rdblue commented on a change in pull request #3425: URL: https://github.com/apache/iceberg/pull/3425#discussion_r756344268
########## File path: site/docs/snapshot-tag-branch.md ########## @@ -0,0 +1,168 @@ +<!-- + - 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 + - (the "License"); you may not use this file except in compliance with + - the License. You may obtain a copy of the License at + - + - http://www.apache.org/licenses/LICENSE-2.0 + - + - Unless required by applicable law or agreed to in writing, software + - 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. + --> + +# Snapshot Tagging and Branching + +Iceberg snapshot tagging and branching feature provides users more functionalities in managing Iceberg snapshot lifecycle. +Users can assign tags to snapshots, create new branches, set the current branch for read and write, 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 provide 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, default to the current value of table property `history.expire.min-snapshots-to-keep` when this value is evaluated | +| **`max-snapshot-age-ms`** | `long` | The duration before a snapshot tagged or in a branch could be expired by any automatic snapshot expiration process, default to the current value of table property `history.expire.max-snapshot-age-ms` when this value is evaluated | + +#### 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) + \ + H (b3) +``` + +| Policy Type | Max Age | Min to Keep | Snapshots Affected | +|------------------|---------------|-------------|--------------------------| +| global | 5 hours | 4 | A, B, C, D, E, F, G, H | +| 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 | Review comment: Min to keep must be at least 1 because it includes the referenced snapshot. -- This is an automated message from the Apache Git Service. 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