rdblue commented on code in PR #3188: URL: https://github.com/apache/iceberg/pull/3188#discussion_r841295696
########## format/view-spec.md: ########## @@ -0,0 +1,264 @@ +<!-- + - 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. + --> + +# Iceberg View Spec + +## Background and Motivation + +Most compute engines (e.g. Trino and Apache Spark) support views. A view is a logical table that can be referenced by future queries. Views do not contain any data. Instead, the query stored by the view is executed every time the view is referenced by another query. + +Each compute engine stores the metadata of the view in its proprietary format in the metastore of choice. Thus, views created from one engine can not be read or altered easily from another engine even when engines share the metastore as well as the storage system. This document standardizes the view metadata for ease of sharing the views across engines. + +## Goals + +* A common metadata format for view metadata, similar to how Iceberg supports a common table format for tables. +* The view metadata format specification + * Includes storage format as well as APIs to write/read the metadata. + * Supports versioning of views to track how a view evolved over time. + +## Overview + +View metadata storage mirrors how Iceberg table metadata is stored and retrieved. View metadata is maintained in metadata files. All changes to view state create a new view metadata file and completely replace the old metadata using an atomic swap. Like Iceberg tables, this atomic swap is delegated to the metastore that tracks tables and/or views by name. The view metadata file tracks the view schema, custom properties, current and past versions, as well as other metadata. +Each metadata file is self-sufficient. It contains the history of the last few operations performed on the view and can be used to roll back the view to a previous version. + +### Metadata Location + +An atomic swap of one view metadata file for another provides the basis for making atomic changes. Readers use the version of the view that was current when they loaded the view metadata and are not affected by changes until they refresh and pick up a new metadata location. + +Writers create view metadata files optimistically, assuming that the current metadata location will not be changed before the writer’s commit. Once a writer has created an update, it commits by swapping the view's metadata file pointer from the base location to the new location. + +## Specification + +### Terms + +* **Schema** -- Names and types of fields in a view. +* **Version** -- The state of a view at some point in time. + +### View Metadata + +The view version metadata file has the following fields: + +| Required/Optional | Field Name | Description | +|-------------------|------------|-------------| +| Required | format-version | An integer version number for the view format. Currently, this must be 1. Implementations must throw an exception if the view's version is higher than the supported version. | +| Required | location | The view's base location. This is used to determine where to store manifest files and view metadata files. | +| Required | current-version-id | Current version of the view. Set to ‘1’ when the view is first created. | +| Optional | properties | A string to string map of view properties. This is used for metadata such as "comment" and for settings that affect view maintenance. This is not intended to be used for arbitrary metadata. | +| Required | versions | An array of structs describing the last known versions of the view. Controlled by the table property: “version.history.num-entries”. See section [Versions](#versions). | Review Comment: Probably don't need "last" in "last known". Just "known versions of the view" should be sufficient. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: dev-unsubscr...@iceberg.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
