One challenge of dynamic documents is that some code chunks may take a long time to run, and these chunks may not be modified or updated frequently. In this case, caching can be very helpful. The basic idea is, a chunk will not be re-executed as long as it has not been modified since the last run, and old results will be directly loaded instead.
Cache is not a new idea — both the packages cacheSweave and weaver have implemented it based on Sweave, with the former using filehash and the latter using *.RData images; cacheSweave also supports lazy-loading of objects based on filehash. The knitr package directly uses internal base R functions to save (tools:::makeLazyLoadDB()) and lazy-load objects (lazyLoad()
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