src/product/README.udc 1.1

Index: src/product/README.udc
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RCS file: src/product/README.udc
diff -N src/product/README.udc
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+++ src/product/README.udc	4 Mar 2010 21:51:39 -0000	1.1
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+
+udcCache means URL-Data-Cache
+
+BBI files use the udcCach.
+BBI means Big Binary Indexed and includes
+file types such as BigBed (.bb) and BigWig (.bw).
+
+UCSC BAM file support may also use the udcCache
+
+The udcCache allows tracks that are either installed tracks
+or custom tracks of the above mentioned types to cache data 
+that they have already fetched via URL.  This allows data to 
+reside elsewhere and only download the parts
+needed on demand.  The datablocks are usually
+compressed and have an efficient random access
+index. They are accessed from a remote location
+via URLs such as HTTP, HTTPS, FTP.
+
+By default, udcCache stores files in /tmp/udcCache
+
+Howver, you may include the following in your hg.conf
+and then let your regular trash cleaning scripts
+clean out the old udcCache automatically as well:
+
+# directory for temporary bbi file caching
+udc.cacheDir=../trash/udcCache
+
+Notice that this path is relative to your 
+cgi executable directory which is the 
+current directory when the cgi starts up.
+On some systems this directory is called cgi-bin/.
+