About warning 'Connection pool is full' why it is occurring?

[Forest John]

The problem

I use 2 machines for android Appium test

• Test script machine that is using python-client. on OSX Big sur
• Selenium grid hub on Centos7 VM
• Appium server with AVD connected on OSX Big sur

But I get the following warning too much, 4~6 times per 10 seconds.
WARNING urllib3.connectionpool:connectionpool.py:275 Connection pool is full, discarding connection: <HUB_URL>

Environment

• Selenium grid docker image: selenium/hub:3.141.59-20210804
  https://github.com/SeleniumHQ/docker-selenium/tree/selenium-3#version-3
• Appium Desktop version: 1.21.0
• Electron: 7.2.4
• Node.js: 12.8.1
• Mobile platform/version under test: Android API30 Emulator
• Real device or emulator/simulator: Emulator
• Appium CLI or Appium.app|exe: Appium.app

Client logs

WARNING urllib3.connectionpool:connectionpool.py:275 Connection pool is full, discarding connection: <HUB_URL> WARNING urllib3.connectionpool:connectionpool.py:275 Connection pool is full, discarding connection: <HUB_URL> WARNING urllib3.connectionpool:connectionpool.py:275 Connection pool is full, discarding connection: <HUB_URL> WARNING urllib3.connectionpool:connectionpool.py:275 Connection pool is full, discarding connection: <HUB_URL>

Also asked to Appium-python github:

https://github.com/appium/python-client/issues/627