1. MongoDB Connection Failures

Understanding the Issue

MongoDB clients fail to connect to the database, resulting in timeout errors or authentication failures.

Root Causes

• MongoDB service is not running.
• Firewall or network rules blocking connections.
• Authentication issues due to incorrect credentials.

Fix

Check if MongoDB service is running:

sudo systemctl status mongod

Restart MongoDB if it is not running:

sudo systemctl restart mongod

Ensure MongoDB is listening on the correct IP and port:

netstat -tulnp | grep mongod

Verify firewall rules allowing MongoDB connections:

sudo ufw allow 27017/tcp

2. Slow MongoDB Queries

Understanding the Issue

Queries take longer than expected, impacting application performance.

Root Causes

• Missing indexes causing full collection scans.
• Large dataset retrieval without pagination.
• Inefficient query patterns.

Fix

Use indexes to speed up queries:

db.collection.createIndex({ fieldName: 1 })

Analyze slow queries using the query profiler:

db.setProfilingLevel(2)
db.system.profile.find().sort({ millis: -1 }).limit(5)

Use projection to fetch only necessary fields:

db.users.find({}, { name: 1, email: 1 })

3. High MongoDB Memory and CPU Usage

Understanding the Issue

MongoDB consumes excessive memory and CPU, leading to slow performance.

Root Causes

• Large working set exceeding available RAM.
• Frequent writes causing lock contention.
• Inefficient indexing leading to high CPU usage.

Fix

Monitor memory and CPU usage:

db.serverStatus().mem
top

Reduce the working set by archiving old data:

db.oldCollection.renameCollection("archived_oldCollection")

Optimize indexing strategies:

db.collection.createIndex({ lastLogin: 1 })

4. MongoDB Replication Lag

Understanding the Issue

Replica set members lag behind the primary node, causing inconsistencies.

Root Causes

• Network latency between replica set members.
• Secondary nodes overwhelmed with read operations.
• Insufficient write concern settings.

Fix

Check replication lag:

rs.printSlaveReplicationInfo()

Increase the oplog size to retain more replication history:

db.adminCommand({ replSetResizeOplog: 1, size: 10240 })

Balance read operations between primary and secondary nodes:

db.getMongo().setReadPref("secondaryPreferred")

5. Data Consistency Issues

Understanding the Issue

Data inconsistencies appear across multiple nodes in a replica set.

Root Causes

• Delayed replication updates causing stale reads.
• Write conflicts due to lack of transactions.
• Incorrectly configured write concern settings.

Fix

Ensure transactions are used for atomic updates:

const session = db.getMongo().startSession();
session.startTransaction();
db.collection.updateOne({ _id: 1 }, { $set: { status: "completed" } });
session.commitTransaction();

Use writeConcern to ensure strong consistency:

db.collection.insertOne({ name: "John" }, { writeConcern: { w: "majority" } })

Conclusion

MongoDB is a powerful and scalable database, but troubleshooting connection failures, slow queries, high memory usage, replication lag, and data consistency issues is essential for optimal performance. By implementing indexing, monitoring resource utilization, and ensuring proper replication settings, administrators can maintain a stable MongoDB environment.

FAQs

1. Why is my MongoDB connection failing?

Ensure the MongoDB service is running, check firewall rules, and verify authentication credentials.

2. How do I optimize slow MongoDB queries?

Use indexes, analyze slow queries with the profiler, and fetch only necessary fields.

3. Why is MongoDB using high memory and CPU?

Reduce working set size, optimize indexing strategies, and archive old data.

4. How do I fix MongoDB replication lag?

Check network latency, increase oplog size, and distribute read operations.

5. How do I ensure data consistency in MongoDB?

Use transactions, configure write concern settings, and monitor replication status.