When deploying MongoDB replica sets, there appears to be conflicting information in the official documentation regarding minimum member requirements. The deployment tutorial states:

"A replica set requires three distinct systems..."

While the core replication documentation mentions:

"Most replica sets consist of two or more mongod instances..."

In production environments, three members are indeed required for a proper replica set with:

• 1 Primary node
• 1 Secondary node
• 1 Arbiter (or another secondary for fault tolerance)

However, MongoDB technically allows temporary two-member configurations for development/testing purposes:

// Example of a 2-member replica set (DEV ONLY)
rs.initiate({
  _id: "rs0",
  members: [
    { _id: 0, host: "mongodb0.example.net:27017" },
    { _id: 1, host: "mongodb1.example.net:27017" }
  ]
})

Production: Always use odd number of members (3,5,7) for:

• Automatic failover
• Data redundancy
• Proper election consensus

Development: Two members can work temporarily but with limitations:

• No automatic failover (requires manual intervention)
• Vulnerable to split-brain scenarios
• Not recommended beyond basic testing

Here's a proper 3-member configuration:

// Production-grade replica set configuration
rs.initiate({
  _id: "rsProd",
  members: [
    { _id: 0, host: "mongo1.prod.net:27017" },
    { _id: 1, host: "mongo2.prod.net:27017" },
    { _id: 2, host: "mongo3.prod.net:27017", arbiterOnly: true }
  ],
  settings: {
    chainingAllowed: false,
    heartbeatTimeoutSecs: 10,
    electionTimeoutMillis: 10000
  }
})

When planning your topology:

• Never deploy single-node "replica sets" - they defeat the purpose
• Two-member sets can work temporarily during maintenance
• For cloud deployments, consider using 3 data-bearing nodes instead of arbiters
• Network partitions require majority to maintain primary

For absolute clarity: 3 members is the minimum for production, while 2 members is technically possible but comes with significant operational risks.

The MongoDB documentation contains two seemingly conflicting statements regarding replica set minimum members:

• Deployment tutorial states: "A replica set requires three distinct systems"
• Core replication documentation states: "Most replica sets consist of two or more mongod instances"

Both statements are technically correct but serve different purposes:

// Minimum production configuration (recommended)
rs.initiate({
   _id: "rs0",
   members: [
      { _id: 0, host: "mongodb0.example.net:27017" },
      { _id: 1, host: "mongodb1.example.net:27017" },
      { _id: 2, host: "mongodb2.example.net:27017" }
   ]
})

While possible for development, a 2-member set has critical constraints:

• No automatic failover (requires arbiter for election quorum)
• Vulnerable to split-brain scenarios
• Violates CAP theorem's partition tolerance

The 3-member configuration provides:

// Safe primary election behavior
rs.conf().members = [
   { _id: 0, host: "primary.example.com:27017", priority: 2 },
   { _id: 1, host: "secondary1.example.com:27017", priority: 1 },
   { _id: 2, host: "secondary2.example.com:27017", priority: 1, arbiterOnly: true }
]

For resource-constrained environments:

• Two data-bearing members + one arbiter (lightweight)
• Two members + delayed secondary for disaster recovery

Always use three members in production:

// Docker compose example for local testing
version: '3'
services:
  mongo1:
    image: mongo
    command: mongod --replSet rs0 --port 27017
  mongo2:
    image: mongo
    command: mongod --replSet rs0 --port 27018
  mongo3:
    image: mongo
    command: mongod --replSet rs0 --port 27019