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1. Non-windowed topic materialized to key-value store. This is the most common case and has already been covered by table() API. 
   
2. Non-windowed topic materialized to window store. This is a fallacious requirement because we could easily use aggregate() API to generate a window store based on non-windowed topic.
3. Windowed topic (KStream changelog) materialized to key-value store. This is also a rare requirement to discuss, because the natural difference between key-value store and window store is that window store sets a retention of the data. By materializing windowed topic to key-value we lost the control on the TTL, which leads to wrong representation of the changelog data.
4. Windowed topic  (KStream changelog) materialized to window store. This is a missing requirement which needs to be addressed by our new API. Currently it's very hard to share a changelog between stream applications, and it could be really useful to share the same state store across applications by this API.

Public Interfaces

The current KTable APIs are defined in the StreamsBuilder class:

public synchronized <K, V> KTable<K, V> table(final String topic, final Consumed<K, V> consumed, final Materialized<K, V, KeyValueStore<Bytes, byte[]>> materialized)
public synchronized <K, V> KTable<K, V> table(final String topic);
public synchronized <K, V> KTable<K, V> table(final String topic, final Consumed<K, V> consumed);
public synchronized <K, V> KTable<K, V> table(final String topic, final Materialized<K, V, KeyValueStore<Bytes, byte[]>> materialized);

Through Materialized struct, we could pass in a KeyValueStore<Bytes, byte[]> struct as the local state store. In fact, underlying KTable class by default stores data in a key-value store backed up by RocksDB. We want to also support window store which is a very natural requirement if we are materializing a windowed topic with windowed key.

Proposed Changes

We would like to add eight new APIs to support window store and session store as underlying storage option for windowed topic.

public synchronized <K, V> KTable<K, V> table(final String topic, final Consumed<K, V> consumed, final Materialized<K, V, KeyValueStore<Bytes, byte[]>> materialized)
public synchronized <K, V> KTable<K, V> table(final String topic);
public synchronized <K, V> KTable<K, V> table(final String topic, final Consumed<K, V> consumed);
public synchronized <K, V> KTable<K, V> table(final String topic, final Materialized<K, V, KeyValueStore<Bytes, byte[]>> materialized);

// New APIs: window store materialization to KTable
public synchronized <K, V> KTable<Windowed<K>, V> windowedTable(final String topic, final Consumed<K, V> consumed, final Materialized<K, V, WindowStore<Bytes, byte[]>> materialized);
public synchronized <K, V> KTable<Windowed<K>, V> windowedTable(final String topic, final Duration windowSize);
public synchronized <K, V> KTable<Windowed<K>, V> windowedTable(final String topic, final Consumed<K, V> consumed);
public synchronized <K, V> KTable<Windowed<K>, V> windowedTable(final String topic, final Materialized<K, V, WindowStore<Bytes, byte[]>> materialized);

// New APIs: session store materialization to KTable
public synchronized <K, V> KTable<Windowed<K>, V> sessionTable(final String topic, final Consumed<Windowed<K>, V> consumed, final Materialized<K, V, SessionStore<Bytes, byte[]>> materialized);
public synchronized <K, V> KTable<Windowed<K>, V> sessionTable(final String topic, final Duration windowSize);
public synchronized <K, V> KTable<Windowed<K>, V> sessionTable(final String topic, final Consumed<Windowed<K>, V> consumed);
public synchronized <K, V> KTable<Windowed<K>, V> sessionTable(final String topic, final Materialized<K, V, SessionStore<Bytes, byte[]>> materialized);

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Proposed Changes

As the new API suggests, we are tailing from a windowed changelog topic to materialize the data as a KTable of type <Windowed<K>, V> for processing. Internally, the Consumed struct will be converted to <Windowed<K>, V> to correctly deserialize the changelog records. For Materialized the serde type will still be <K, V> because the window store needs raw key serdes and automatically wrapped with windowed key serde (Checkout WindowedKeySchema.toStoreKeyBinary). These details however, are hided from end user. After KIP-393 we have built the constructor which could wrap around a general key serde to make it a window serde, so stream user doesn't need to worry about the type casting, providing raw key serdes should be suffice.

A `windowSize` duration is required to properly initialize the windowed serde. This is because the underlying storage for windowed records are only storing window start timestamp for space efficiency. When using the new API without configuring Consumed or Materialized, it is required to explicitly pass in the window size. This means user must be aware of the windowed topic window size in order to properly deserialize the topic.

As we are talking, the Consumed and Materialized classes will also be added with windowSize type.

public class Consumed<K, V> {
	...
	protected Duration windowSize; // new
	...
	public static <K, V> Consumed<K, V> with(final Duration windowSize); // new
}

public class Materialized<K, V, S> {
	...
	protected Duration windowSize; // new
	...
	public static <K, V, S> Materialized<K, V, S> withWindowSize(final Duration windowSize); // new
}

So when both Materialized and Consumed are equipped with `windowSize`, the Consumed `windowSize` will take precedence, as it is the primary owner for deserializing the input topic.

Compatibility, Deprecation, and Migration Plan

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