[GitHub] [iceberg] namrathamyske commented on a diff in pull request #6723: Docs: Separate page for Branching and Tagging

["namrathamyske (via GitHub)" <gi...@apache.org>]

namrathamyske commented on code in PR #6723:
URL: https://github.com/apache/iceberg/pull/6723#discussion_r1169050760


##########
docs/branching-and-tagging.md:
##########
@@ -0,0 +1,116 @@
+---
+title: "Branching and Tagging"
+url: branching
+aliases:
+    - "tables/branching"
+menu:
+    main:
+        parent: Tables
+        weight: 0
+---
+
+<!--
+ - Licensed to the Apache Software Foundation (ASF) under one or more
+ - contributor license agreements.  See the NOTICE file distributed with
+ - this work for additional information regarding copyright ownership.
+ - The ASF licenses this file to You under the Apache License, Version 2.0
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+ -
+ -   http://www.apache.org/licenses/LICENSE-2.0
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+ - distributed under the License is distributed on an "AS IS" BASIS,
+ - WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ - See the License for the specific language governing permissions and
+ - limitations under the License.
+ -->
+
+# Branching and Tagging
+
+## Overview
+
+Iceberg table metadata maintains a log of snapshots which represent the changes applied to a table.
+Snapshots are fundamental in Iceberg as they are the basis for reader isolation and time travel queries.
+For controlling metadata size and storage costs, Iceberg provides snapshot lifecycle management procedures such as [`expire_snapshots`](../../spark/spark-procedures/#expire-snapshots) for removing unused snapshots and no longer neccessary data files based on table snapshot retention properties.
+
+**For more sophisticated snapshot lifecycle management, Iceberg supports branches and tags which are named references to snapshots with their own independent lifecycles. This lifecycle is controlled by branch and tag level retention policies.** 
+Branches are independent lineages of snapshots and point to the head of the lineage. 
+Branches and tags have a maximum reference age property which control when the reference to the snapshot itself should be expired.
+Branches have retention properties which define the minimum number of snapshots to retain on a branch as well as the maximum age of individual snapshots to retain on the branch. 
+These properties are used when the expireSnapshots procedure is run. 
+For details on the algorithm for expireSnapshots, refer to the [spec](../../../spec#snapshot-retention-policy).
+
+## Use Cases
+
+Branching and tagging can be used for handling GDPR requirements and retaining important historical snapshots for auditing.
+Branches can also be used as part of data engineering workflows, for enabling experimental branches for testing and validating new jobs.
+See below for some examples of how branching and tagging can facilitate these use cases.
+
+### Historical Tags
+
+Tags can be used for retaining important historical snapshots for auditing purposes.
+
+![Historical Tags](../img/historical-snapshot-tag.png)
+
+The above diagram demonstrates retaininig important historical snapshot with the following retention policy, defined 
+via Spark SQL.
+
+1. Retain the latest 5 days of snapshots for 1 week. This can be achieved by setting a branch retention policy where 5 days worth of snapshots will be kept, and the branch reference itself will be retained for 1 week. 
+```sql
+-- Create a branch historical-branch which will all snapshots be written to. Historical-branch will be retained for 7 days, and 5 days worth of snapshots will be retained
+ALTER TABLE prod.db.table CREATE BRANCH historical-branch AS OF VERSION 1 RETAIN 7 DAYS WITH RETENTION 5 DAYS
+```
+
+2. Retain 1 snapshot per week for 1 month. This can be achieved by tagging the weekly snapshot and setting the tag retention to be a month.
+snapshots will be kept, and the branch reference itself will be retained for 1 week. 
+```sql
+-- Create a tag for the first end of week snapshot. Retain the snapshot for a week
+ALTER TABLE prod.db.table CREATE TAG 'EOW-01' AS OF VERSION 7 RETAIN 7 DAYS
+```
+
+3. Retain 1 snapshot per month for 6 months. This can be achieved by tagging the monthly snapshot and setting the tag retention to be 6 months.
+```sql
+-- Create a tag for the first end of month snapshot. Retain the snapshot for 6 months
+ALTER TABLE prod.db.table CREATE TAG 'EOM-01' AS OF VERSION 30 RETAIN 180 DAYS
+```
+
+4. Retain 1 snapshot per year forever. This can be achieved by tagging the annual snapshot. The default retention for branches and tags is forever.
+```sql
+-- Create a tag for the end of the year and retain it forever.
+ALTER TABLE prod.db.table CREATE TAG 'EOY-2023' AS OF VERSION 365
+```
+
+### Audit Branch
+
+![Audit Branch](../img/audit-branch.png)
+
+The above diagram shows an example of using an audit branch for validating a write workflow. 
+
+1. Create `audit-branch` starting from snapshot 3, which will be written to and retained for 1 week.
+```sql
+ALTER TABLE db.table CREATE BRANCH `audit-branch` AS OF VERSION 3 RETAIN 7 DAYS
+```
+2. Writes are performed on a separate `audit-branch` independent from the main table history.
+```sql
+-- WAP Branch write
+SET spark.wap.branch = audit

Review Comment:
   this should be `SET spark.wap.branch = audit-branch` ?



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