PersistentProperty attribute was introduced in XSockets 5. Applying this attribute will store the value of the property and then re-populate the value when the client reconnects.
By default the reading and writing of property values is handled by the
IPersistentPropertyStorage module. However, since this is just a plugin you can overwrite it with your own functionality and store the values in any way you see fit. The example of a custom property storage will be using Azure Table Storage.