[GitHub] [iceberg] jackye1995 commented on a change in pull request #3425: Spec: Add snapshot tagging and branching

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

jackye1995 commented on a change in pull request #3425:
URL: https://github.com/apache/iceberg/pull/3425#discussion_r761604768



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File path: site/docs/spec.md
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@@ -593,16 +625,16 @@ Table metadata consists of the following fields:
 | _optional_ | _required_ | **`default-spec-id`**| ID of the "current" spec that writers should use by default. |
 | _optional_ | _required_ | **`last-partition-id`**| An integer; the highest assigned partition field ID across all partition specs for the table. This is used to ensure partition fields are always assigned an unused ID when evolving specs. |
 | _optional_ | _optional_ | **`properties`**| A string to string map of table properties. This is used to control settings that affect reading and writing and is not intended to be used for arbitrary metadata. For example, `commit.retry.num-retries` is used to control the number of commit retries. |
-| _optional_ | _optional_ | **`current-snapshot-id`**| `long` ID of the current table snapshot. |
+| _optional_ | _optional_ | **`current-snapshot-id`**| `long` ID of the current table snapshot; must be the same as the current ID of the `main` branch in `refs`. |
 | _optional_ | _optional_ | **`snapshots`**| A list of valid snapshots. Valid snapshots are snapshots for which all data files exist in the file system. A data file must not be deleted from the file system until the last snapshot in which it was listed is garbage collected. |
 | _optional_ | _optional_ | **`snapshot-log`**| A list (optional) of timestamp and snapshot ID pairs that encodes changes to the current snapshot for the table. Each time the current-snapshot-id is changed, a new entry should be added with the last-updated-ms and the new current-snapshot-id. When snapshots are expired from the list of valid snapshots, all entries before a snapshot that has expired should be removed. |
 | _optional_ | _optional_ | **`metadata-log`**| A list (optional) of timestamp and metadata file location pairs that encodes changes to the previous metadata files for the table. Each time a new metadata file is created, a new entry of the previous metadata file location should be added to the list. Tables can be configured to remove oldest metadata log entries and keep a fixed-size log of the most recent entries after a commit. |
 | _optional_ | _required_ | **`sort-orders`**| A list of sort orders, stored as full sort order objects. |
 | _optional_ | _required_ | **`default-sort-order-id`**| Default sort order id of the table. Note that this could be used by writers, but is not used when reading because reads use the specs stored in manifest files. |
+|            | _optional_ | **`refs`** | A map of snapshot references. The map keys are the unique snapshot reference names in the table, and the map values are snapshot reference objects. There is always a `main` branch reference pointing to the `current-snapshot-id` even if the `refs` map is null. |

Review comment:
       makes sense. In that case do you have any additional concern related to the spec change?




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