Fast Updates with MongoDB (update-in-place)



One nice feature with MongoDB is that updates can happen “in place” – the database does not have to allocate and write a full new copy of the object.

This can be highly performant for frequent update use cases.  For example, incrementing a counter is a highly efficient operation.  We need not fetch the document from the server, we can simply send an increment operation over:

db.my_collection.update( { _id : ... }, { $inc : { y : 2 } } ); // increment y by 2

MongoDB disk writes are lazy.  If we receive 1,000 increments in one second for the object, it will only be written once.  Physical writes occur a couple of seconds after the operation.

One question is what happens when an object grows.  If the object fits in its previous allocation space, it will update in place.  If it does not, it will be moved to a new location in the datafile, and its index keys must be updated, which is slower.  Because of this, Mongo uses an adaptive algorithm to try to minimize moves on an update.  The database computes a padding factor for each collection based on how often items grow and move.  The more often the objects grow, the larger the padding factor will be; when less frequent, smaller.

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