Caching a hash - am I missing something? - modperl

This is a discussion on Caching a hash - am I missing something? - modperl ; Hi, This might be a little off topic, I hope it's OK to post. I'm not positive if mod_perl matters or not because it's a little confusing to me. I've taken over some pretty old code that I'm updating and ...

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Thread: Caching a hash - am I missing something?

  1. Caching a hash - am I missing something?

    Hi,



    This might be a little off topic, I hope it's OK to post. I'm not positive
    if mod_perl matters or not because it's a little confusing to me.



    I've taken over some pretty old code that I'm updating and making mp2
    content handlers out of. The main script is a standard cgi script
    "start.cgi" there is nothing special in the apache conf for it.





    SetHandler perl-script

    PerlFixupHandler My::Fixup

    PerlResponseHandler ModPerl::PerlRun

    PerlOptions +ParseHeaders

    DirectoryIndex start.cgi

    Options +ExecCGI +Indexes

    allow from all





    start.cgi calls a custom module (use CustomModule which exports a bunch of
    subs, for example foobar and all over the place in the subs that are
    exported from CustomModule I see code like





    sub foobar {

    my $key = @_;



    if ($cache{$key}) {

    return $cache{$key};

    } else {

    my $do_some_query = xxxx;

    $cache{$key} = $do_some_query_results

    return $cache{$key};

    }

    }



    My question is isn't the "else" in foobar always going to be true anyplace
    where start.cgi is calling "&foobar('somekey')"??????

    I don't understand how %cache could already be populate from a previous
    browser request or something? I'm I just missing something stupid?



    FYI this was all running before on a version of Debian with old apache and
    old mod_perl (early 1.99 and 2.0.x). I've updated mod_perl to 2.0.2 and
    apache to 2.2 and everything still works.



    TIA!!

    -Chris

















  2. Re: Caching a hash - am I missing something?

    On Tue, Aug 19, 2008 at 5:35 PM, Chris Faust wrote:
    > Hi,
    >
    >
    >
    > This might be a little off topic, I hope it's OK to post. I'm not positive
    > if mod_perl matters or not because it's a little confusing to me.
    >
    >
    >
    > I've taken over some pretty old code that I'm updating and making mp2
    > content handlers out of. The main script is a standard cgi script
    > "start.cgi" there is nothing special in the apache conf for it.
    >
    >
    >
    >
    >
    > SetHandler perl-script
    >
    > PerlFixupHandler My::Fixup
    >
    > PerlResponseHandler ModPerl::PerlRun
    >
    > PerlOptions +ParseHeaders
    >
    > DirectoryIndex start.cgi
    >
    > Options +ExecCGI +Indexes
    >
    > allow from all
    >
    >

    >
    >
    >
    > start.cgi calls a custom module (use CustomModule which exports a bunch of
    > subs, for example foobar and all over the place in the subs that are
    > exported from CustomModule I see code like
    >
    >
    >
    >
    >
    > sub foobar {
    >
    > my $key = @_;
    >
    >
    >
    > if ($cache{$key}) {
    >
    > return $cache{$key};
    >
    > } else {
    >
    > my $do_some_query = xxxx;
    >
    > $cache{$key} = $do_some_query_results
    >
    > return $cache{$key};
    >
    > }
    >
    > }
    >
    >
    >
    > My question is isn't the "else" in foobar always going to be true anyplace
    > where start.cgi is calling "&foobar('somekey')"??????
    >
    > I don't understand how %cache could already be populate from a previous
    > browser request or something? I'm I just missing something stupid?


    %cache is defined outside the scope of the sub so it will persist for
    the lifetime of the apache server. The very first time
    foobar('somekey') is called it will do the query lookup, the next time
    it will return from cache.

    >
    >
    >
    > FYI this was all running before on a version of Debian with old apache and
    > old mod_perl (early 1.99 and 2.0.x). I've updated mod_perl to 2.0.2 and
    > apache to 2.2 and everything still works.
    >
    >
    >
    > TIA!!
    >
    > -Chris
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >
    >




    --
    ~Tyler


  3. Re: Caching a hash - am I missing something?

    W. Tyler Gee wrote:
    > On Tue, Aug 19, 2008 at 5:35 PM, Chris Faust wrote:
    >> Hi,
    >>
    >>
    >>
    >> This might be a little off topic, I hope it's OK to post. I'm not positive
    >> if mod_perl matters or not because it's a little confusing to me.
    >>
    >>
    >>
    >> I've taken over some pretty old code that I'm updating and making mp2
    >> content handlers out of. The main script is a standard cgi script
    >> "start.cgi" there is nothing special in the apache conf for it.
    >>
    >>
    >>
    >>
    >>
    >> SetHandler perl-script
    >>
    >> PerlFixupHandler My::Fixup
    >>
    >> PerlResponseHandler ModPerl::PerlRun
    >>
    >> PerlOptions +ParseHeaders
    >>
    >> DirectoryIndex start.cgi
    >>
    >> Options +ExecCGI +Indexes
    >>
    >> allow from all
    >>
    >>

    >>
    >>
    >>
    >> start.cgi calls a custom module (use CustomModule which exports a bunch of
    >> subs, for example foobar and all over the place in the subs that are
    >> exported from CustomModule I see code like
    >>
    >>
    >>
    >>
    >>
    >> sub foobar {
    >>
    >> my $key = @_;
    >>
    >>
    >>
    >> if ($cache{$key}) {
    >>
    >> return $cache{$key};
    >>
    >> } else {
    >>
    >> my $do_some_query = xxxx;
    >>
    >> $cache{$key} = $do_some_query_results
    >>
    >> return $cache{$key};
    >>
    >> }
    >>
    >> }
    >>
    >>
    >>
    >> My question is isn't the "else" in foobar always going to be true anyplace
    >> where start.cgi is calling "&foobar('somekey')"??????
    >>
    >> I don't understand how %cache could already be populate from a previous
    >> browser request or something? I'm I just missing something stupid?

    >
    > %cache is defined outside the scope of the sub so it will persist for
    > the lifetime of the apache server. The very first time
    > foobar('somekey') is called it will do the query lookup, the next time
    > it will return from cache.
    >

    I believe the above is almost, but not totally true. It should probably
    be "for the lifetime of this particular apache child".
    Each apache child process has it's own copy of the above code, and it's
    own copy of the above "global" (*) %cache hash. Thus whether the first
    or second part of the if will run, depends of the previous history of
    the particular apache child which handles the current request. If this
    particular child has already previously accessed the same hash key, it
    will server it from (it's own) cache, and otherwise it will execute the
    query to create the key (in it's own cache).
    Apache children are created, and die, as directed by the main Apache
    process configuration, and HTTP requests are served more or less at
    random, by whichever child is available when the request comes in.
    Is is thus quite possible for instance that the first 10 requests for a
    particular hash key would each be handled by a different apache child,
    and would each result in a query; then the 11th request would be handled
    by a child that has already accessed this same key before, and thus
    served from cache; the 13th request would be handled by a brand-new
    apache child just created, thus would re-do the query, etc..

    I this would be useful, I believe that it would be possible to avoid
    this, by "priming" the mod_perl module during the initial start of
    Apache, before it forks into children. Then each new child (being a
    copy of the main apache) would start it's life with a number of keys
    already in its cache. Of course this would work only if the contents of
    the keys of the hash are never modified while apache is running.
    And it probably involves some delicate mod_perl programming to do the
    priming process.

    (*) like the previous contributor, I guess that the %cache hash is
    somehow global, because it is not declared within the sub that you show.
    Whether it really is though, may depend on other code that we don't
    see here. But anyway, "global" would mean only "global to this apache
    child", not to the whole apache server.

    And something that I don't know at all, is how this all works out with a
    threaded apache, such as under Windows e.g.

    André


  4. RE: Caching a hash - am I missing something?

    Thanks Andre and Tyler.

    Believe it or not I never even thought of that until you guys pointed it out (that the hash would live for the the life of the apache child). I can be such a fool sometimes

    In this case I assume %cache is global - it only appers in that sub, there is no "my %cache = ()" any place else in the pm file and there is no "Use strict".

    I guess I'll just keep it the same way except I'll "use strict" and I'll declare "my %cache = ()" outside that sub.

    Thanks Guys!!
    -Chris

    ________________________________

    From: André Warnier [mailto:aw@ice-sa.com]
    Sent: Wed 8/20/2008 3:28 AM
    To: mod_perl list
    Cc: W. Tyler Gee
    Subject: Re: Caching a hash - am I missing something?



    W. Tyler Gee wrote:
    > On Tue, Aug 19, 2008 at 5:35 PM, Chris Faust wrote:
    >> Hi,
    >>
    >>
    >>
    >> This might be a little off topic, I hope it's OK to post. I'm not positive
    >> if mod_perl matters or not because it's a little confusing to me.
    >>
    >>
    >>
    >> I've taken over some pretty old code that I'm updating and making mp2
    >> content handlers out of. The main script is a standard cgi script
    >> "start.cgi" there is nothing special in the apache conf for it.
    >>
    >>
    >>
    >>
    >>
    >> SetHandler perl-script
    >>
    >> PerlFixupHandler My::Fixup
    >>
    >> PerlResponseHandler ModPerl::PerlRun
    >>
    >> PerlOptions +ParseHeaders
    >>
    >> DirectoryIndex start.cgi
    >>
    >> Options +ExecCGI +Indexes
    >>
    >> allow from all
    >>
    >>

    >>
    >>
    >>
    >> start.cgi calls a custom module (use CustomModule which exports a bunch of
    >> subs, for example foobar and all over the place in the subs that are
    >> exported from CustomModule I see code like
    >>
    >>
    >>
    >>
    >>
    >> sub foobar {
    >>
    >> my $key = @_;
    >>
    >>
    >>
    >> if ($cache{$key}) {
    >>
    >> return $cache{$key};
    >>
    >> } else {
    >>
    >> my $do_some_query = xxxx;
    >>
    >> $cache{$key} = $do_some_query_results
    >>
    >> return $cache{$key};
    >>
    >> }
    >>
    >> }
    >>
    >>
    >>
    >> My question is isn't the "else" in foobar always going to be true anyplace
    >> where start.cgi is calling "&foobar('somekey')"??????
    >>
    >> I don't understand how %cache could already be populate from a previous
    >> browser request or something? I'm I just missing something stupid?

    >
    > %cache is defined outside the scope of the sub so it will persist for
    > the lifetime of the apache server. The very first time
    > foobar('somekey') is called it will do the query lookup, the next time
    > it will return from cache.
    >

    I believe the above is almost, but not totally true. It should probably
    be "for the lifetime of this particular apache child".
    Each apache child process has it's own copy of the above code, and it's
    own copy of the above "global" (*) %cache hash. Thus whether the first
    or second part of the if will run, depends of the previous history of
    the particular apache child which handles the current request. If this
    particular child has already previously accessed the same hash key, it
    will server it from (it's own) cache, and otherwise it will execute the
    query to create the key (in it's own cache).
    Apache children are created, and die, as directed by the main Apache
    process configuration, and HTTP requests are served more or less at
    random, by whichever child is available when the request comes in.
    Is is thus quite possible for instance that the first 10 requests for a
    particular hash key would each be handled by a different apache child,
    and would each result in a query; then the 11th request would be handled
    by a child that has already accessed this same key before, and thus
    served from cache; the 13th request would be handled by a brand-new
    apache child just created, thus would re-do the query, etc..

    I this would be useful, I believe that it would be possible to avoid
    this, by "priming" the mod_perl module during the initial start of
    Apache, before it forks into children. Then each new child (being a
    copy of the main apache) would start it's life with a number of keys
    already in its cache. Of course this would work only if the contents of
    the keys of the hash are never modified while apache is running.
    And it probably involves some delicate mod_perl programming to do the
    priming process.

    (*) like the previous contributor, I guess that the %cache hash is
    somehow global, because it is not declared within the sub that you show.
    Whether it really is though, may depend on other code that we don't
    see here. But anyway, "global" would mean only "global to this apache
    child", not to the whole apache server.

    And something that I don't know at all, is how this all works out with a
    threaded apache, such as under Windows e.g.

    André





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