InitialWrite - softwareunderground/xsdf GitHub Wiki
Starting the write process
Many storage systems force you to know the exact setup of the data object you want to write before you can actually write data. The problem is that this is something you may not know at that point in time. Therefore, an important service that the system should provide is to make sure it can receive data not just unordered, but also collecting the geometry while writing.
This on-the-fly establishing of geometry is not an easy task, but it is such a life-saver for many entry-level situations that it is deemed very important.