[FIXED] Connection refused by postgresql docker container when connecting via Flask docker container

Issue

I have flask application that connects to PostgreSQL and works well in a single installation. However when I containerise it using docker into separate containers, flask app is unable to connect to PostgreSQL and the error received is psycopg2.OperationalError: could not connect to server: Connection refused

provide below is the code for app.py

app = Flask(__name__)

app.config['basedir'] = os.path.abspath(os.path.dirname(__file__))
app.config['DEVELOPMENT'] = True
app.config['DEBUG'] = True

POSTGRES = {
    'user': 'postgres',
    'pw': 'postgres',
    'db': 'postgres',
    'host': 'localhost',
    'port': '5432',
}
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://%(user)s:%(pw)s@%(host)s:%(port)s/%(db)s' % POSTGRES
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

api = Api(app)

from database.postgresql import db
db.init_app(app)

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000, debug=True)

the app is deployed using app server uwsgi and provided below is the app.ini file contents

[uwsgi]
module=wsgi:app
wsgi-file = app.py
callable = app
socket = :8080
processes = 4
threads = 2
master = true
chmod-socket = 660
vacuum = true
die-on-term = true

the error stack is as follows

  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 2336, in _wrap_pool_connect
    return fn()
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 364, in connect
    return _ConnectionFairy._checkout(self)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 778, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 495, in checkout
    rec = pool._do_get()
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/impl.py", line 140, in _do_get
    self._dec_overflow()
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.raise_(
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/impl.py", line 137, in _do_get
    return self._create_connection()
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 309, in _create_connection
    return _ConnectionRecord(self)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 440, in __init__
    self.__connect(first_connect_check=True)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 661, in __connect
    pool.logger.debug("Error on connect(): %s", e)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.raise_(
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 656, in __connect
    connection = pool._invoke_creator(self)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/strategies.py", line 114, in connect
    return dialect.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 493, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.9/site-packages/psycopg2/__init__.py", line 127, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not connect to server: Connection refused
    Is the server running on host "localhost" (127.0.0.1) and accepting
    TCP/IP connections on port 5432?
could not connect to server: Cannot assign requested address
    Is the server running on host "localhost" (::1) and accepting
    TCP/IP connections on port 5432?

Solution

There is a problem with your postgres configuration in app.py.

You're not trying to connect to your postgres container, rather your Flask container's localhost. You'll need to specify the address of the postgres container instead. Then it will work perfectly.

You can read more about how network communication in Docker works here.

Answered By - MB95