Status

Current state:  Under Discussion 

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Further details and a more detailed case for headers can be seen here : A Case for Kafka Headers

Public Interfaces

This KIP has the following public interface changes:

1. Add a new headers length and value (byte[]) to the core message format.

2. Add Create a new Headers classclass 

   1. It contains a map of key, value

      Add a headers (

      map(Typed

      MultiMap<String,

      Typed))
   2. It exposes accessor methods, which convert into typed values (this is akin to JMS/AMQP headers/property accessors)
      1. void setBoolean(String, Boolean)
      2. boolean getBoolean(String)
      3. void setByte(String, Byte)
      4. Byte getByte(String)
      5. void setChar(String, Char)
      6. Char getChar(String)
      7. void setShort(String, Short)
      8. Short getShort(String)
      9. void setInteger(String, Integer)
      10. Integer getInteger(String)
      11. void setLong(String, Long)
      12. Long getLong(String)
      13. void setFloat(String, Float)
      14. Float getFloat(String)
      15. void setDouble(String, Double)
      16. Double getDouble(String)
      17. void setString(String, String)
      18. String getString(String)
      19. void setBytes(String, byte[])
      20. byte[] getBytes(String)
   3. As noted Key interface is of type string, if this key though can be converted to a more compact typed representational form, for compactness it will be converted e.g. 
      1. "12" would be converted to Typed object of type=short and value=12 (int16)
      2. "true" would be converted to Typed object of type=boolean(true) and value=null
3. Add a new Typed class
   1. To hold the typed representations

4. 1. Object>) field to Headers class

   2. Add accessor methods on the Headers class - void put<T>(String) and a Collection<Object> get(String)

5. Add a headers field to ProducerRecord and ConsumerRecord. 

6. Add Add accessor methods on the Producer/ConsumerRecord to get lazy initialised and lazy deserialized Headers Object.Headers getHeaders()
   ;
   1. Add ProduceRequest/ProduceResponse V4 which uses the new message format.
7. Add FetchRequest/FetchResponse V4 which uses the new message format.
8. The serialisation of the [typedString, typedbyte[]] header set array will on the wire using a strict format
9. Each headers value will be typed, where primitive types and string will be natively supported and support for custom types with a byte[] typecustom serialisable by the interceptors/plugins that use the header.

For more detail information of the above changes, please refer to the Proposed Changes section.

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MessageAndOffset => Offset MessageSize Message
  Offset => int64  
  MessageSize => int32
  
  Message => Crc MagicByte Attributes Timestamp KeyLength Key HeadersLength Headers ValueLength Value
    Crc => int32
    MagicByte => int8  <---------------------- Bump up magic byte to 2
    Attributes => int8 <---------------------- Use Bit 4 as boolean flag for if headers present
    Timestamp => int64
    KeyLength => int32
    Key => bytes
    (optional) HeadersLength => int32 <--------------- NEW [optional] size of the byte[] of the serialised headers if headers
    (optional) Headers => bytes <--------------------- NEW [optional] serialised form of the headers Map<int, byte[]>
    ValueLength => int32
    Value => bytes

Wire protocol of the headers bytes (if above mentioned attributes bit flag is true)

The below is for the headers wire protocol.

• A key can occur multiple times, clients should expect to handle this, this can be represented as a multi map, or an array of values per key in clients.
  
  

Headers (bytes) => SetArray(KeyLength, Key, ValueLength, Value)
  Set =>
	KeyKeyLength => int32 byte [] <-----------------NEW size of the byte[] of the serialised key value
	Key => Typedbytes <---------------------- NEW int key serialised string (UTF8) bytes of the header key
    ValueValueLength => int32 <-------------- NEW size of the byte[] of the serialised header value
    Value => Typedbytes <-------------------- NEW serialised form of the typed header value -> TypeValue interpretation of this below.
 
TypedTypedValue (bytes) => (Type, Value)
	Type => byte <---------------------- 0x00 = null
										 0x01 = boolean=true
                                         0x02 = boolean=false
                                         0x03 0x01 = byte
                                         0x040x02 = char
                                         0x050x03 = short (int16)
                                         0x060x04 = int (int32)
                                         0x070x05 = long (int64)
                                         0x080x06 = float
                                         0x090x07 = double
                                         0x0A0x08 = string
                                         0x0B0x09 = byte[]
    Value => bytes <-------------------- NEW byte array holding the corresponding value in byte array form,  
										 0x00-0x002 - null and boolean this will be zero length
                                         0x03-0x09 - type length, will be the length of the byte array
                                         0x0A-0x0B - string and byte this will contain the leading int32 length of the byte array.
 
 

Compatibility, Deprecation, and Migration Plan

• Current client users should not be affected, this is new api methods being added to client apis
• Message version allows clients expecting older message version, would not be impacted 
  • older producers simply would produce a message without headers
    • new consumer would simply get a message with empty headers
  • older consumers simply would consume a message oblivious to there being any headers
• Message version migration would be handled as like in KIP-32

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• Core message size reduction
  • remove overhead of 4 bytes for KeyLength when no headers using attributes bit
  • reduce overhead of 4 bytes for KeyLength by using variable length encoded int
  • reduce overhead of 4 bytes for ValueLength by using variable length encoded int
• Broker side interceptors
  
  • with headers we could start introducing broker side message interceptors to append meta data or handle messages
• Single Record consumer API
  • There is many uses cases where single record consumer/listener api is more user friendly - this is evident by the fact spring kafka have already created a wrapper, it would be good to support this natively.

Rejected Alternatives

Map<Int, byte[]> Headers added to the Producer/ConsumerRecord

Map<String, String> Headers added to the

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ConsumerRecord

The concept is similar to the above proposed but with a few more disadvantages.

• Benefits

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