21.4 Durable Reader State

Durable reader state allows a DataReader to locally store its state in disk and remember the data that has already been processed by the application1 The circumstances under which a data sample is considered “processed by the application” are described in the sections that follow.. When an application restarts, each DataReader configured to have durable reader state automatically reads its state from disk. Data that has already been processed by the application before the restart will not be provided to the application again.

Important: The DataReader does not persist the full contents of the data in its historical cache; it only persists an identification (e.g. sequence numbers) of the data the application has processed. This distinction is not meaningful if your application always uses the ‘take’ methods to access your data, since these methods remove the data from the cache at the same time they deliver it to your application. (See 41.3.1 Read vs. Take) However, if your application uses the ‘read’ methods, leaving the data in the DataReader's cache after you've accessed it for the first time, those previously viewed samples will not be restored to the DataReader's cache in the event of a restart.