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A new configuration for producers json.decimal.serialization.format will be introduced to the JsonConverter configuration to help control whether to produce source converters will serialize decimals in numeric or binary formats. The valid values will be "BASE64" (default, to maintain compatibility) and "NUMERIC".

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The changes will be scoped nearly entirely to the JsonConverter, which will be able to deserialize a DecimalNode when the schema is defined as a decimal. Namely, the converter will no longer throw an exception if the incoming data is a numeric node but the schema is specified decimal (logical type). If json.decimal.serialization.format is set to BASE64, the serialization path will remain the same. If it is set to NUMERIC, the JSON value being produced serialized will be a number instead of a text value.

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There are the following combinations that could occur during migration:

• Legacy ProducerSource Converter, Upgraded ConsumerSink Converter: this scenario is okay, as the upgraded consumer sink converter will be able to read the implicit BINARY format
• Upgraded Producer Source Converter with NUMERIC serialization, Upgraded ConsumerSink Converter: this scenario is okay, as the upgraded consumer sink converter will be able to read (deserialize) the numeric serialization
• Upgraded Producer Source Converter with BASE64 serialization, Legacy ConsumerSink Converter: this scenario is okay as the upgraded producer source converter will be able to read serialize binary data as today in BASE64 format
• Upgraded Producer Source Converter with NUMERIC serialization, Legacy ConsumerSink Converter: this is the only scenario that is not okay and will cause issues since the legacy consumers sink converter cannot consumer deserialize NUMERIC data. 

Because of this, users must take care to first ensure that all consumers sink connectors have upgraded to the new converter code before upgrading producerssource connectors to make use of the NUMERIC serialization format in JsonConverter.

There is also concern of data changing in the middle of the stream:

• Legacy → Upgraded BASE64: this will not cause any change in the data in the topic
• Legacy → Upgraded NUMERIC: this will cause a all new values to be serialized using NUMERIC format and will cause issues unless consumers sink converters are upgraded
• Upgraded BINARY → Upgraded NUMERIC: this is identical to above
• Upgraded NUMERIC → Upgraded BASE64: this will not cause a new issue since if the numeric format was already working, all consumers sink converters would be able to read binary format as well
• Upgraded NUMERIC → (Rollback) Legacy: this is identical to above

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• The original KIP suggested supporting an additional representation - base10 encoded text (e.g. `{"asText":"10.2345"}`). While it is possible to automatically differentiate NUMERIC from BASE10 and BASE64, it is not always possible to differentiate between BASE10 from BASE64. Take, for example, the string "12" - this is both a valid decimal (12) and a valid hex string which represents a decimal (1.8). This causes issues because it is impossible to disambiguate between BASE10 and BASE64 without an additional config - furthermore, this makes the migration from one to the other nearly impossible because it would require that all consumers stop consuming and producers stop producing and atomically updating the config on all of them after deploying the new code, or waiting for the full retention period to pass - neither option is viable. The suggestion in the KIP is strictly an improvement over the existing behavior, even if it doesn't support all combinations.
• Encoding the serialization in the schema for Decimal LogicalType. This is good because it means that the deserializer will be able to decode based on the schema and one converter can handle different topics encoded differently as long as the schema is in line. The problem is that this is specific only to only JSON and changing the LogicalType is not the right place.