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RE: dynamic module

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  • Stephen Bardsley
    Hi John: It s ironic how similar our applications seem! I have a perl module that wraps around DBI, and get this, it is called Db.pm also. Not to worry, it
    Message 1 of 8 , Sep 16, 2002
      Hi John:

      It's ironic how similar our applications seem!

      I have a perl module that wraps around DBI, and
      get this, it is called Db.pm also. Not to worry,
      it has a unique namespace prefix, so they will not
      clash if ever they meet. A good thing for modules
      I think. BTW -- I use the selective query tricks too.
      It's a big help.

      If you are able to use global vars (use vars) in
      your subs with no problem, then I've some work ahead
      of me. I think it points to the fact that either I have
      a systematic problem with my code or I have uncovered a
      bug/feature in some of the underlying code.

      I'm still open for suggestions on what might cause a
      global to be unreliable in subs or other .asp
      scripts. Thanks again.

      Regards,
      Steve

      > -----Original Message-----
      > From: Brat Wizard [mailto:brat@...]
      > Sent: Tuesday, September 17, 2002 12:50 AM
      > To: Stephen Bardsley; Apache::ASP Mailing List
      > Subject: Re: dynamic module
      >
      >
      > > I want my variable to be valid and defined for the duration of a Session.
      >
      > Okay, that's what the session object is for, as you appear to already know. ;)
      >
      > > > example) is to use Apache::DBI. Apache::DBI will override the normal DBI
      >
      > > I am using and have used Apache::DBI with good results. I'm sorry, I
      > > didn't mean to imply "on-the-fly" as from one script to the next. To be
      > > more precise I meant from one Session to the next.
      >
      > Okay-- so if I'm understanding you correctly, connecting to the database each
      > time the script is executed (a la Script_OnStart) is okay, so what is the
      > problem with say storing the datasource parameters as session data and then
      > using it to reconstitute the handle each time? You can't cache the handle
      > itself, but you _can_ cache the name of the database connected to (and
      > whatever host/user info you need) as session scalars (or a hash-- or even
      > just store the actual DBI data source string itself if that's all you really
      > need). Then just use that information the next time around to re-connect to
      > the database.. Basically check to see if we've been connected before (ie. is
      > $..dbname defined, or whatever). If not, store it. When you're done, close
      > the connection and zap the data in the session variable (or abandon the
      > session altogether). Then go through the process all over again for the next
      > session..
      >
      >
      > You didn't say, but perhaps you are concerned about caching selected data from
      > a query and that's what the real bottom-line issue is?? (as in: why it would
      > be interesting/useful to be able to store a db handle in the session object
      > anyway...)
      >
      > This is a trick I often use.. if the query is expensive (a full-text search or
      > something) and I don't want to have to rerun the query each time to get the
      > next chunk of data-- what I will do is run the query and ONLY select the
      > actual key(s) to the data itself in whatever order they're needed. (eg.
      > SELECT key_field FROM table WHERE blah blah ORDER BY blah blah) Then blast
      > the keys into a file-- several ways to do this quickly & effectively, the
      > easiest is to simply create a key array and use Storable to freeze it in a
      > file. Then you can just thaw it out later and use the list.
      >
      > By using a key array, you can construct a query retrieving _only_ the items
      > needed for the specific page which is generally very fast (much much faster
      > usually) compared to running the query all over again (eg. SELECT
      > fields_i_need FROM table WHERE key=key1 or key=key2 or key=key3..etc) They
      > are already in the correct order so you don't need the overhead of an "order
      > by" clause.
      >
      > When you're through, dump the key array whenever its convenient (or don't if
      > you don't care about disk space-- the frozen files are small, you can reap
      > them at your leisure).
      >
      > If I'm displaying paged data, a simple rule I often employ is that either a
      > new sort order (clicking on a column header for instance) or retrieving page
      > zero will dump the cache & rerun the query (making a new key array). It
      > generally works quite well, and I've been very pleasantly surprised at how
      > small the frozen cache files are, even for large queries. And slurping up a
      > frozen array is pretty darned quick. If you hide all the mechanical bits
      > inside functions, pretty soon you forget how horrible a kludge this really is
      > and it almost begins to seem normal :) ;)
      >
      >
      > > > The next easiest thing to do is to figure out what database to connect to
      > > > and open it each time the script runs (the Script_OnStart() function is a
      >
      > >
      > > This is kind of what I am doing now, and can't quite get things working.
      > > It seems it may actually be an issue with variable scoping. If I scope
      > > my module's instance variable in the Script_OnStart things seem to work.
      > > That is, I create the instance in Script_OnStart. If I create the
      > > instance in one of global.asa's sub's then it does not seem to work
      > > reliably. Bear in mind I am using global variables in global.asa
      > > by way of "use vars qw( $var2 $var2 $etc );". If this is incorrect
      > > or a better way exists, I'm all ears. Thanks again for your time.
      >
      > Hmm- interesting. I do it in subs all the time with no problems whatsoever. I
      > declare the global variable the same way you do, "use vars qw( $db ... blah
      > blah);" at the top of my global.asa file (and again in EVERY package file
      > that is not a separate module-- not a separate namespace-- I break a lot of
      > my code up into sub-files to keep it more manageable). The scope should be
      > global. Might you have some my variable someplace that's stomping on the
      > global version??
      >
      > I have a Db.pm module that I wrote that basically encapsulates most of the DBI
      > stuff I use-- so keep that in mind in the short example below-- it isn't
      > exactly what you're used to but it will be similar enough. In one of my
      > global.asa files, the flow goes something like this:
      >
      > package Apache::ASP::STORES;
      >
      > use vars qw{ $db ... [more vars] };
      > use lib Apache->request->dir_config('BaseDir')."/lib";
      >
      > use Apache::ASP;
      > use Db;
      >
      > [more use statements]
      >
      > sub Script_OnStart {
      > [..do stuff]
      >
      > connectToDB();
      >
      > [.. do more stuff.. ]
      > }
      >
      > sub connectToDB {
      > my $dbname = <my dbname>; ## actually declared elsewhere
      > my $dbhost = <my host>; ## for me, but you get the idea
      > return if ($db ne undef); ## got a handle thanks, don't need another
      > $db = new Db;
      > $db->Connect($dbname, $dbhost);
      > return $db;
      > }
      >
      >
      > In this case, the $db handle that gets returned is of my own "db" style that I
      > mentioned. But it only wraps around the usual dbh handle and is mostly
      > exposed except for wrapping commonly used constructs to make slapping
      > together code quicker, easier, and a bit more mindless. Its pretty easy
      > actually, even my cat could do it... (as it happens, I walked into the
      > computer room one day and there he was sitting building a database cataloging
      > north american migratory birds and cross-referencing them by recipe... but
      > that's another story altogether ;)
      >
      >
      > Hope this helps-
      >
      > John Whitten
      > brat@...
      > Wizard.Org, Inc.
      >
      >
      >
      > >
      > > > The best way (but not necessarily the easiest) is to use some sort of
      > > > middleware such as SQLrelay or something similar that will sit between
      > > > your perl application and the database, maintain open connections and
      > > > drop or establish new connections as demanded. SQLrelay is a neat thing
      > > > but it is a pain-in-the-ass (IMO) to install and configure. (I should
      > > > also point out that my experience with it is some 6+months old so a newer
      > > > version which may address some of these issues may be available?)
      > > > SQLrelay works with just about any database DBI can work with, and in
      > > > addition can work across servers, and I *think* (perhaps it was a planned
      > > > feature, in any case I never used it myself) it had auto-failover support
      > > > in case one server didn't answer up-- won't swear to that though- perhaps
      > > > it was on the to-do list. In any case, it is pretty clever software to
      > > > say the least.
      > > >
      > > > On Monday 16 September 2002 05:11 pm, Stephen Bardsley spewed into the
      > ether:
      > > > > Greetings:
      > > > >
      > > > > I have been struggling with something and I hope
      > > > > someone can set me straight.
      > > > >
      > > > > <%
      > > > > I have a perl module, which contains a database
      > > > > handle (dbh).
      > > > >
      > > > > I would like to use this module under Apache::ASP.
      > > > > I would like to have users dynamically define a database
      > > > > for the module to connect to. I can/will constrain
      > > > > the lifetime of the module to that of an Apache::ASP
      > > > > $Session.
      > > > >
      > > > > The trouble I am having is that I cannot maintain
      > > > > the coherence of the dbh in the module. Sometimes
      > > > > the dbh is defined and sometimes not. I have used
      > > > > this module successfully in other Apache::ASP
      > > > > applications, but the database has always been
      > > > > statically defined to the module.
      > > > >
      > > > > Is there a way to allow the user to define the
      > > > > database to the module?
      > > > > %>
      > > > >
      > > > > Any suggestions will be greatly appreciated.
      > > > >
      > > > > Steve
      > > > > _____________________
      > > > > Stephen Bardsley
      > > > > RLW Inc.
      > > > > Malta, NY
      > > > >
      > > > > ---------------------------------------------------------------------
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      > > >
      > > > --
      > > >
      > > > -------------------------------------------------------------------------
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      >
      > --
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      > --------------------------------------------------------------------------------
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      > *NEW* Computer Parts & Accessories - Drives - LCD - Systems - Linux
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    • Josh Chamas
      ... I recently discovered a combination of UseStrict setting with PerlWarn On can uncover some nasty errors like my closure variable scoping issues, that could
      Message 2 of 8 , Sep 16, 2002
        Stephen Bardsley wrote:
        > I'm still open for suggestions on what might cause a
        > global to be unreliable in subs or other .asp
        > scripts. Thanks again.
        >

        I recently discovered a combination of UseStrict setting
        with PerlWarn On can uncover some nasty errors like
        my closure variable scoping issues, that could be getting
        mistaken for globals & such. You might give those configs
        a try and see if anything breaks.

        Regards,

        Josh
        ________________________________________________________________
        Josh Chamas, Founder phone:925-552-0128
        Chamas Enterprises Inc. http://www.chamas.com
        NodeWorks Link Checking http://www.nodeworks.com


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      • Brat Wizard
        ... Hmm- I contacted the naming committee, they took the matter under advisement, generated half-a-dozen candidates, handed it off to the selection committee
        Message 3 of 8 , Sep 16, 2002
          On Tuesday 17 September 2002 01:54 am, Stephen Bardsley spewed into the ether:
          > Hi John:
          >
          > It's ironic how similar our applications seem!
          >
          > I have a perl module that wraps around DBI, and
          > get this, it is called Db.pm also.

          Hmm- I contacted the naming committee, they took the matter under advisement,
          generated half-a-dozen candidates, handed it off to the selection committee
          (which by some quirk, only meets the second tuesday following a full moon)
          who selected three finalists then handed everything off to management, legal,
          and marketing research, who in turn rejected one right out (for unspecified
          legal reasons I think it was), and another after two independent (and
          double-blind-- from-birth apparently) focus groups gave it a pass-- until
          finally they settled on the one we didn't use. So we ended up with "Db.pm"
          instead. Your way seems a little easier, I may try that next time ;)

          > If you are able to use global vars (use vars) in
          > your subs with no problem, then I've some work ahead
          > of me. I think it points to the fact that either I have
          > a systematic problem with my code or I have uncovered a
          > bug/feature in some of the underlying code.

          Probably not a code failure. I think Josh may have a clue in his reply (I read
          it before I saw yours).

          > Thanks again.

          No prob, that's what we're all here for-- so many people have helped me, its
          nice to be able to help someone else for a change ;)


          > Regards,
          > Steve

          John

          --

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          *NEW* Computer Parts & Accessories - Drives - LCD - Systems - Linux
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          Write to us: sales@... -- Get your Store Online Today!
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        • Stephen Bardsley
          Josh: I always have UseStrict on, but your suggestion of turning on PerlWarn was like some cold water in the face. I normally have it turned on, but not this
          Message 4 of 8 , Sep 17, 2002
            Josh:

            I always have UseStrict on, but your suggestion of
            turning on PerlWarn was like some cold water in the
            face. I normally have it turned on, but not this time.
            That simple step illuminated a number of silly little
            issues that conspired to put my application in the
            crapper. While it's still not perfect, I am back on
            track adding value and not time.

            Sometimes it's real nice to have someone looking over
            your shoulder, thank you.

            Also, as you probably suspected, this problem was
            apparently related to my other post RE: strange error,
            it has become remarkably better.

            Thanks again.

            Steve

            > -----Original Message-----
            > From: Josh Chamas [mailto:josh@...]
            > Sent: Tuesday, September 17, 2002 2:36 AM
            > To: Stephen Bardsley
            > Cc: Apache::ASP Mailing List
            > Subject: Re: dynamic module
            >
            >
            > Stephen Bardsley wrote:
            > > I'm still open for suggestions on what might cause a
            > > global to be unreliable in subs or other .asp
            > > scripts. Thanks again.
            > >
            >
            > I recently discovered a combination of UseStrict setting
            > with PerlWarn On can uncover some nasty errors like
            > my closure variable scoping issues, that could be getting
            > mistaken for globals & such. You might give those configs
            > a try and see if anything breaks.
            >
            > Regards,
            >
            > Josh
            > ________________________________________________________________
            > Josh Chamas, Founder phone:925-552-0128
            > Chamas Enterprises Inc. http://www.chamas.com
            > NodeWorks Link Checking http://www.nodeworks.com
            >
            >

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