under gevent.

I’ve created an endpoint that execute a pg_sleep faking slow queries, then, I used vegeta for tests my setup with high load.

my MAX_CONNS was 10.

But some of my api response was 500 :

Traceback (most recent call last):
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/danilo/Code/django_db__pool_issues_example/app/sleep_db.py", line 9, in sleep_in_db
    with connection.cursor() as cursor:
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/backends/base/base.py", line 260, in cursor
    return self._cursor()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/backends/base/base.py", line 236, in _cursor
    self.ensure_connection()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/backends/base/base.py", line 220, in ensure_connection
    self.connect()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/backends/base/base.py", line 220, in ensure_connection
    self.connect()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django/db/backends/base/base.py", line 197, in connect
    self.connection = self.get_new_connection(conn_params)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django_db_geventpool/backends/postgresql_psycopg2/base.py", line 49, in get_new_connection
    self.connection = self.pool.get()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django_db_geventpool/backends/postgresql_psycopg2/psycopg2_pool.py", line 48, in get
    conn = self.create_connection()
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/django_db_geventpool/backends/postgresql_psycopg2/psycopg2_pool.py", line 84, in create_connection
    conn = self.connect(*self.args, **self.kwargs)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/psycopg2/__init__.py", line 126, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
  File "/home/danilo/.local/share/virtualenvs/django_db__pool_issues_example-SoJGTr3V/lib/python3.6/site-packages/psycogreen/gevent.py", line 32, in gevent_wait_callback
    state = conn.poll()
django.db.utils.OperationalError: FATAL:  sorry, too many clients already

I’ve uploaded a project to show it. https://github.com/dnp1/django_db_gevent_pool_issues_example

My attack was: echo “GET http://localhost:8000/sleep_in_db” | vegeta attack -duration=10s --rate 100 | tee results.bin | vegeta report

When I watched the database: SELECT * FROM "pg_stat_activity" WHERE "client_addr" = '172.26.0.1' there was 99 connections of my application.

I’m using python 3.6.10 (CPython) under Linux. My postgres is running inside a docker container

[s4ke]

Thanks for the fix @bellini666 . Just had an issue with a lot of extra connections being created and this fixed it.

[bellini666]

@joekohlsdorf from what I can tell, the previous version had a small issue here: https://github.com/jneight/django-db-geventpool/blob/a53a258eec27f76939516792020192c565d6f8ee/django_db_geventpool/backends/postgresql_psycopg2/psycopg2_pool.py#L48

As you can see, when that new_connection was used, it would create a new connection without doing self.size += 1. That could probably lead to more connections being used.

Either way, I don’t know if you saw my update to my PR, but I changed it a lot based on your comment and we are waiting for your feedback so it can get merged. In it I already fixed that issue you are mentioning by making sure that all new connections will increase self.size