Indexes can be built using the MongoDB shell or any of the available drivers. By default, indexes are built in the foreground, blocking all other operations in the database. This is faster but often undesirable, especially in production instances.

We can also build indexes in the background by adding the {background: true} parameter in our index commands in the shell. Background indexes will only block the current connection/thread. We can open a new connection (that is, using mongo in the command line) to connect to the same database:

> db.books.createIndex( { name: 1 }, { background: true } )

Background index building can take significantly more time than foreground, especially if the indexes can't fit in available RAM.

Index early, revisit indexes regularly for consolidation. Queries won't see partial index results. Queries will start getting results from an index only after it is completely created.

Do not use main application code to create indexes as it can impose unpredicted delays. Instead, get a list of indexes from the application and mark these for creation during maintenance windows.