MongoDB Replication
Ensuring data availability and redundancy
🔄 What is MongoDB Replication?
Replication creates multiple copies of your data across different servers. It ensures high availability, data redundancy, and automatic failover if the primary server fails.
// Initialize a replica set
rs.initiate({
_id: "myReplicaSet",
members: [
{ _id: 0, host: "localhost:27017" }
]
})
Output:
{ "ok" : 1 }
Key Replication Concepts
Primary Node
Receives all write operations
// Check replica set status
rs.status()Secondary Nodes
Replicate data from primary
// Add secondary member
rs.add("localhost:27018")Automatic Failover
Elects new primary if needed
// View current primary
rs.isMaster()Oplog
Records all data operations
// View oplog
use local
db.oplog.rs.find()🔹 Setting Up a Replica Set
Replica sets provide redundancy and high availability. Here's how to configure a basic three-member replica set for production use.
// Configure a 3-member replica set
rs.initiate({
_id: "myReplicaSet",
members: [
{ _id: 0, host: "mongodb0.example.net:27017" },
{ _id: 1, host: "mongodb1.example.net:27017" },
{ _id: 2, host: "mongodb2.example.net:27017" }
]
})
// Verify configuration
rs.conf()Output:
{
"_id" : "myReplicaSet",
"version" : 1,
"members" : [ ... ]
}
🔹 Adding Members to Replica Set
You can dynamically add or remove members from your replica set to scale your deployment.
// Add a new secondary member
rs.add("mongodb3.example.net:27017")
// Add an arbiter (voting member only)
rs.addArb("mongodb4.example.net:27017")
// Remove a member
rs.remove("mongodb3.example.net:27017")Output:
{ "ok" : 1 }
🔹 Read Preferences
Control where your application reads data from in a replica set to optimize performance and data consistency.
// Read from primary only (default)
db.collection.find().readPref("primary")
// Read from secondary nodes
db.collection.find().readPref("secondary")
// Read from primary preferred
db.collection.find().readPref("primaryPreferred")
// Read from nearest node
db.collection.find().readPref("nearest")🔹 Monitoring Replica Set Health
Regular monitoring ensures your replica set operates smoothly and helps identify issues early.
// Check replica set status
rs.status()
// View replication lag
rs.printReplicationInfo()
// Check secondary lag
rs.printSecondaryReplicationInfo()
// View current configuration
rs.conf()Output:
configured oplog size: 1024MB log length start to end: 3600secs oplog first event time: Mon Jan 01 2024 oplog last event time: Mon Jan 01 2024