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MongoDB DBA Questions

Mar 26, 20253 min read

Basic Configuration and Logs

  1. What is the purpose of the db.startup_log.find() command in MongoDB?

  2. How do you configure the logpath, datapath, port, and fork options in a MongoDB configuration file?

Storage Engines

  1. Compare the WiredTiger and MMAPv1 storage engines in MongoDB.

  2. What is document-level concurrency, and how does WiredTiger implement it?

  3. Explain Multi-Version Concurrency Control (MVCC) in MongoDB.

  4. How does WiredTiger handle compression, and what are the benefits?

  5. What is a checkpoint in WiredTiger, and why is it important?

Server Status and Monitoring

  1. What information does the db.serverStatus() command provide?

  2. How can you check the storage engine being used in your MongoDB instance using db.serverStatus()?

Journaling

  1. What is journaling in MongoDB, and why is it important for durability?

  2. How does journaling work in WiredTiger?

Locking and Unlocking

  1. What is the purpose of the db.fsyncLock() and db.fsyncUnlock() commands?

  2. When would you use these commands in a production environment?

Capped Collections

  1. How do you create a capped collection in MongoDB?

  2. What are the use cases for capped collections?

Automatic Archival

  1. How can you set up automatic archival in MongoDB?

Document Insertion Time

  1. How can you determine the insertion time of a document using its _id field?

TTL Indexes

  1. How do you create a TTL (Time-To-Live) index in MongoDB?

  2. What is the purpose of the ttlMonitorSleepSec parameter, and how can you modify it?

Query Optimization

  1. How can you determine if a query is using an index?

  2. What do the terms DocumentsScanned and RowsReturned indicate in the context of query execution?

Indexing Strategies

  1. What is the ESR (Equality, Sort, Range) rule for indexing in MongoDB?

  2. How does the ESR rule help in optimizing queries?

Replication

  1. How do you set up a replica set in MongoDB?

  2. What are the key components of a replica set, and how does failover work?

Sharding

  1. Explain the concept of sharding in MongoDB.

  2. What are the benefits of sharding, and how does it improve performance and scalability?

Sample Follow-Up Questions

Storage Engines

  • Can you switch the storage engine from MMAPv1 to WiredTiger in an existing MongoDB deployment? If yes, how?

Journaling

  • What happens if journaling is disabled in MongoDB? What are the risks?

Capped Collections

  • What happens when a capped collection reaches its maximum size or document limit?

TTL Indexes

  • What happens if the ttlMonitorSleepSec parameter is set to a very high value?

Replication

  • How do you handle a scenario where the primary node in a replica set goes down?

Sharding

  • What is a shard key, and how do you choose an effective shard key for your data?

Practical Scenarios

Performance Tuning

  • A query is running slowly. How would you diagnose and optimize it?

Backup and Recovery

  • How would you take a backup of a MongoDB database, and what steps would you follow to restore it?

Scaling

  • Your MongoDB database is experiencing high read and write traffic. How would you scale it to handle the load?

Security

  • How do you enable authentication and authorization in MongoDB? What are some best practices for securing a MongoDB deployment?

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