jackye1995 commented on a change in pull request #3425:
URL: https://github.com/apache/iceberg/pull/3425#discussion_r755717248
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File path: site/docs/snapshot-tag-branch.md
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+# Snapshot Tagging and Branching
+
+Iceberg snapshot tagging and branching feature offers user a Git-like
experience in manging table snapshots.
+Users can assign tags to snapshots, create branches and configure customized
retention policy for them.
+
+## Example use cases
+
+### Time-based Snapshot tagging
+
+Users can leverage Iceberg snapshot tagging to keep multiple versions of the
table across different points in time.
+For example, a table can be configured to keep all snapshots within 24 hours,
then 1 tagged snapshot per day, per week, per month, etc.
+The daily snapshots are retained for 1 week, weekly snapshots are retained for
1 month, monthly snapshots are retained for 1 year, etc.
+
+### Critical snapshot maintenance branch
+
+There are snapshots that are critical for legal or business reasons, such as
the yearly snapshots used for financial auditing.
+Because they are kept for an extended period of time (maybe even forever),
data files in the table are commonly compacted and encrypted with periodic key
rotation.
+Occasionally, rows in the snapshot also have to be deleted or updated to
satisfy GDPR requirements.
+Users can create an Iceberg branch for such snapshots to maintain its
independent lifecycle.
+
+### Experimental Branch
+
+An experimental branch is useful for many user groups, including:
+
+1. Data scientists and ML researchers can easily create an Iceberg branch to
experiment with table data without worrying about polluting the main table
snapshot.
+2. Data engineers can perform production AB testing against the experimental
branch to ensure the correctness of certain table updates.
+3. Data producers can perform test load in a table in an experimental branch,
and then append all the loaded files back to the main branch (similar to Git
cherry-pick).
+
+!!!Note
+ Iceberg does not plan to offer a Git-like merge operation through
branching.
+ Merging arbitrary changes requires a lot of work to keep track of the
intent of the commit and the context.
+ Merging in a table is actually committing a transaction. The expectation
is different from a merge in Git, where the lack of a conflict is the
definition of "correct".
+ In a table, the lack of a file conflict does not mean that the transaction
can be committed.
+ In addition, longer transaction lengths from branch-like behavior
dramatically increases the likelihood that the transaction could fail.
+ The merge feature would likely be supported through multi-table
transaction in the future.
+
+## Snapshot Reference
+
+In version control systems like git, branch and tag are both references of
commits.
+In Iceberg, we use a similar concept of **Snapshot Reference** to implement
branching and tagging.
+
+Each Iceberg table metadata contains a list of `refs` (references), and a
`current-branch` indicating the current branch to use.
+When user creates an Iceberg table, the first commit belongs to the default
`main` branch.
+Each snapshot reference has a uniquely identifiable name across all references
of a table.
+A snapshot can have multiple references. The exact snapshot reference spec is
documented at the [Spec](../spec/#snapshot-reference) page.
+Here we will provide some more explanations to the concepts in snapshot
reference.
+
+### Reference Type
+
+There are clearly 2 types of snapshot reference, which are `branch` and `tag`.
Their key differences are:
+
+- **New commit**: when a new snapshot is added as a child of a referenced
snapshot, tag remains on the old snapshot, but branch reference moves to the
child.
+
+- **Retention policy**: retention policy affects all the snapshots in a
branch, but only a single tagged snapshot. (More details in the next section)
+
+### Retention Policy
+
+Iceberg offers a [snapshot expiration
procedure](../spark-procedures/#expire_snapshots) to clean up snapshots that
are not needed to free up storage space.
+Retention policy can be configured both globally and on snapshot reference to
provide highly flexible customization to the expiration behavior.
+
+#### Global snapshot retention policy
+
+Global snapshot retention policy can be set through the following table
properties:
+
+| Property | Default | Description
|
+| ------------------------------------ | ------------------ |
------------------------------------------------------------- |
+| history.expire.max-snapshot-age-ms | 432000000 (5 days) | Default max age
of snapshots to keep while expiring snapshots |
+| history.expire.min-snapshots-to-keep | 1 | Default min
number of snapshots to keep while expiring snapshots |
+
+#### Snapshot reference retention policy
+
+Similarly, snapshot reference has the properties below to provider finer grain
control:
+
+| Property | Type | Description |
+|------------------------------|-----------|-------------|
+| **`min-snapshots-to-keep`** | `int` | For `branch` type only, the
minimum number of snapshots to keep in a branch |
+| **`max-snapshot-age-ms`** | `long` | The duration before a snapshot
tagged or in a branch could be expired by any automatic snapshot expiration
process |
+
+#### Policy evaluation mechanism
+
+When a snapshot expiration process starts, it follows the steps described
below:
+
+1. form an expiration candidate pool containing all snapshots
+2. for each snapshot reference, evaluate the associated policy and move
snapshots out of the candidate pool
+3. apply global retention policy to and move snapshots out of the candidate
pool
+4. when multiple snapshots can be chosen to be moved out, newer snapshots win
+4. after evaluation, expire all snapshots that are still in the candidate pool
+
+#### Policy evaluation example
+
+Here is an example for how an Iceberg snapshot expiration procedure evaluates
what snapshots to expire.
+
+Suppose we have the following snapshot graph and retention policies configured:
+
+```
+A -> B -> C (main)
+ \ (dev)
+ D -> E (b1)
+ \
+ F -> G (b2)
+```
+
+| Policy Type | Max Age | Min to Keep | Snapshots Affected |
+|------------------|---------------|-------------|-----------------------|
+| global | 5 hours | 4 | A, B, C, D, E, F, G |
+| branch/main | 3 days | 1 | A, B, C |
+| branch/b1 | 2 days | 2 | A, D, E |
+| branch/b2 | 1 day | 0 | A, D, F, G |
+| tag/dev | forever | N/A | C |
+
+Assume that we have a process continuously running the snapshot expiration
procedure, we would have the results below as time progresses:
+
+##### Day 1: F and G are expired
+
+On day 1, the global and branch b2 max age has passed, affecting A, D, F, G.
+A andD cannot be expired due to branch b1 policy, so only F and G are expired.
Review comment:
If a snapshot with reference is dropped, the reference is also dropped,
so branch b2 will be removed.
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