MongoDB Replica Set: oplog, Heartbeat & Elections Explained

Minimal MRE

# Start replica set
mongod --replSet rs0 --port 27017 --dbpath /data/rs0-1 --bind_ip localhost --fork --logpath /data/rs0-1/mongod.log
mongod --replSet rs0 --port 27018 --dbpath /data/rs0-2 --bind_ip localhost --fork --logpath /data/rs0-2/mongod.log
mongod --replSet rs0 --port 27019 --dbpath /data/rs0-3 --bind_ip localhost --fork --logpath /data/rs0-3/mongod.log

# Initialize replica set
mongosh --port 27017
rs.initiate({ _id:"rs0", members:[
  {_id:0, host:"localhost:27017"},
  {_id:1, host:"localhost:27018"},
  {_id:2, host:"localhost:27019"}
]})

Problems with Master-Slave Replication

oplog is stored in the oplog.$main collection in the local database. oplog is idempotent; regardless of how many times it executes, the result is consistent.

oplog fields:

• ts: Operation timestamp
• h: Unique identifier for the operation
• op: Operation type (i/u/d/c/n)
• ns: Collection the operation targets
• o: Operation content

Note: Master-slave replication is no longer supported after MongoDB 4.0.

Replica Set

What is a Replica Set

A replica set is a cluster of MongoDB instances that have the same data set. Replica sets provide data redundancy backup and store data copies on multiple servers.

Replica Set Advantages

1. High availability: Automatic failover
2. Data security and disaster recovery: Real-time data synchronization
3. Read-write separation: Primary handles writes, secondaries handle read-only operations
4. O&M advantages: Rolling upgrades, geographic distribution

Architecture Principles

oplog Log Structure

{
  "ts": Timestamp(1446011584, 2),
  "h": NumberLong("1687359108795812092"),
  "op": "i",
  "ns": "test.nosql",
  "o": { "_id": ObjectId(...), "name": "mongodb" }
}

Heartbeat Detection Mechanism

• Heartbeat frequency: Default every 2 seconds
• Timeout setting: Default 10 seconds
• State maintenance: Each node maintains a state mapping table

Election Mechanism

Trigger election timing:

• First initialization of replica set
• Secondary node weight is higher than primary node
• Secondary node discovers no primary in cluster
• Primary node cannot access majority of members

Quick Reference

Parameter	Default Value	Description
heartbeatIntervalMillis	2000ms	Heartbeat frequency
heartbeatTimeoutSecs	10s	Heartbeat timeout threshold
electionTimeoutMillis	10000ms	Election timeout
Max members	50	Replica set member limit
Max voting members	7	Member voting limit

Check commands:

• db.getReplicationInfo(): View oplog status
• rs.printSecondaryReplicationInfo(): View secondary replication status
• rs.status(): View overall replica set status