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Re: [rest-discuss] Information hiding?

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  • Nick Gall
    My favorite expression of information hiding is this passage from the closing section of David Parnas s seminal essay, On the Criteria To Be Used in
    Message 1 of 14 , Feb 28, 2013
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      My favorite expression of information hiding is this passage from the closing section of David Parnas's seminal essay, "On the Criteria To Be Used in Decomposing Systems into Modules": "...it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which are likely to change. Each module is then designed to hide such a decision from the others.
      Parnas D.L. (December 1972). "On the Criteria To Be Used in Decomposing Systems into Modules". Comm ACM 15 (12): 1053–8. doi:10.1145/361598.361623

      In other words, information hiding is the hiding of design decisions--especially those design decisions likely to change. Information hiding is NOT just about hiding implementation details; it is much more about hiding (an at least indirecting--a form of hiding) design issues. But here we are 40 years later, still starting our designs by decomposing systems into steps in a process diagram.

      -- Nick

      Nick Gall
      Phone: +1.781.608.5871
      Other Contact Info: http://bit.ly/nickgall


      On Thu, Feb 28, 2013 at 3:13 AM, Roy T. Fielding <fielding@...> wrote:
      On Feb 27, 2013, at 3:23 PM, Eric J. Bowman wrote:

      > Alan Dean wrote:
      >>
      >> I can see *"The resource implementation details are hidden behind the
      >> interface."* in [1] but that doesn't seem to be in conflict with
      >> *"allows information hiding through a generic interface"* to me.
      >>
      >
      > So "information hiding" is shorthand for "implementation-detail hiding"?
      > I guess I was thinking "uses SQL" is an implementation detail, while
      > the results of a SQL query would be "information," and I was missing
      > some subtlety.

      Information hiding is one of the first software engineering
      principles learned when systems became large enough to require
      modularity.

      http://www.itmweb.com/essay550.htm

      ....Roy



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    • Eric J. Bowman
      ... There s the subtlety I was missing; thanks, Roy. -Eric
      Message 2 of 14 , Feb 28, 2013
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        "Roy T. Fielding" wrote:
        >
        > Information hiding is one of the first software engineering
        > principles learned when systems became large enough to require
        > modularity.
        >
        > http://www.itmweb.com/essay550.htm
        >

        There's the subtlety I was missing; thanks, Roy.

        -Eric
      • Eric J. Bowman
        ... My lack of formal training may be an advantage, because that s not how I did it. I m writing a modular httpd, where a NIO listener just makes
        Message 3 of 14 , Feb 28, 2013
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          Nick Gall wrote:
          >
          > But here we are 40 years later, still starting our designs by
          > decomposing systems into steps in a process diagram.
          >

          My lack of formal training may be an advantage, because that's not how
          I did it. I'm writing a modular httpd, where a NIO "listener" just
          makes connections, and passes the socket's file descriptor off to a
          vhost process; I define a vhost as one or more IP's / FQDN's which
          share a configuration, the vhost is just a threaded cache incapable of
          generating content.

          Static files are served by a standalone FastCGI app, which maintains
          multiplexed connections to those vhosts which use it. That's three
          modules whose boundaries were determined by design decision -- the
          listener is shaking out as procedural, vhost as OOP, file server as
          functional, written in different languages.

          >
          > Parnas D.L. (December 1972). "On the Criteria To Be Used in
          > Decomposing Systems into Modules". Comm ACM 15 (12): 1053–8.
          > doi:10.1145/361598.361623
          >

          Guess I'd better give that a read while I can still start over. Just
          because I don't know what I'm doing, doesn't mean I can't learn! So
          thanks for (unknowingly) pointing me the right direction. I wasn't even
          going to mention my httpd project, except this is topical and timely to
          what it is I'm doing.

          -Eric
        • Markus Lanthaler
          Thanks for the great pointer Nick. While hiding functionality isn t really surprising giving REST s uniform interface it is much less apparent when looking
          Message 4 of 14 , Feb 28, 2013
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            Thanks for the great pointer Nick.

             

            While hiding “functionality” isn’t really surprising giving REST’s uniform interface it is much less apparent when looking at the data model - at least for me.

             

            Sure, you can decouple your internal data model from your external one by creating (and standardizing) a media type. But what are guidelines to create such a type? Which data do you expose and which do you hide? What if you decide to remove some data in the future or it is just not reasonable anymore to expose it?

             

             

            Cheers,

            Markus

             

             

            --

            Markus Lanthaler

            @markuslanthaler

             

             

             

            From: rest-discuss@yahoogroups.com [mailto:rest-discuss@yahoogroups.com] On Behalf Of Nick Gall
            Sent: Thursday, February 28, 2013 4:01 PM
            To: Rest List
            Subject: Re: [rest-discuss] Information hiding?

             

            My favorite expression of information hiding is this passage from the closing section of David Parnas's seminal essay, "On the Criteria To Be Used in Decomposing Systems into Modules": "...it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which are likely to change. Each module is then designed to hide such a decision from the others.

            Parnas D.L. (December 1972). "On the Criteria To Be Used in Decomposing Systems into Modules". Comm ACM 15 (12): 1053–8. doi:10.1145/361598.361623

             

            In other words, information hiding is the hiding of design decisions--especially those design decisions likely to change. Information hiding is NOT just about hiding implementation details; it is much more about hiding (an at least indirecting--a form of hiding) design issues. But here we are 40 years later, still starting our designs by decomposing systems into steps in a process diagram.

             

            -- Nick


            Nick Gall
            Phone: +1.781.608.5871
            Other Contact Info: http://bit.ly/nickgall

          • Keith Hassen
            Very interesting topic. :) This might just be a philosophical tangent, but ... I wonder if some of the lessons from evolutionary theory would be applicable ...
            Message 5 of 14 , Feb 28, 2013
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              Very interesting topic. :)

              This might just be a philosophical tangent, but ... I wonder if some of the lessons from evolutionary theory would be applicable ... systems that can interact successfully (APIs? agents of the web?), produce offspring (e.g. web systems that learn from each other and produce new, more flexible systems), with the occasional mutation (experimentation by design).  You may start in a very dark place but provided you define the right fitness goals (the measure of success), it could be that the answer to these kinds of questions are necessarily hidden from us and are best solved by encouraging interaction and growth. :)

              K


              On 2013-02-28, at 1:00 PM, Markus Lanthaler wrote:

               

              Thanks for the great pointer Nick.

               

              While hiding “functionality” isn’t really surprising giving REST’s uniform interface it is much less apparent when looking at the data model - at least for me.

               

              Sure, you can decouple your internal data model from your external one by creating (and standardizing) a media type. But what are guidelines to create such a type? Which data do you expose and which do you hide? What if you decide to remove some data in the future or it is just not reasonable anymore to expose it?

               

               

              Cheers,

              Markus

               

               

              --

              Markus Lanthaler

              @markuslanthaler

               

               

               

              From: rest-discuss@yahoogroups.com [mailto:rest-discuss@yahoogroups.com] On Behalf Of Nick Gall
              Sent: Thursday, February 28, 2013 4:01 PM
              To: Rest List
              Subject: Re: [rest-discuss] Information hiding?

               

              My favorite expression of information hiding is this passage from the closing section of David Parnas's seminal essay, "On the Criteria To Be Used in Decomposing Systems into Modules": "...it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which are likely to change. Each module is then designed to hide such a decision from the others.

              Parnas D.L. (December 1972). "On the Criteria To Be Used in Decomposing Systems into Modules". Comm ACM 15 (12): 1053–8. doi:10.1145/361598.361623

               

              In other words, information hiding is the hiding of design decisions--especially those design decisions likely to change. Information hiding is NOT just about hiding implementation details; it is much more about hiding (an at least indirecting--a form of hiding) design issues. But here we are 40 years later, still starting our designs by decomposing systems into steps in a process diagram.

               

              -- Nick


              Nick Gall
              Phone: +1.781.608.5871
              Other Contact Info: http://bit.ly/nickgall



            • Nick Gall
              On Thu, Feb 28, 2013 at 1:00 PM, Markus Lanthaler ... Parnas gave this general advice in his same paper: A data structure, its internal linkings, accessing
              Message 6 of 14 , Feb 28, 2013
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                On Thu, Feb 28, 2013 at 1:00 PM, Markus Lanthaler <markus.lanthaler@...> wrote:

                Sure, you can decouple your internal data model from your external one by creating (and standardizing) a media type. But what are guidelines to create such a type? Which data do you expose and which do you hide? What if you decide to remove some data in the future or it is just not reasonable anymore to expose it?


                Parnas gave this general advice in his same paper: "A data structure, its internal linkings, accessing procedures and modifying procedures are part of a single module. They are not shared by many modules as is conventionally done. This notion is perhaps just an elaboration of the assumptions behind the papers of Balzer [9] and Mealy [10]. Design with this in mind is clearly behind the design of BLISS [11]."

                What I like about this advice is it gets across that "module" as the unit of change according to Parnas does not mean a function, or a method, or a subroutine. A module means the entire set of dependencies that should change together. I think only aspect-oriented folk really get Parnas's sense of modularity.

                Of course this begs the question of which parts of a data model, especially a shared one like a media type, "change together." I know of only a few general, cross-domain, principles regarding this issue. The rate of change of different parts of a data model are IME very domain-specific. That's why good data modeling is not a technical exercise, it is a business (or similar domain) exercise--only domain experts have a deep sense of the varying rates of change of the aspects of their domain.

                That said, one of the few powerful general principles for factoring interfaces into "modules" whose components are more likely to change at the same rate comes from the AWWWv1: http://www.w3.org/TR/webarch/#orthogonal-specs . "Identification, interaction, and representation are orthogonal concepts, meaning that technologies used for identification, interaction, and representation may evolve independently." Or as I like to put it: IFaPs (Identifiers, Formats, & Protocols) should be as loosely coupled as possible because they are likely to change at different rates. Protocols change the slowest, identifiers next, and formats the most quickly.

                "Architects can mature from being artists of space to become artists of time." — Stewart Brand

                -- Nick

              • Markus Lanthaler
                ... Yeah.. that s one of the aspects that is still not 100% clear to me. (Most) media types not only define a serialization (representation) format but also
                Message 7 of 14 , Mar 1 3:14 AM
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                  On Thursday, February 28, 2013 8:56 PM, Nick Gall wrote:

                  > That said, one of the few powerful general principles for
                  > factoring interfaces into "modules" whose components are
                  > more likely to change at the same rate comes from the
                  > AWWWv1: http://www.w3.org/TR/webarch/#orthogonal-specs.
                  > "Identification, interaction, and representation are
                  > orthogonal concepts, meaning that technologies used for
                  > identification, interaction, and representation may evolve
                  > independently." Or as I like to put it: IFaPs (Identifiers,
                  > Formats, & Protocols) should be as loosely coupled as possible
                  > because they are likely to change at different rates. Protocols
                  > change the slowest, identifiers next, and formats the most quickly.

                  Yeah.. that's one of the aspects that is still not 100% clear to me. (Most)
                  media types not only define a serialization (representation) format but also
                  interaction models and sometimes specific semantic. Some are domain-specific
                  (to various degrees) while others are completely generic.

                  I tend more towards the use of completely domain-agnostic media types. Of
                  course, such media types need to provide some mechanism to express concrete
                  semantics (and thus hypermedia controls). The form doesn't really matter..
                  whether, e.g., XML-namespaces are used or a profile is associated with a
                  JSON file (preferably as media type parameter so that it's usable in conneg)
                  doesn't make a difference. I think this separation of concerns makes it much
                  easier to evolve systems in the long term since the parts may change at
                  different rates.

                  Maybe Roy could share his view on this as well.. I know he ran out of time
                  doing so a couple of years ago :-)


                  Thanks a lot,
                  Markus


                  --
                  Markus Lanthaler
                  @markuslanthaler
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