ServerSelectionTimeoutError on mongo replica set using pymongo - mongodb

Question

I am trying to connect to a MongoDB replica set using pymongo, but I keep getting the error: pymongo.errors.ServerSelectionTimeoutError: No replica set members match selector. In the error message it's also specified that my topology type is ReplicaSetNoPrimary, which is odd, as connecting with mongo bash shows a clear primary.
Note that the replica set works fine and is usable via mongo bash on the master node.
Also, I have added firewall rules to allow both inbound and outbound traffic on the specified ports, just to make sure this isn't the issue.
I am using docker-compose for the cluster. The file:
version: "3.9"
services:
mongo-master:
image: mongo:latest
container_name: mongo_master
volumes:
- ./data/master:/data/db
ports:
- 27017:27017
command: mongod --replSet dbrs & mongo --eval rs.initiate(`cat rs_config.json`)
stdin_open: true
tty: true
mongo-slave-1:
image: mongo:latest
container_name: mongo_slave_1
volumes:
- ./data/slave_1:/data/db
ports:
- 27018:27017
command: mongod --replSet dbrs
stdin_open: true
tty: true
mongo-slave-2:
image: mongo:latest
container_name: mongo_slave_2
volumes:
- ./data/slave_2:/data/db
ports:
- 27019:27017
command: mongod --replSet dbrs
stdin_open: true
tty: true
The rs_config.json file used above:
{
"_id" : "dbrs",
"members" : [
{
"_id" : 0,
"host" : "mongo_master:27017",
"priority" : 10
},
{
"_id" : 1,
"host" : "mongo_slave_1:27017"
},
{
"_id" : 2,
"host" : "mongo_slave_2:27017"
}
]
}
The error raises on the last line here:
self.__client = MongoClient(["localhost:27017", "localhost:27018", "localhost:27019"], replicaset="dbrs")
self.__collection = self.__client[self.__db_name][collection.value]
self.__collection.insert_one(dictionary_object)
I ommitted some code for brevity, but you can assume all class attributes and dictionary_object are well defined according to pymongo docs.
Also please note that I have tried many different ways to initialize MongoClient, including a connection string (as in the docs), and the connect=False optional parameter as advised in some blogs. The issue persists...
Edit: I tried adding "mongo_master" to my etc/hosts file pointing at 127.0.0.1 and changing the connection string from localhost to that, and it works with the replica set. This is a bad workaround but maybe can help in figuring out a solution.
Thanks in advance for any help!

Answer 1

To get a connection to a MongoDB replicaset from an external client, you must be able to resolve the hostnames from the local client.
https://docs.mongodb.com/manual/tutorial/deploy-replica-set/#connectivity
Ensure that network traffic can pass securely between all members of the set and all clients in the network.
So, add the following to your /etc/hosts file:
127.0.0.1 mongodb-1
127.0.0.1 mongodb-2
127.0.0.1 mongodb-3
To be able to connect both internally and externally, you will need to run each MongoDB service on different ports.
The following script will initiate a 3-node MongoDB replicaset and run a test client. I recommend using the Bitnami image as it takes care of the replset initiation for you. (Borrowing heavily from this configuration)
#!/bin/bash
PROJECT_NAME=replset_test
MONGODB_VERSION=4.4
PYTHON_VERSION=3.9.6
PYMONGO_VERSION=4.0.1
cd "$(mktemp -d)" || exit
cat << EOF > Dockerfile
FROM python:${PYTHON_VERSION}-slim-buster
COPY requirements.txt /tmp/
RUN pip install -r /tmp/requirements.txt
COPY ${PROJECT_NAME}.py .
CMD [ "python", "./${PROJECT_NAME}.py" ]
EOF
cat << EOF > requirements.txt
pymongo==${PYMONGO_VERSION}
EOF
cat << EOF > ${PROJECT_NAME}.py
from pymongo import MongoClient
connection_string = 'mongodb://root:password123#mongodb-1:27017,mongodb-2:27018,mongodb-3:27019/mydatabase?authSource=admin&replicaSet=replicaset'
client = MongoClient(connection_string)
db = client.db
db['mycollection'].insert_one({'a': 1})
record = db['mycollection'].find_one()
if record is not None:
print(f'{__file__}: MongoDB connection working using connection string "{connection_string}"')
EOF
cp ${PROJECT_NAME}.py ${PROJECT_NAME}_external.py
cat << EOF > docker-compose.yaml
version: '3.9'
services:
mongodb-1:
image: docker.io/bitnami/mongodb:${MONGODB_VERSION}
ports:
- 27017:27017
environment:
- MONGODB_ADVERTISED_HOSTNAME=mongodb-1
- MONGODB_PORT_NUMBER=27017
- MONGODB_REPLICA_SET_MODE=primary
- MONGODB_ROOT_PASSWORD=password123
- MONGODB_REPLICA_SET_KEY=replicasetkey123
volumes:
- 'mongodb_master_data:/bitnami/mongodb'
mongodb-2:
image: docker.io/bitnami/mongodb:${MONGODB_VERSION}
ports:
- 27018:27018
depends_on:
- mongodb-1
environment:
- MONGODB_ADVERTISED_HOSTNAME=mongodb-2
- MONGODB_PORT_NUMBER=27018
- MONGODB_REPLICA_SET_MODE=secondary
- MONGODB_INITIAL_PRIMARY_HOST=mongodb-primary
- MONGODB_INITIAL_PRIMARY_ROOT_PASSWORD=password123
- MONGODB_REPLICA_SET_KEY=replicasetkey123
mongodb-3:
image: docker.io/bitnami/mongodb:${MONGODB_VERSION}
ports:
- 27019:27019
depends_on:
- mongodb-1
environment:
- MONGODB_ADVERTISED_HOSTNAME=mongodb-3
- MONGODB_PORT_NUMBER=27019
- MONGODB_REPLICA_SET_MODE=secondary
- MONGODB_INITIAL_PRIMARY_HOST=mongodb-primary
- MONGODB_INITIAL_PRIMARY_ROOT_PASSWORD=password123
- MONGODB_REPLICA_SET_KEY=replicasetkey123
${PROJECT_NAME}:
container_name: ${PROJECT_NAME}
build: .
depends_on:
- mongodb-1
- mongodb-2
- mongodb-3
volumes:
mongodb_master_data:
driver: local
EOF
docker rm --force $(docker ps -a -q --filter name=mongo) 2>&1 > /dev/null
docker rm --force $(docker ps -a -q --filter name=${PROJECT_NAME}) 2>&1 > /dev/null
docker-compose up --build -d
python ${PROJECT_NAME}.py
docker ps -a -q --filter name=${PROJECT_NAME}
docker logs $(docker ps -a -q --filter name=${PROJECT_NAME})
If all is ok you will get an output confirming both internal and external connectivity:
/tmp/tmp.QM9tQPE8Dj/replset_test.py: MongoDB connection working using connection string "mongodb://root:password123#mongodb-1:27017,mongodb-2:27018,mongodb-3:27019/mydatabase?authSource=admin&replicaSet=replicaset"
d53e8c41ad20
//./replset_test.py: MongoDB connection working using connection string "mongodb://root:password123#mongodb-1:27017,mongodb-2:27018,mongodb-3:27019/mydatabase?authSource=admin&replicaSet=replicaset"