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• Making Materialized implicit suggests that a Materialized is a context object whereas it's not.
• The user of the API will by default just not care about implicit parameters and assume that they are provided.
• The user of the API might declare it's own Materialized in an implicit val since the parameter is implicit and then be caught by other unwanted functions that also takes Materialized as implicit.
• This a long shot view but in Scala 3 giving implicits explicitly looks like:
  groupedStream.count().explicitly(Materialized.as("store-name"))
  So it's clear tha

All of that applies to Consumed and Produced too.

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org.apache.kafka.streams.scala.kstream.KGroupedStream.count()
org.apache.kafka.streams.scala.kstream.KGroupedStream.reduce()
org.apache.kafka.streams.scala.kstream.KGroupedStream.aggregate()
org.apache.kafka.streams.scala.kstream.KGroupedTable.count()
org.apache.kafka.streams.scala.kstream.KGroupedTable.reduce()
org.apache.kafka.streams.scala.kstream.KGroupedTable.aggregate()
org.apache.kafka.streams.scala.ImplicitConversions.materializedFromSerde()
org.apache.kafka.streams.scala.StreamsBuilder.stream()
org.apache.kafka.streams.scala.StreamsBuilder.table()
org.apache.kafka.streams.scala.StreamsBuilder.globalTable()

Proposed Changes

def count()(implicit keySerde: Serde[K], valueSerde: Serde[V])
def count(implicit materialized: Materialized[K, Long, ByteArrayKeyValueStore])

def reduce(reducer: (V, V) => V)(implicit keySerde: Serde[K], valueSerde: Serde[V])
def reduce(reducer: (V, V) => V, materialized: Materialized[K, V, ByteArrayKeyValueStore])

def aggregate[VR](initializer: => VR)(aggregator: (K, V, VR) => VR)(implicit keySerde: Serde[K], valueSerde: Serde[V])
def aggregate[VR](materialized: Materialized[K, VR, ByteArrayKeyValueStore], initializer: => VR)(aggregator: (K, V, VR) => VR)

defstream[K, V](topic: String)(implicit keySerde: Serde[K], valueSerde: Serde[V])
defstream[K, V](topic: String, consumed: Consumed[K, V])

deftable[K, V](topic: String)(implicit keySerde: Serde[K], valueSerde: Serde[V])
deftable[K, V](topic: String, consumed: Consumed[K, V])

defglobalTable[K, V](topic: String)(implicit keySerde: Serde[K], valueSerde: Serde[V])
defglobalTable[K, V](topic: String, consumed: Consumed[K, V])

def through(topic: String)(implicit keySerde: Serde[K], valueSerde: Serde[V])
def through(topic: String, produced: Produced[K, V])

def to(topic: String)(implicit keySerde: Serde[K], valueSerde: Serde[V])
def to(topic: String, produced: Produced[K, V])

def to(extractor: TopicNameExtractor[K, V])(implicit keySerde: Serde[K], valueSerde: Serde[V])
def to(extractor: TopicNameExtractor[K, V], produced: Produced[K, V])

This way we require only the Serdes if the Materialized (or Consumed, or Produced) is not given explicitly.TBD

Compatibility, Deprecation, and Migration Plan

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