Garbage collection initial text. Needs more work.

pep-0458.txt

@@ -682,6 +682,21 @@ server failure.
 __ https://en.wikipedia.org/wiki/Transaction_log
 
 
+Cleaning up old metadata
+------------------------
+
+Prior versions of snapshot, targets, and timestamp metadata does not need to 
+be kept indefinitely.  (Root files must be indefinitely retained.)
+However, a client that performs an update MUST be able
+to retrieve a consistent set of versions of the files on the repository.
+For example, if a client downloads a snapshot file, it should retrieve the versions
+of targets metadata that existed when that snapshot file was created, even if
+the targets metadata has been updated concurrently with the client requests.
+Fortunately, the use of hash / version delegations handle this case automatically
+since clients request targets unambiguously.  Once no clients could reasonably
+be requesting outdated targets, snapshot or timestamp files, those files may be
+removed to save space.  Thus files that were obsoleted some reasonable time
+in the past (such as 1 hour) may be safely discarded.
 
 Revoking Trust in Projects and Versions
 =======================================