Take a look at the $push and $addToSet[1] update operators in mongodb. They are both for atomically adding one or more elements to an array in an object. On the

Take a look at the $push and $addToSet[1] update operators in mongodb. They are both for atomically adding one or more elements to an array in an object. On the other hand, MongoDB doesn't support server-side set union or intersection so if you are using those features redis would still probably be a good use there. [1] http://www.mongodb.org/display/DOCS/Updating#Updating-%24addToSetand%24each http://www.mongodb.org/display/DOCS/Updating#Updating-%24add...