A data access system includes proxy servers for caching "local" copies of selected data sets stored on data servers to which users request access. Each proxy server is adapted to generate a subscription request message in respect of each identified cached data set for submission to a conventional "publish & subscribe" data distribution system. The data distribution system is arranged with access to "published" updates to data sets, made available from respective data servers. Upon receipt of a published updated data set having an identifier matching that in an earlier-received subscription request, the data distribution system forwards the data set to the subscribing proxy server to enable update to the respective cached copy. Thus, a proxy server, having decided to cache a particular data set, need only issue a subscription request message in order to receive all subsequent updates, as they become available, until choosing to remove the data set from the cache.