1. A method performed on a computing device, the method comprising: obtaining an indication that a file has changed; checking whether the changed file is in a protected namespace, where files in the protected namespace are designated for back up to backup storage; if the changed file is in the protected namespace, then: maintaining, in an index, references to an object that represents versions of the changed file, adding to the index a new token for each new token found in the changed file, adding, to the index, references to the changed file for each new token found in the changed file; and if the changed file is not in a protected namespace, then: deleting, from the index, prior references to the changed file, where the prior references existed prior to when the changed file was changed deleting, from the index, prior tokens that reference only the changed file, where the prior tokens existed prior to when the changed file was changed, adding, to the index for any token found in the changed file, a new token, and updating, in response to each token in the index that corresponds to a token in the changed file, the each token to reference the changed file.