Question: What port does MongoDB replication use?

Answer

MongoDB uses a variety of ports for its operations, with the default MongoDB server port being 27017. However, when it comes to replication, there isn't a separate, fixed 'replication port' that MongoDB uses. Instead, replication in MongoDB occurs over the same port that the MongoDB server instance uses for all its network communication.

When setting up a MongoDB replica set, all members of the replica set communicate with each other using the MongoDB server port (27017 by default, but this can be configured). This means the primary node and all secondary nodes in a replica set will use their respective server's configured port for replication activities such as data synchronization, heartbeats, and election processes.

Example Configuration

Here is an example of how you might configure a simple MongoDB replica set on a single machine for testing purposes, using different ports for each instance:

# Start the primary MongoDB instance on port 27017
mongod --port 27017 --dbpath /srv/mongodb/rs0-0 --replSet rs0

# Start a secondary MongoDB instance on port 27018
mongod --port 27018 --dbpath /srv/mongodb/rs0-1 --replSet rs0

# Start another secondary MongoDB instance on port 27019
mongod --port 27019 --dbpath /srv/mongodb/rs0-2 --replSet rs0

In this configuration, each mongod instance starts on a different port (27017, 27018, 27019) on the same machine, and all are part of the same replica set named rs0. Despite the different ports, they communicate with each other for replication using these ports, proving that MongoDB does not designate a specific 'replication port'.

Conclusion

To ensure successful replication between MongoDB instances, it's essential to ensure that all instances within a replica set can communicate over their configured ports. This often involves configuring firewalls and network security groups to allow traffic on the MongoDB ports between the servers hosting MongoDB instances.