davidradl commented on code in PR #29090: URL: https://github.com/apache/flink/pull/29090#discussion_r3979861261
########## docs/content/docs/concepts/sql-table-concepts/interpreting_changelog.md: ########## @@ -0,0 +1,326 @@ +--- +title: "Interpreting Flink's Changelog" +weight: 3 +type: docs +--- +<!-- +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. +--> + +# Interpreting Flink's Changelog + +A Flink table can change over time: rows get inserted, updated, or deleted. This is internally encoded internally as changelogs inside the Flink SQL engine. When you consume that table as a stream — after writing the table to another system, through `toChangelogStream`, a table connector, or your own service reading the output — each row carries a change flag telling you what kind of change it represents. This page describes how to read that stream correctly so you're able to built a table representation out of it. + +## The four row kinds + +Every row in a changelog stream has a `RowKind`: + +| Kind | Symbol | Meaning | +|---|---|---| +| `INSERT` | `+I` | A new row was added. | +| `UPDATE_BEFORE` | `-U` | The previous value of a row that is about to be updated. | +| `UPDATE_AFTER` | `+U` | The new value of a row that was updated. | +| `DELETE` | `-D` | A row was removed. | + +## Changelog Modes + +Under the hood, the Flink SQL engine is always operating with changelogs in one of three modes: append, upsert and retract. This depends on the operations present in your query and each of these modes has it's own characteristic. + +**Append Mode `{+I}`** +- All messages are insert-only. +- Every insertion message is an immutable fact. +- Messages can be distributed in an arbitrary fashion across partitions and processors because they are unrelated. + +**Upsert Mode `{+I, +U, -D}`** +- Messages can contain updates leading to an updating table. +- Updates are related using a key (i.e. the *upsert key*). +- Every message is either an upsert or delete message for a result under the upsert key. +- Messages for the same upsert key should land at the same partition and processor. +- Deletions can contain only values for upsert key columns (i.e. *partial deletes*) or values for + all columns (i.e. *full deletes*). +- The mode is also known as *partial image* in the literature because `-U` messages are missing. + +**Retract Mode `{+I, -U, +U, -D}`** +- Messages can contain updates leading to an updating table. +- Every insertion or update event is a fact that can be "undone" (i.e. retracted). +- Updates are related by all columns. In simplified words: The entire row is kind of the key but duplicates are supported. + For example: `+I['Bob', 42]` is related to `-D['Bob', 42]` and `+U['Alice', 13]` is related to `-U['Alice', 13]`. Review Comment: when you say "is related to" it would be good to clarify. Are you taking about they will be potentially processed together in a partition, or effects the same row in the updating table? -- 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: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
