[FLINK-34682][cdc][docs] Add "Understand Flink CDC API" page for Flink CDC docs

This closes #3162.

docs/content/docs/developer-guide/understand-flink-cdc-api.md

@@ -5,6 +5,7 @@ type: docs
 aliases:
   - /developer-guide/understand-flink-cdc-api
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+
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@@ -23,3 +24,100 @@ KIND, either express or implied.  See the License for the
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+
+# Understand Flink CDC API
+
+If you are planning to build your own Flink CDC connectors, or considering
+contributing to Flink CDC, you might want to hava a deeper look at the APIs of
+Flink CDC. This document will go through some important concepts and interfaces
+in order to help you with your development.
+
+## Event
+
+An event under the context of Flink CDC is a special kind of record in Flink's
+data stream. It describes the captured changes in the external system on source
+side, gets processed and transformed by internal operators built by Flink CDC,
+and finally passed to data sink then write or applied to the external system on
+sink side.
+
+Each change event contains the table ID it belongs to, and the payload that the
+event carries. Based on the type of payload, we categorize events into these
+kinds:
+
+### DataChangeEvent
+
+DataChangeEvent describes data changes in the source. It consists of 5 fields
+
+- `Table ID`: table ID it belongs to
+- `Before`: pre-image of the data
+- `After`: post-image of the data
+- `Operation type`: type of the change operation
+- `Meta`: metadata of the change
+
+For the operation type field, we pre-define 4 operation types:
+
+- Insert: new data entry, with `before = null` and `after = new data`
+- Delete: removal of data, with `before = removed` data and `after = null`
+- Update: update of existed data, with `before = data before change`
+  and `after = data after change`
+- Replace:
+
+### SchemaChangeEvent
+
+SchemaChangeEvent describes schema changes in the source. Compared to
+DataChangeEvent, the payload of SchemaChangeEvent describes changes in the table
+structure in the external system, including:
+
+- `AddColumnEvent`: new column in the table
+- `AlterColumnTypeEvent`: type change of a column
+- `CreateTableEvent`: creation of a new table. Also used to describe the schema
+  of
+  a pre-emitted DataChangeEvent
+- `DropColumnEvent`: removal of a column
+- `RenameColumnEvent`: name change of a column
+
+### Flow of Events
+
+As you may have noticed, data change event doesn't have its schema bound with
+it. This reduces the size of data change event and the overhead of
+serialization, but makes it not self-descriptive Then how does the framework
+know how to interpret the data change event?
+
+To resolve the problem, the framework adds a requirement to the flow of events:
+a `CreateTableEvent` must be emitted before any `DataChangeEvent` if a table is
+new to the framework, and `SchemaChangeEvent` must be emitted before any
+`DataChangeEvent` if the schema of a table is changed. This requirement makes
+sure that the framework has been aware of the schema before processing any data
+changes.
+
+{{< img src="/fig/flow-of-events.png" alt="Flow of Events" >}}
+
+## Data Source
+
+Data source works as a factory of `EventSource` and `MetadataAccessor`,
+constructing runtime implementations of source that captures changes from
+external system and provides metadata.
+
+`EventSource` is a Flink source that reads changes, converts them to events
+, then emits to downstream Flink operators. You can refer
+to [Flink documentation](https://nightlies.apache.org/flink/flink-docs-stable/docs/dev/datastream/sources/)
+to learn internals and how to implement a Flink source.
+
+`MetadataAccessor` serves as the metadata reader of the external system, by
+listing namespaces, schemas and tables, and provide the table schema (table
+structure) of the given table ID.
+
+## Data Sink
+
+Symmetrical with data source, data sink consists of `EventSink`
+and `MetadataApplier`, which writes data change events and apply schema
+changes (metadata changes) to external system.
+
+`EventSink` is a Flink sink that receives change event from upstream operator,
+and apply them to the external system. Currently we only support Flink's Sink V2
+API.
+
+`MetadataApplier` will be used to handle schema changes. When the framework
+receives schema change event from source, after making some internal
+synchronizations and flushes, it will apply the schema change to
+external system via this applier.