A server system receives a first request for a data item from a web application on a client device, wherein the first request includes an identifier of the data item. In response to the first request, the server system calculates a first checksum of the data item and sends the data item and the first checksum to the web application, for storage in the local database. The server system receives a second request for the data item from the web application, calculates a second checksum of the data item and compares the first and second checksums. The server system, determines that the data item has been updated by comparing the first checksum to the second checksum; and in response to determining that the data item has been updated, the server system a reply to the web application a indicating that the data item has been updated.