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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