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+# Caching System
+
+The Versioned Data tool caching system operates in two stages, one covering server folders that contain data store version retrievals, and the second covering Galaxy's "Versioned Data" data library folder behaviour.  The "Versioned Data" library folder files mainly link to the server folders having the respective content, so cache management consists of creating/deleting the server's versioned data as well as Galaxy's link to it.  As well the "Versioned Data" library has a "Workflow Cache" folder which stores all derivative (output) workflow datasets as requested by Versioned Data tool users.
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+The program that periodically clears out the Galaxy link and server data cache is called *versioned_data_cache_clear.py*, and should be set up by a Galaxy administrator to run on a monthly basis, say.  It leaves each database's latest version in the cache.
+
+---
+
+### Galaxy behaviour when user history references to missing cached data occur
+
+If a server data store version folder is deleted, the galaxy data library and user history references to it will be broken.  The user will experience a "Not Found" display in their workspace when clicking on a linked datastore like that in their history.  The user can simply rerun the "Versioned Data Retrieval" tool to restore the data.
+
+![message when selected dataset file no longer exists on the server](bad_history_item.png)
+
+In the example above, clicking on the view (eye) icon for "155:16s_rdp.fasta" triggered this message.  To remedy the situation, the user simply reruns the "154: Versioned Data Retrieval" step below to regenerate the dataset(s).