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I am interested in "focused crawling" (crawling web pages of some specific topic and ignoring all the others) and have written a "focused crawler" recently. Perl is a reasonable alternative to writing web crawler for its LWP module and CPAN. However, when I planed to implement multithread strategy in crawling, I was confused by perl 5.8.4's "threads" module, especially threads::shared. How is a object reference shared by multiple threads? I want to utilize "Cache::File::Heap" module to sort the urls in "crawling frontier" by heuristic prediction of its "harvest outcome". Below is the relevant code part:
#!/usr/bin/perl -w use strict; use threads; use threads::shared; use Cache::File::Heap; my $heap = Cache::File::Heap->new('frontier'); my $heap_lock : shared = 0; ... sub go {#crawling thread's control flow .... #xtracted best promising url { lock $heap_lock; my($value, $url) = $heap->extract_minimum; } ... #after downloading and extract hyperlinks { lock $heap_lock; $heap->add($value, $url) for } ... } my @threads; for(1..10) { push @threads, threads->new(\&go); } for(@threads) { $_->join; }
All is fine, just untill all the threads joined by main thread and main thread exists. Following error message appears: Scalar leaks : -1 Segmentation fault. My question is : How to share object reference (such as Cache::File::Heap) ?? Cache::File::Heap is the wrapper of BerkeleyDB's BTREE, is BerkeleyDB thread-safe?

In reply to Multithread Web Crawler by xuqy

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