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[fitsbits] Start of the 'INHERIT' Public Comment Period
There are two different issues being discussed he
1) What is a convention, and what is our role in documenting them, and 2) Is the INHERIT convention (most recently) a good idea Regarding 1), I suggest that a convention is not an official recommended standard, and if we are confused about this fact, we cannot proceed to document conventions. I suspect that if we examine any convention carefully in the same way that we do broad FITS standards, we will find plenty of things to be concerned about which each individual convention. Probably if this were not the case, and it were generally useful, it would long ago have been promoted as a general standard. Nontheless, conventions can be quite useful for solving more limited problems. The FITS registry of conventions should not "recommend" or "discourage" any convention. If we start making such distinctions are we are starting to repeat the standards process. I suggest it is better to merely document them uniformly. We can attach some of the discussions of the review groups to inform potential adopters of any issues. It is not our job however, to revisit the design of each convention, or we are repeating the standards process (and we will probably throw out 80% of them). Regarding 2), INHERIT is an established convention in current use, and as such should be documented. It is absolutely fine if we do so with various caveats about possible issues. Regarding whether INHERIT is a good idea: I can comment on that a bit as I had a lot to do with creating this way back when. The point is not a reduction in file size as Bill suggests, but to avoid duplicating information in the way the MEF file is stored. Duplicating information in a complex data structure is bad, and causes problems with, for example, dynamic updates. At run time, when an individual extension is accessed, the inherited information is supposed to be included, and the header is restored to its full logical size. Hence if one "imcopies" a single extension, the inheritance is resolved and the result is a self-contained FITS object. I agree with Steve that this is a simple example of a broader problem of associating relational entities. FITS is in essence a relational system; every FITS object (even an image) is actually a table. INHERIT is a simple means for specifying the relationship between two or more tables composed as an MEF. A FITS MEF is a simple container with one level of structure. Every extension logically inherits from the "global header" (primary HDU). One can resolve the inheritance to simplify access to an individual extension, but this is problematic as it is very easy to get into a situation where updates affecting the entire MEF object do not propagate to all the extensions. The real problem with INHERIT is that it is a simplistic solution to what is a more general problem. On the other hand, it *is* simple, and is adequate for such complex data where we need to aggregate a number of primary data objects (images, tables) into a container. Like most conventions, it does not fully address the underlying problem. - Doug |
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