1. A method for incrementally updating a multi-user document index distributed over a plurality of index servers, the method performed by one or more computing devices, the method comprising: generating an index mutation comprising one or more tokens, wherein the index mutation is based at least in part on one or more identified differences between a new version of a document and a previous version of the document; appending the index mutation and an associated time-based value to a log; reading the time-based value associated with the index mutation from the log and comparing the associated time-based value to a time-based value associated with an index of the document, the index of the document stored at an index server of the plurality of index servers; wherein the time-based value associated with the index of the document stored at the index server represents a last-update time of the index of the document stored at the index server; based at least in part on the reading and the comparing, providing the one or more tokens of the index mutation read from the log to the index server at which the index of the document is stored; and wherein the providing the one or more tokens to the index server causes the index server to use the one or more tokens to update the index of the document stored at the index server.
2. The method of claim 1 , wherein at least one of the one or more tokens is identified in the new version of the document but not the previous version of the document.