psql: FATAL: 剩余的连接槽是为非复制超级用户连接保留的

Question

I want to understand when this error occurs and how to resolve it. I checked pg_stat_activity and pg_locks but couldnt figure out which process is exhausing the connections

We are using sqlalchemy to connect to database as below

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine_url = f'{dbms}://{username}:{password}@{hostname}:{port}/{database}'

engine = create_engine(engine_url, pool_size=20, max_overflow=10)
Session = sessionmaker(bind=engine)

db_session = Session()

This is an extension of Heroku "psql: FATAL: remaining connection slots are reserved for non-replication superuser connections"

Answer 1

I am adding the client process id in application name of sqlalchemy engine url. This will be stored under application_name in pg_stat_activity and it will be helpful to debug from where did this connection was opened.

import os
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

client_hostname = os.environ.get('HOSTNAME', 'UNKNOWN')
client_pid = os.getpid()


engine_url = f'{dbms}://{username}:{password}@{hostname}:{port}/{database}?application_name={client_hostname}_{client_pid}'

engine = create_engine(engine_url, pool_size=20, max_overflow=10)
Session = sessionmaker(bind=engine)

db_session = Session()

This shows where the connection was originated from and the application name has a limit of 64 chars , so we can include any custom information that you want to pass on to db stats