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#95: Development of CF 1.5 Data Model -----------------------------+---------------------------------------------- Reporter: markh | Owner: [email protected] Type: task | Status: new Priority: medium | Milestone: Component: cf-conventions | Version: Resolution: | Keywords: -----------------------------+---------------------------------------------- Comment (by bnl): Hi Folks We now have two threads running on the global/variable attribute dilemma: (1) do such things have any semantic difference? and (2) what to do with data where both global and variable attributes with the same name exist (the precedence argument). Taking the second first: I think the consensus of the argument thus far is that we *may* have data where Jonathan's interpretation of precedence (aka replace) might give the wrong results, and that in any case, we need to build up provenance as we manipulate data, which means we want to keep adding information to attributes as we process data. What then should be the best practice to do the latter? Multiple attributes with the same name, or add/aggregate information into the same attribute? Effectively we can't have multiple attributes with the same name, so "best practice" is to add provenance to "the" history comment as software processes data. Note that in the previous paragraph I didn't mention whether the attributes were "file" or "variable". The same argument applies to both. So now we can think of file/variable attribute conflict as just being a special case of having to deal with aggregating information as software processes data (and that will now go past history, we might be adding authors sources etc). There are three possible conflict cases to deal with: 1. Global attribute and variable attribute of the same name exists, and they are complementary, that is, the right answer is to join them together in some sense. 1. Global attribute and variable attribute of the same name exists, but the intent was that the global attribute has been overridden by the variable attribute. 1. Global attribute and variable attribute of the same name exists, but the intent was that the global attribute overides the variable attribute. The third we can discard. If such data exists, the file is not CF compliant. The first two are clearly at least consistent with some interpretations of CF, but frankly, we can't know what was intended. We can fix that in a future version, but we are where we are. Returning to my very first enumeration, then we have the semantic difference issue. I believe that my previous interventions, plus the argument above about workflow, suggest that there is no semantic difference between global and variable attributes, we simply have to deal with the problem above: two possible interpretations of how to process the attributes. I suggest for existing CF data (up to 1.7 I suppose), we recommend software simply aggregate them, but tag the global one as "inherited global %s %s"%(filename,timestamp). A human will have to disambiguate any problems that arise from the correct behaviour having been replace not aggregate. This does not affect the data model! For future version of CF, we choose which of the above interpretations we want, and make it unambiguous. Either way, we simply treat global and variable attributes as having no different semantics. -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/95#comment:96> CF Metadata <http://cf-pcmdi.llnl.gov/> CF Metadata This message came from the CF Trac system. To unsubscribe, without unsubscribing to the regular cf-metadata list, send a message to "[email protected]" with "unsubscribe cf-metadata" in the body of your message.
