[GitHub] [iceberg] rdblue commented on pull request #2961: Core: add snapshot tags interface

[GitBox <gi...@apache.org>]

rdblue commented on pull request #2961:
URL: https://github.com/apache/iceberg/pull/2961#issuecomment-903937976


   I agree with much of what @rymurr said. I think it is good that snapshots are immutable. I think of the tags (and later, branches) feature as a way of referring to snapshots and keeping track of them, just like references in git.
   
   For the spec, what I've been thinking is to add a new map that tracks snapshots with names as keys and an object of data. Something like this:
   
   ```
     "named-snapshots": {
       "current": { "snapshot-id": 1234..., "type": "branch" },
       "dev": { "snapshot-id": 1235..., "type": "branch" },
       "monthly-2021-06-01": { "snapshot-id": 1233..., "type": "tag", "lifecycle": "snapshot" },
       "audit-2021-q2": { "snapshot-id": 1232..., "type": "tag", "lifecycle": "independent" }
     }
   ```
   
   Things to note about this example:
   * The current version is a normal branch. We'd need to keep `current-version-id` until v3
   * The "type" determines what can be done. Branches can be updated just like "current" and tags are static
   * Tags need information about lifecycle. Some tags are just convenience names that should be removed when a snapshot is expired (lifecycle=snapshot) but others are snapshots that should be retained forever (lifecycle=independent)
   * Using a JSON object for the tag/branch metadata allows us to evolve it more easily, unlike `map<string, long>`


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