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	/**
     * Get an iterator over keys which have the specified prefix. The type of the prefix can be different from that of
     * the key. That's why, callers should also pass a serializer for the prefix to convert the prefix into the
     * format in which the keys are stored underneath in the stores
     * @param prefix The prefix.
     * @param prefixKeySerializer Serializer for the Prefix key type
     * @param <PS> Prefix Serializer type
     * @param <P> Prefix Type.
     * @return The iterator for keys having the specified prefix.
     */
    <PS extends Serializer<P>, P> KeyValueIterator<K, V> prefixScan(P prefix, PS prefixKeySerializer);

This method would also have a default implementation that throws an UnsupportedOperationException for source compatibility with existing state stores.

Proposed Changes

All the classes which implement ReadOnlyKeyValueStore and which support prefixScan would implement this method. Rest of the classes would throw throw use the default implementation which throws an UnsupportedOperationException. 

As an example, plz review the following code for RocksDB store:

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Since the proposal adds the prefixScan method at ReadOnlyKeyValueStore interface level, we will need to either implement the method for those stores which support prefixScan or make them throw UnSupportedOperationException.. For source compatibility purposes, rest of the stores would use the default implementation which throws UnsupportedOperationException

Rejected Alternatives

The first alternative to add a separate interface called PrefixSeekableStore and have only relevant stores implement it was rejected. This is because it was not compatible with the way users of Kafka Streams define state stores in the application. 

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