A system is arranged to receive data which is written to a data store by a writer. A controller is able to read data from the data store. That controller is able to control the rate at which data is read from the data store with respect to the rate at which data is written to the data store. A query function receives a stream of said data substantially in real time and when said stream of data is unavailable in real time, the query function is able to subsequently obtain that unavailable data from said data store.