Ideas for Further Expansion (Perl & LWP)

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12.4. Ideas for Further Expansion

In its current form, this bot is a passable implementation framework for a Type Three Requester spider that checks links on typical HTML web sites. In actual use, you would want to fine tune its heuristics. For example, if you want to check the validity of lots of URLs to sites that don't implement HEAD, you'd want to improve on the logic that currently just considers those URLs a lost cause; or you might want to add code that will skip any attempt at HEADing a URL on a host that has previously responded to any HEAD request with a "Method Not Supported" error, or has otherwise proven uncooperative.

If you wanted the spider to check large numbers of URLs, or spider a large site, it might be prudent to have some of its state saved to disk (specifically @schedule, %seen_url_before, %points_to, and %notable_url_error); that way you could stop the spider, start it later, and have it resume where it left off, to avoid wastefully duplicating what it did the last time. It would also be wise to have the spider enforce some basic constraints on documents and requests, such as aborting any HTML transfer that exceeds 200K or that seems to not actually be HTML, or by having the spider put a maximum limit on the number of times it will hit any given host (see the no_visits( ) method mentioned in the LWP::RobotUA documentation, and specifically consider $bot->no_visits($url->host_port)).

Moreover, the spider's basic behavior could be altered easily by changing just a few of the routines. For example, to turn it into a robot that merely checks URLs that you give it on the command line, you need only redefine one routine like this:

sub near_url { 0; }   # no URLs are "near", i.e., spiderable

Conversely, to turn it into a pure Type Four Requester spider that recursively looks for links to which any web pages it finds link, all it takes is this:

sub near_url { 1; }   # all URLs are "near", i.e., spiderable

But as pointed out earlier in this chapter, that is a risky endeavor. It requires careful monitoring and log analysis, constant adjustments to its response-processing heuristics, intelligent caching, and other matters regrettably beyond what can be sufficiently covered in this book.


12.3. Example: A Link-Checking Spider		A. LWP Modules