Frequently Asked Questions

• FAQ: MongoDB Fundamentals
  • What platforms does MongoDB support?
  • Is MongoDB offered as a hosted service?
  • How does a collection differ from a table?
  • How do I create a database and a collection?
  • How do I define or alter the collection schema?
  • Does MongoDB support SQL?
  • Does MongoDB support transactions?
  • Does MongoDB handle caching?
  • How does MongoDB address SQL or Query injection?
• FAQ: Indexes
  • How do I create an index?
  • How does an index build affect database performance?
  • How do I see what indexes exist on a collection?
  • How can I see if a query uses an index?
  • How do I determine which fields to index?
  • How can I see the size of an index?
  • How do write operations affect indexes?
• FAQ: Concurrency
  • What type of locking does MongoDB use?
  • How granular are locks in MongoDB?
  • How do I see the status of locks on my mongod instances?
  • Does a read or write operation ever yield the lock?
  • What locks are taken by some common client operations?
  • Which administrative commands lock the database?
  • Which administrative commands lock a collection?
  • Does a MongoDB operation ever lock more than one database?
  • How does sharding affect concurrency?
  • How does concurrency affect a replica set primary?
  • How does concurrency affect secondaries?
  • Does MongoDB support transactions?
  • What isolation guarantees does MongoDB provide?
• FAQ: Sharding with MongoDB
  • Is sharding appropriate for a new deployment?
  • Can I select a different shard key after sharding a collection?
  • Why are my documents not distributed across the shards?
  • How does mongos detect changes in the sharded cluster configuration?
  • What does writebacklisten in the log mean?
  • How does mongos use connections?
• FAQ: Replication and Replica Sets
  • What kind of replication does MongoDB support?
  • Does replication work over the Internet and WAN connections?
  • Can MongoDB replicate over a “noisy” connection?
  • Why use journaling if replication already provides data redundancy?
  • What information do arbiters exchange with the rest of the replica set?
  • Is it normal for replica set members to use different amounts of disk space?
  • Can I rename a replica set?
• FAQ: MongoDB Storage
  • Storage Engine Fundamentals
  • Can you mix storage engines in a replica set?
  • WiredTiger Storage Engine
  • Data Storage Diagnostics
• FAQ: MongoDB Diagnostics
  • Where can I find information about a mongod process that stopped running unexpectedly?
  • Does TCP keepalive time affect MongoDB Deployments?
  • Why does MongoDB log so many “Connection Accepted” events?
  • What tools are available for monitoring MongoDB?
  • Memory Diagnostics for the WiredTiger Storage Engine
  • Sharded Cluster Diagnostics