Create prune.md

design/object-reset/prune.md

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+# Object Prune Design
+
+_Presented as [Option 9 in README](README.md#option-9-use-existing-tools-to-make-current-version-correct-apply-a-prune-transaction-to-purge-files-that-are-not-in-the-current-version-of-the-object)_
+
+## Summary
+
+Use existing tools to make current version "correct". 
+Apply a "PRUNE" transaction to purge files that are not in the current version of the object.
+
+### PRO
+- leverages existing tools
+- depositor can preview purge candidates in the Merritt UI (and in an enhanced API)
+- depositor initiates the PRUNE
+- provenance can be recorded when the PRUNE is applied
+  - should we record a new version to save this provenance information?
+
+### CON:
+- fix is a 2 step process  
+
+## Process Description
+
+### Repair option 1 (for depositors)
+- use mrt-delete files to correct the current object
+
+### Repair option 2 (for depositors and Merritt Team) 
+- run a Merritt ADD using a storage-generated ingest manifest for reconstruction
+ - generate the baseline manifest from the current version
+ - if we expose the storage manifest paths as an input format, what restrictions do we need to set on the use of these patterns?  Or, should the use of these paths be a privileged operation?
+ - question - can the manifest be modified so that a rename effectively takes place?
+
+### Repair option 3 (for Merritt Team) 
+- run a Merritt ADD using a storage-generated ingest manifest for reconstruction
+ - generate the baseline manifest from ALL versions
+ - this would allow versioning mistakes to be created
+
+## Submit the Repair manifest as a Merritt ADD
+
+## Review the correction in the Merritt UI
+- The Merritt Object API could add a JSON array of "prune candidates" from prior versions
+
+## Apply a Merritt PRUNE transaction
+      - Submit this through ingest with no payload
+### Prune options
+- prune any file key that has not been pulled forward to the current version
+- prune any file key that has not been pulled forward to the current version AND that has a duplicate checksum on a different key
+
+## Storage actions
+- delete prune-able keys from primary storage
+- remove pruned keys from the storage manifest (or mark them as pruned)
+- trigger an inventory rebuild
+- trigger a re-replication of the entire object
+
+## Billing Process Implications
+- The current billing process does not account for content deletion.  If a motivation for fixing content is to reduce cost, we will need to add features to decrement billing numbers
+  - tombstone deleted file data to a new database table
+  - run a monthy reconciliation process (as needed by collection?) to account for content removal
+- Create a report to total prune-able counts at a collection level