Which operations lock the database?
原文：http://docs.mongodb.org/manual/faq/concurrency/#what-type-of-locking-does-mongodb-use
The following table lists common database operations and the types of locks they use.
| Issue a query | Read lock |
| Get more data from a cursor | Read lock |
| Insert data | Write lock |
| Remove data | Write lock |
| Update data | Write lock |
| Map-reduce | Read lock and write lock, unless operations are specified as non-atomic. Portions of map-reduce jobs can run concurrently. |
| Create an index | Building an index in the foreground, which is the default, locks the database for extended periods of time. |
| db.eval() | Write lock. db.eval() blocks all other JavaScript processes. |
| eval | Write lock. If used with the nolock lock option, the eval option does not take a write lock and cannot write data to the database. |
| aggregate() | Read lock |