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RE: Should information be encoded into identifiers?

• From: "Michael Kay" <mike@saxonica.com>
• To: "'Costello, Roger L.'" <costello@mitre.org>,<xml-dev@l...>
• Date: Sat, 6 Mar 2010 00:08:46 -0000

Ah, that old question....

I tend to the view:

(a) by all means generate identifiers by applying some algorithm to the
properties of an object, but

(b) never rely on that algorithm to infer information from the identifier.

Why? Resilience to change. Usually you want the identifier to be stable even
when the accidental properties of the object change. I once worked for a
company where male employees had even personnel numbers, and female
employees had odd personnel numbers. Then there was a merger, and the old
personnel number acquired a prefix to make it unique, and the odd/even rule
was abandoned.

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay  

> -----Original Message-----
> From: Costello, Roger L. [mailto:costello@mitre.org] 
> Sent: 05 March 2010 21:58
> To: xml-dev@lists.xml.org
> Subject:  Should information be encoded into identifiers?
> 
> Hi Folks,
> 
> Should identifiers be dumb? That is, no meaning can be 
> ascribed to identifiers; they are completely random.
> 
> Or, should information be encoded into identifiers? What 
> information should be encoded into them? 
> 
> There are precedents for encoding information into identifiers:
> 
> 1. In the U.S. each auto is identified by a Vehicle 
> Identification Number (VIN). Encoded within each VIN is a 
> wealth of information, including the make and model of the 
> auto, the plant where it was manufactured, and the vehicle's 
> options.[1]
> 
> 2. Books are identified by ISBNs. Encoded within each ISBN is 
> a wealth of information, including the country, publisher, 
> and the relative size of the publisher.[2]
> 
> 3. UUIDs are used in many applications. Encoded within some 
> UUIDs are the date/time stamp of when the UUID was created, 
> and the network address of the machine which created the UUID.[3]
> 
> I suspect there are other examples of identifiers that have 
> information encoded into them.
> 
> What are the advantages of encoding information into an 
> identifier? What are the disadvantages?
> 
> /Roger
> 
> [1] Format of VIN: http://en.wikipedia.org/wiki/VIN
> 
> [2] Format of ISBN: http://www.xfront.com/isbn.xsd
> 
> [3] Fomat of UUID: http://www.ietf.org/rfc/rfc4122.txt
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• References:
  • Should information be encoded into identifiers?
    • From: "Costello, Roger L." <costello@mitre.org>