jackye1995 commented on a change in pull request #3425:
URL: https://github.com/apache/iceberg/pull/3425#discussion_r758833172
##########
File path: site/docs/spec.md
##########
@@ -566,6 +566,38 @@ Notes:
1. An alternative, *strict projection*, creates a partition predicate that
will match a file if all of the rows in the file must match the scan predicate.
These projections are used to calculate the residual predicates for each file
in a scan.
2. For example, if `file_a` has rows with `id` between 1 and 10 and a delete
file contains rows with `id` between 1 and 4, a scan for `id = 9` may ignore
the delete file because none of the deletes can match a row that will be
selected.
+#### Snapshot Reference
+
+Iceberg tables keep track of branches and tags using snapshot references.
+Tags are labels for individual snapshots. Branches are mutable named
references that can be updated by committing a new snapshot as the branch's
referenced snapshot using the [Commit Conflict Resolution and
Retry](#commit-conflict-resolution-and-retry) procedures.
+
+The snapshot reference object records all the information of a reference
including snapshot ID, reference type and [Snapshot Retention
Policy](#snapshot-retention-policy).
+
Review comment:
I think I mentioned this a bit in the design doc's reply, we can always
add a reference pointer when there is a need to have a separated retention
policy construct, so it's fully backwards compatible. For exmaple:
```
"branchA": {
"max-ref-age-ms": 10000
},
"branchB": {
"retention-policy-id": 2
}
```
But for now having a separated retention policy object feels a bit overkill.
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