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Virtuous Lazyness For Your Data Doing it once andknowing youve done it. Steve LembarkWorkhorse Computing lembark@wrkhors.com

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Data wants to be free, it also wants to be very expensive.● Data you never use is free; accessing gets expensive.● Managing datas cost requires controlling its lifecycle.● Mis­managing the lifecycle causes problems: ● Caching table lookups in forked httpd crashes your database. ● Pre­processing messages files kills your startup times. ● Reading XML configuration data you never use wastes most of your testing budget. ● Your tests fail because of a connection failure to a database server that you dont use in the test.● Fixing these problems requires using lazy data.

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False Lazyness● We have all see the two most common data management stratagies: ● You load everything at startup to avoid checking it every time it is used. ● You check everything every time before using it to avoid loading it up front.● Both approaches ignore important knowledge: ● You know when the data is needed. ● You know that the data was loaded.

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One alternative: Scalar Cache● $cache ||= read_cache_values; ● Seems nice: You know that $cache is populated. ● This requires dereferencing a hashref throughout the program, which is expensive. ● What youd rather do is just use $cache{ $foobar } without having to check it every time.● Even checking if( ! %cache ) is expensive – arrays are much cheaper to check.

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True Lazyness: Do Something Once● Truly Lazy data means loading you data when you need it and knowing that it is loaded.● Perl gives us – of course – more than one way: ● Trampoline objects and subroutines. ● “state” variables, introduced in v5.10.● Trampolines are flyweight objects – data structures or subroutines that transform themselves when used.● State variables are assigned only once, at runtime, the first time they are used.

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Follow The Bouncing Object● Object::Trampoline – flyweight data you dont know isnt there.● These delay calling the “real” constructor until the object is actually used. ● Your constructor gets called before the first method call. ● At that point it can cache, parse, or compute whatever it needs.● Spreads the cost of loading data set each over the lifetime of a process.

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Example: Delay Expensive XML● Use an initializer to read and parse the XML.● The Object::Trampoline calls your constructor once to transform the object the first time you call a method.● Parsing the XML is pushed off until you actually use the data.● Requires using a hashref – and possibly methods – to access the data.

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package XML::Message;use Object::Trampoline;...sub new{ Object::Trampline->install( XML::Message, $path );}sub install{ my $error = &construct; $error->initialize( @_ )}sub initialize{ my ( $err, $path ) = @_; %$err = %{ XMLin $path => @lots_of_args }; $err}# calling translate bounces the trampoline exactly oncesub translate { … }

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Tramploline Subroutine● Similar to a trampoline object: A portion of the code runs once and replaces itself.● One­shot code initializes the cache, which can now be a simple hash.● Symbol::qualify_to_ref make this painless in Perl. ● An anonymous sub manages the cache. ● A named subroutine loads the cache, replaces itself with the manager, and re­dispatches to the manager.

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The Simplest Versionmy %foo_cache = (); ● Minimal code my $handler= sub includes:{ my $foo = shift; ● Closed­over cache, %foo_cache{ $foo } Or die “Bogus foo: $foo unknown” ● Subref to permenant }; cache manager,sub do_something{ ● Initial subroutine. %cache = initialize_foo_cache; ● The initial subroutine my $ref = qualify_to_ref do_something; initializes, installs, *$ref = $handler; and re­dispatches. goto &$handler}

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BEGIN Blocks Are CleanerBEGIN{ my $name = foo_handler; ● The block isolates my $ref = qualify_to_ref $name; my %cache = (); cache, ref, and my $handler sub variables; = sub { allows re­cycling %cache{ $_[0] } or die ... }; the ref. *$ref = sub ● This is also rather { %cache = init_the_cache; amenable to *$ref = $handler; installation by module. goto &$handler }}

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Sub::Trampolinesub install_trampoline{ my ( $name, $init, $mgr ) = @_; ● Aside from the actual my $caller = caller; assignment, init code is my $ref = qualify_to_ref $name, $caller; identical. *$ref ● Simply pass the name, = sub { manager, and init $init->(); assignment. &$ref = $mgr; ● The module can call goto &$mgr } $init, replace itself.} ● Caller defines the cache and handler.

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Using Sub::Trampolineuse Sub::Trampoline;my %cache1 = ();my @cache2 = ();my $cache1_mgr= sub { $cache1{ $_[0] } or croak "Unknown $_[0]" };my $cache2_mgr= sub { first { $_ eq $_[0] } @cache2 } };my $init_cache1 = sub { %cache1 = select_from_hell }sub init_cache2 { @cache2 = XMLin $nasty_messy_xml_struct }install_trampoline( subone => $init_cache1, $cache1_mgr );install_trampoline( known => &init_cache2, $cache2_mgr );my $value1 = subone $key1;if( known $value ){ … }else{ carp “Unknown: $value” }

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True One­Shot: Empty $handler● If you want to run something exactly once but dont know where it might be called initially: my $manager = sub(){};● You can also substitute a trampoline object with a constructor that does the work and no methods.● Calling the object once constructs it, after which the classes constructor can stub itself.● Useful for sharing the cache variable: the init populates it once and stubs itself to do nothing more.

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Cycling The Cache● There are times when you want to purge and re­ initialize the cache● A trampoline object with populate and use subs that flip­flop can handle this easily.● Re­assigning a trampoline re­initializes the cache: $cache = Object::Trampoline->init_cache( $class => @argz ) if $age > $time_max;

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v5.10 Introduced “state” Variables● Scoped like a lexical.● Assinged once at runtime.● Maintain value within a single lexical context throughout the program.● Assign the cache or assign a flag variable with the side effect of populating the cache.● Currently supports only scalars.

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Obvious case: Assign the cache.● Assign the cache at runtime: sub cache_mangler { state $cache = init_cache; … }● $cache will be assigned at runtime, the first time cache_mangler is called.● The value will be retained between calls.● Catch: $cache is only available within cache_mangler, not outside of it.

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Initialize a Shared Cacheuse v5.10; ● Subs may want to share my %cache = (); a cache.sub init ● $y and $z are assigned { %k or %k = …; at most once per } executison when foo or sub foo { state $y = init; … } bar are called.sub bar { state $z = init; … }… ● The sanity check in init only needs to be handled my $foo = foo bletch;my $bar = bar blort; at most per subroutine.

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Summary● True lazyness includes managing data.● Preloading it all or testing it at each step are not lazy.● Object::Trampoline provides one way.● Trampoline subroutines offer another approach.● v5.10 introduced state variables which provide a few ways to initialize something once.