Frequently Asked Questions

• FAQ: MongoDB Fundamentals
  • What kind of database is MongoDB?
  • Do MongoDB databases have tables?
  • Do MongoDB databases have schemas?
  • What languages can I use to work with MongoDB?
  • Does MongoDB support SQL?
  • What are typical uses for MongoDB?
  • Does MongoDB support transactions?
  • Does MongoDB require a lot of RAM?
  • How do I configure the cache size?
  • Does MongoDB require a separate caching layer for application-level caching?
  • Does MongoDB handle caching?
  • Are writes written to disk immediately, or lazily?
  • What language is MongoDB written in?
  • What are the limitations of 32-bit versions of MongoDB?
• FAQ: MongoDB for Application Developers
  • What is a namespace in MongoDB?
  • How do you copy all objects from one collection to another?
  • If you remove a document, does MongoDB remove it from disk?
  • When does MongoDB write updates to disk?
  • How do I do transactions and locking in MongoDB?
  • How do you aggregate data with MongoDB?
  • Why does MongoDB log so many “Connection Accepted” events?
  • Does MongoDB run on Amazon EBS?
  • Why are MongoDB’s data files so large?
  • How do I optimize storage use for small documents?
  • When should I use GridFS?
  • How does MongoDB address SQL or Query injection?
  • How does MongoDB provide concurrency?
  • What is the compare order for BSON types?
  • When multiplying values of mixed types, what type conversion rules apply?
  • How do I query for fields that have null values?
  • Are there any restrictions on the names of Collections?
  • How do I isolate cursors from intervening write operations?
  • When should I embed documents within other documents?
  • Where can I learn more about data modeling in MongoDB?
  • Can I manually pad documents to prevent moves during updates?
• FAQ: The mongo Shell
  • How can I enter multi-line operations in the mongo shell?
  • How can I access different databases temporarily?
  • Does the mongo shell support tab completion and other keyboard shortcuts?
  • How can I customize the mongo shell prompt?
  • Can I edit long shell operations with an external text editor?
• 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?
  • Which operations lock the database?
  • Which administrative commands lock the database?
  • 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?
  • What kind of concurrency does MongoDB provide for JavaScript operations?
  • Does MongoDB support transactions?
  • What isolation guarantees does MongoDB provide?
  • Can reads see changes that have not been committed to disk?
• FAQ: Sharding with MongoDB
  • Is sharding appropriate for a new deployment?
  • How does sharding work with replication?
  • Can I change the shard key after sharding a collection?
  • What happens to unsharded collections in sharded databases?
  • How does MongoDB distribute data across shards?
  • What happens if a client updates a document in a chunk during a migration?
  • What happens to queries if a shard is inaccessible or slow?
  • How does MongoDB distribute queries among shards?
  • How does MongoDB sort queries in sharded environments?
  • How does MongoDB ensure unique _id field values when using a shard key other than _id?
  • I’ve enabled sharding and added a second shard, but all the data is still on one server. Why?
  • Is it safe to remove old files in the moveChunk directory?
  • How does mongos use connections?
  • Why does mongos hold connections open?
  • Where does MongoDB report on connections used by mongos?
  • What does writebacklisten in the log mean?
  • How should administrators deal with failed migrations?
  • What is the process for moving, renaming, or changing the number of config servers?
  • When do the mongos servers detect config server changes?
  • Is it possible to quickly update mongos servers after updating a replica set configuration?
  • What does the maxConns setting on mongos do?
  • How do indexes impact queries in sharded systems?
  • Can shard keys be randomly generated?
  • Can shard keys have a non-uniform distribution of values?
  • Can you shard on the _id field?
  • What do moveChunk commit failed errors mean?
  • How does draining a shard affect the balancing of uneven chunk distribution?
• FAQ: Replication and Replica Sets
  • What kinds of replication does MongoDB support?
  • What do the terms “primary” and “master” mean?
  • What do the terms “secondary” and “slave” mean?
  • How long does replica set failover take?
  • Does replication work over the Internet and WAN connections?
  • Can MongoDB replicate over a “noisy” connection?
  • What is the preferred replication method: master/slave or replica sets?
  • What is the preferred replication method: replica sets or replica pairs?
  • Why use journaling if replication already provides data redundancy?
  • Are write operations durable if write concern does not acknowledge writes?
  • How many arbiters do replica sets need?
  • What information do arbiters exchange with the rest of the replica set?
  • Which members of a replica set vote in elections?
  • Do hidden members vote in replica set elections?
  • Is it normal for replica set members to use different amounts of disk space?
• FAQ: MongoDB Storage
  • What are memory mapped files?
  • How do memory mapped files work?
  • How does MongoDB work with memory mapped files?
  • What are page faults?
  • What is the difference between soft and hard page faults?
  • What tools can I use to investigate storage use in MongoDB?
  • What is the working set?
  • Why are the files in my data directory larger than the data in my database?
  • How do I reclaim disk space?
  • How can I check the size of a collection?
  • How can I check the size of indexes?
  • How do I know when the server runs out of disk space?
• FAQ: Indexes
  • Should you run ensureIndex() after every insert?
  • How do you know what indexes exist in a collection?
  • How do you determine the size of an index?
  • What happens if an index does not fit into RAM?
  • How do you know what index a query used?
  • How do you determine what fields to index?
  • How do write operations affect indexes?
  • Will building a large index affect database performance?
  • Can I use index keys to constrain query matches?
  • Using $ne and $nin in a query is slow. Why?
  • Can I use a multi-key index to support a query for a whole array?
  • How can I effectively use indexes strategy for attribute lookups?
• FAQ: MongoDB Diagnostics
  • Where can I find information about a mongod process that stopped running unexpectedly?
  • Does TCP keepalive time affect sharded clusters and replica sets?
  • What tools are available for monitoring MongoDB?
  • Memory Diagnostics
  • Sharded Cluster Diagnostics

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