ReddRadar
Agent skill
mongodb-replication-sharding
Master MongoDB replication, replica sets, and sharding for distributed deployments. Learn failover, shard keys, and cluster management. Use when setting up high availability or scaling horizontally.
Install this agent skill to your Project
npx add-skill https://github.com/pluginagentmarketplace/custom-plugin-mongodb/tree/main/skills/mongodb-replication
SKILL.md
MongoDB Replication & Sharding
Master distributed MongoDB architectures.
Quick Start
Replica Set Setup
# Start mongod instances with replica set config
mongod --replSet rs0 --port 27017
mongod --replSet rs0 --port 27018
mongod --replSet rs0 --port 27019
# Initiate replica set
mongo mongodb://localhost:27017
> rs.initiate({
_id: 'rs0',
members: [
{ _id: 0, host: 'localhost:27017', priority: 1 },
{ _id: 1, host: 'localhost:27018', priority: 0.5 },
{ _id: 2, host: 'localhost:27019', priority: 0 }
]
})
# Check replica set status
> rs.status()
Replication Concepts
Primary: Accepts reads and writes
Secondary: Replicates from primary, serves reads only
Arbiter: Participates in elections, no data
Write Concerns
// Unacknowledged
await collection.insertOne(doc, { writeConcern: { w: 0 } })
// Acknowledged (single node)
await collection.insertOne(doc, { writeConcern: { w: 1 } })
// Majority
await collection.insertOne(doc, { writeConcern: { w: 'majority' } })
// Majority with timeout
await collection.insertOne(doc, {
writeConcern: { w: 'majority', wtimeout: 5000 }
})
Read Preferences
// Read from primary only (default)
find().setReadPreference('primary')
// Read from primary, failover to secondary
find().setReadPreference('primaryPreferred')
// Read from secondary if available
find().setReadPreference('secondary')
// Read from secondary, failover to primary
find().setReadPreference('secondaryPreferred')
// Read from nearest node
find().setReadPreference('nearest')
Sharding
Enable Sharding
# Start config servers
mongod --configsvr --dbpath /data/config0 --port 27019
mongod --configsvr --dbpath /data/config1 --port 27020
mongod --configsvr --dbpath /data/config2 --port 27021
# Start mongos router
mongos --configdb localhost:27019,localhost:27020,localhost:27021
# Start shard servers
mongod --shardsvr --dbpath /data/shard0 --port 27017
mongod --shardsvr --dbpath /data/shard1 --port 27018
Shard Key Design
// Enable sharding on database
sh.enableSharding('myapp')
// Shard collection with key
sh.shardCollection('myapp.users', { email: 1 })
// Hash-based sharding (better distribution)
sh.shardCollection('myapp.logs', { userId: 'hashed' })
// Compound shard key
sh.shardCollection('myapp.events', { tenantId: 1, timestamp: 1 })
Check Sharding Status
// Get sharding status
sh.status()
// Get shard information
db.adminCommand({ listShards: 1 })
// Get chunk distribution
sh.balancerStatus()
Failover & High Availability
Replica Set Elections
// Trigger election (step down)
rs.stepDown()
// View election status
rs.status()
// Check oplog
db.oplog.rs.find().sort({ ts: -1 }).limit(5)
Monitor Replication Lag
// Check replica set members lag
rs.status().members
// Monitor oplog length
db.oplog.rs.find().limit(1).sort({ $natural: -1 })
Transactions (MongoDB 4.0+)
// Multi-document transaction
const session = client.startSession();
try {
await session.withTransaction(async () => {
await users.insertOne({ name: 'John' }, { session });
await accounts.insertOne({ userId: '...', balance: 100 }, { session });
});
} finally {
await session.endSession();
}
Python Examples
from pymongo import MongoClient
# Connect to replica set
client = MongoClient('mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0')
# Write concern
collection.insert_one(
{'name': 'John'},
write_concern=WriteConcern(w='majority')
)
# Read preference
collection.find().with_options(
read_preference=ReadPreference.SECONDARY
)
Monitoring
✅ Check replica set status regularly ✅ Monitor replication lag ✅ Watch chunk migration progress ✅ Monitor oplog size ✅ Alert on member failures ✅ Track failover events ✅ Monitor balancer activity ✅ Check data distribution
Recommended Agent Skills
Expand your agent's capabilities with these related and highly-rated skills.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-find-queries
Master MongoDB find queries with filters, projections, sorting, and pagination. Learn query operators, comparison, logical operators, and real-world query patterns. Use when retrieving data from MongoDB collections.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-index-creation
Master MongoDB index creation and types. Learn single-field, compound, unique, text, geospatial, and TTL indexes. Optimize query performance dramatically with proper indexing.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-atlas-setup
Master MongoDB Atlas cloud setup, cluster configuration, security, networking, backups, and monitoring. Get production-ready cloud database in minutes. Use when setting up cloud MongoDB, configuring clusters, or managing Atlas.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-crud-operations
Master MongoDB CRUD operations, document insertion, querying, updating, and deletion. Learn BSON format, ObjectId, data types, and basic operations. Use when working with documents, collections, and fundamental MongoDB operations.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-indexing-optimization
Master MongoDB indexing and query optimization. Learn index types, explain plans, performance tuning, and query analysis. Use when optimizing slow queries, analyzing performance, or designing indexes.
pluginagentmarketplace/custom-plugin-mongodb
mongodb-schema-design
Master MongoDB schema design and data modeling patterns. Learn embedding vs referencing, relationships, normalization, and schema evolution. Use when designing databases, normalizing data, or optimizing queries.
Didn't find tool you were looking for?