Keen readers may recall a post from April 2008 about my website-archiving forays working with Web Curator Tool, the workflow database, used for programming Heritrix, the crawler which does the harvesting of websites.
Other UKWAC partners and myself have since found that Heritrix sometimes has a problem, described by some as ‘collateral harvesting’. This means it can gather links, pages, resources, images, files and so forth from websites we don’t actually want to include in the finished archived item.
Often this problem is negligible, resulting in a few extra KB of pages from adobe.com or google.com for example. Sometimes though it can result in large amounts of extraneous material, amounting to several MB or even GB of digital content (for example if the crawler somehow finds a website full of .avi files.)
I have probably become overly preoccupied with this issue, since I don’t want to increase our sponsor (JISC)’s overheads by occupying their share of the server space with unnecessarily bloated gathers, nor clutter up the shared bandwidth by spending hours gathering pages unnecessarily.
Web Curator Tool allows us two options for dealing with collateral harvesting. One of them is to use the Prune Tool on the harvested site after the gather has run. The Prune Tool allows you to browse the gather’s tree structure, and to delete a single file or an entire folder full of files which you don’t want.
The other option is to apply exclusion filters to the title before the gather runs. This can be a much more effective method. The method is to enter a little bit of code in the ‘Exclude Filters’ box of a title’s profile. The basic principle is using the code .* for exclusions. .*www.aes.org.* will exclude that entire website from the gather. .*/images/.* will exclude any path containing a folder named ‘images’.
So far I generally find myself making two types of exclusion:
(a) Exclusions of websites we don’t want. As noted with collateral harvesting, Heritrix is following external links from the target a little too enthusiastically. It’s easy to identify these sites with the Tree View feature in WCT. This view also lets you know the size of the folder that has resulted from the external gathering. This has helped me make decisions; I tend to target those folders where the size is 1MB or larger.
(b) Exclusions of certain pages or folders within the Target which we don’t want. This is where it gets slightly trickier, and we start to look in the log files of client-server requests for instances where the browser is staying in the target, but performing actions like requesting the same page over and over. This can happen with database-driven sites, CMS sites, wikis, and blogs.
I believe I may have had a ‘breakthrough’ of sorts with managing collateral harvesting with at least one brand of wiki, and will report on this for my next post.