1. A scalable method for delivering ordered data updates from a plurality of data sources, the method comprising: receiving, at a computer system, data updates from a plurality of data sources; assigning, by the computer system, a global sequence identifier to each data update, wherein the global sequence identifier assigned to a data update indicates a position of the data update in a global sequence of data updates comprising the data updates received from each of the plurality of data sources; assigning, by the computer system, a previous global sequence identifier to each data update, wherein the previous global sequence identifier assigned to a data update is the global sequence identifier assigned to a preceding data update received from the same data source; identifying a low water mark, wherein the low water mark is a maximum of the global sequence identifiers for which all of the data updates in the global sequence having a lesser global sequence identifier have been delivered; determining, by the computer system for each data update, whether the preceding data update from the same data source has been delivered without requiring independent delivery status information to be maintained for each of the plurality of data sources, the determination comprising comparing the previous global sequence identifier of each data update with the low water mark; and delivering each data update in response to a determination that the preceding data update from the same data source has been delivered.