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Status

Current state:  Under Discussion Adopted

Discussion thread: here 

JIRA: KAFKA-4208

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

Amendments made during implementation, and on KIP-118 being pulled are highlighted orange, changes reviewed during PR and notification sent to dev mailing lists.

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. Create a Header Interface and implementing class

   1. Interface

      Code Block
      public interface Header { String key(); byte[] value(); }

   2. Implementation Detail

      1. Add a String key field to Header implementing class

      2. Add a byte[] value field to Header implementing class

3. Create a Headers Interface and implementing class 
   1. Headers will be mutable
      1. For the Producer, after send and post interceptors it will be turned into a read only immutable instance.
      2. This will be done by the invoking "close()" method, this method is not exposed in the api, but an implementation detail.

   2. Interface

      Code Block

      public interface Headers extends Iterable<Header> {      /** Headers append(Iterable<Header> headers);   * Adds a header (key inside), returning if the operation succeeded. * If headers is in read-only, will always fail the operation with throwing IllegalStateException. */ Headers appendadd(Header header) throws IllegalStateException;   /** * Adds a header by Headers remove(Iterable<Header> headers); key and value, returning if the operation succeeded. * If headers is in read-only, will always fail the operation with throwing IllegalStateException. */ Headers remove(Header header); add(String key, byte[] value) throws IllegalStateException;   /** * Removes ALL HEADERS for the Get just the headers for the given keygiven key returning if the operation succeeded. * If headers is in read-only, will always fail the operation with throwing IllegalStateException. */ Iterable<Header>Headers headersremove(String key) throws IllegalStateException;   /** * Returns JUST TurnsONE (the very last) mutableheader instancefor intothe readgiven onlykey, immutableif instance. * this is invoked on send after the interceptorspresent. */  Header lastHeader(String key)   /** * Returns ALL headers for the given key, if present. */ HeadersIterable<Header> closeheaders(String key);   }

   3. Implementation Detail

      1. Add accessor methods on the Headers class 

         1. Headers append(Iterable<Header> headers)
         2. Headers appendHeaders add(Header header)Headers remove(Iterable<Header> headers
         3. Headers add(String key, byte[] value)
         4. Headers remove(Header header)
         5. Header lastHeader(String key)
         6. Iterable<Header> headers(String key)
            
            1. Inline with ConsumerRecords interface returning the subset of ConsumerRecord for a given topic.
               1. public Iterable<ConsumerRecord<K, V>> records(String topic)
         7. close method will turn headers object from mutable into immutable
         8. implement Iterable<Header>
      2. interceptors and k,v serialisers are expected to add headers during the produce intercept stage.
4. Add a headers field to ProducerRecord and ConsumerRecord. 
5. Add constructor(s) of Producer/ConsumerRecord to allow passing in of Iterable<Header> 
   1. use case is MirrorMakers able to copy headers.
6. Add accessor methods on the Producer/ConsumerRecord Headers headers()

   1. Code Block
      public class ProducerRecord<K, V> {    ...   ProducerRecord(K key, V value, Iterable<Header> headers, ...)   ... public Headers headers(); ... }

      Code Block
      public class ConsumerRecord<K, V> { ... ConsumerRecord(K key, V value, Iterable<Header> headers, ...) ... public Headers headers(); ... }

7. Changes needed, will piggyback onto V3 of ProduceRequest and V4 of V5 of FetchRequest which were introduced in KIP-98
8. The serialisation of the [String, byte[]] header array will on the wire using a strict format
9. Each headers value will be custom serialisable by the interceptors/plugins/serdes that use the header.

10. Expose headers to De/Serializers - extended interface added, for lack of default methods available in java 8

    Code Block
    public interface ExtendedDeserializer<T> extends Deserializer<T> { T deserialize(String topic, Headers headers, byte[] data); }

    Code Block
    public interface ExtendedSerializer<T> extends Serializer<T> { byte[] serialize(String topic, Headers headers, T data); }

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

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Wire protocol change - add array of headers to end of the message format

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Message =>
        Length => uintVarvarint
        Attributes => int8
        TimestampDelta => intVarvarlong
        OffsetDelta => uintVarvarint
        KeyLen => intVarvarint
        Key => data
        ValueLen => intVarvarint 
        Value => data
        Headers => [Header] <------------ NEW Added Array of headers
        
Header =>
		Length => uintVar <-------------------------------- NEW length of individual header
        KeyLen => uintVar <-------------------------------- NEW length of key bytes
        Key => datastring (utf8) <------------------------------- NEW UTF8 encoded string as(uses data (byte[]varint length)
        Value => databytes  <------------------------------------ NEW header value as data (byte[]uses varint length)

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 which doesn't support headers
    • upcasting would result in a message without headers
    • new consumer would simply get a message and as no headers on send would have none.
  • older consumers simply would consume a message they would not be aware of any headers
    • down casting would remove headers from the message as older message format doesn't have these in protocol.
• Message version migration would be handled as like in KIP-32

Out of Scope

Some additional features/benefits were noticed and discussed on the above but are deemed out of scope and should be tackled by further KIPS.

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