Frequently Asked Questions¶
- FAQ: MongoDB Fundamentals
- What platforms does MongoDB support?
- 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
- 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?
- 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 does the term “primary” mean?
- What does the term “secondary” mean?
- How long does replica set failover take?
- 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?
- 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?
- Can I rename a replica set?
- FAQ: MongoDB Storage
- 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 MMAPv1 Storage Engine
- Memory Diagnostics for the WiredTiger Storage Engine
- Sharded Cluster Diagnostics