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  • 1. CHI: Universal caching for Perl Jonathan Swartz YAPC|10 2009 Pittsburgh, PA
  • 2. What is CHI? • Standard interface and implementation for Perl caches - aka 'DBI for caching' • Inspired by Cache::Cache, but improves performance and extendibility
  • 3. Why do we need it? • CPAN modules that need cache can simply ask for a CHI handle • Implement generic caching features in a single place • Make new cache backends trivial to create
  • 4. Modules that just need "a cache"' • Templating: HTML::Mason, Template::Toolkit • Web frameworks: Catalyst, CGI::Application • Sessions: CGI::Session, Apache::Session • ORMs: Data::ObjectDriver, Rose::DB::Object • Code processing: Perl::Tidy, Perl::Critic • Startup impaired: Class::MOP/Moose
  • 5. Basic API # Choose a driver # my $cache = CHI->new( driver => 'Memory' ); my $cache = CHI->new( driver => 'File', cache_root => '/path/to/root' ); my $cache = CHI->new( driver => 'FastMmap', root_dir => '/path/to/root', cache_size => '1k' ); # Get and set items # my $customer = $cache->get($name); if ( !defined $customer ) { $customer = get_customer_from_db($name); $cache->set( $name, $customer, "10 minutes" ); } $cache->remove($name);
  • 6. Current CHI drivers • Memory - In-process memory • File - Hierarchical, one file per entry • FastMmap - Mmap'ed files • Memcached • DBI • BerkeleyDB • CacheCache - Any existing Cache::Cache driver
  • 7. Driver skeleton package CHI::Driver::MyDriver; use Moose; extends 'CHI::Driver'; has 'attr' => ( ... ); sub fetch { my ( $self, $key ) = @_; sub store { my ( $self, $key, $data ) = @_; sub remove { my ( $self, $key ) = @_; sub clear { my ($self) = @_; sub get_keys { my ($self) = @_; sub get_namespaces { my ($self) = @_;
  • 8. Avoiding miss stampedes • Problem: Many processes try to recompute cache when it expires • Solution 1: Probabilistic expiration my $cache = CHI->new(..., expires_variance => 0.20); • Solution 2: Busy locks my $cache = CHI->new(..., busy_lock => '30 sec');
  • 9. Recomputation latency • Problem: Client waits while data is recomputed • Solution 1: Background recomputation my $customer = $cache->compute( $name, sub { get_customer_from_db($name) }, "10 minutes" ); • Solution 2: Externally initiated recomputation use LWP::Simple; foreach my $url (@common_urls) { # recompute_cache temporarily reduces expiration get("http://www.mysite.com/$url?recompute_cache=1"); }
  • 10. Multilevel caches • Several caches can be chained together • Example 1: L1 cache in front of memcached my $cache = CHI->new( driver => 'Memcached', servers => [ "10.0.0.15:11211", ... ], l1_cache => { driver => 'Memory', max_size => '1MB' } ); • Example 2: Mirror cache to migrate cache directories my $cache = CHI->new( driver => 'File', root_dir => '/old/root' mirror_cache => { driver => 'File', root_dir => '/new/root' } );
  • 11. To do • Cache statistics - collection and reporting • Registry of storage types (like Rose::DB) with memoization • Background recomputation • Close performance gap between CHI and native • Plugins for Catalyst, CGI::Session, etc.
  • 12. Additional resources • Download CHI from CPAN - http://cpan.uwinnipeg.ca/dist/CHI • Search for drivers - http://cpan.uwinnipeg.ca/search?query=CHI::Driver • Subscribe to perl-cache mailing list - http://groups.google.com/group/perl-cache-discuss