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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 key and value, Headers remove(Iterable<Header> headers); 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 Get just the headers for the given keyfor the given 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 mutable instance into read only immutable instancevery last) header for the given key, if present. * this is invoked on send after the interceptors/  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. Mutation methods will return a new instance. *** under discussion ***
         8. close method will turn headers object from mutable into immutable *** under discussion ***
         9. 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()
   Add mutation methods for headers on the Producer/ConsumerRecord returning a new instance of Producer/ConsumerRecord with modified headers *** under discussion ***
   

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

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

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

   4. 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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   • Adds the ability for producers to set standard header key=value value pairs
   • No incompatible client api change (only new methods)
   • Allows users to specify the serialisation of the header value per header
   • Provides a standardised interface to eco systems of tools that then can grow around the feature

   The disadvantage of this proposal is:

   • Change to the message object

    

   Key duplication and ordering

   • Duplicate headers with the same key must be supported.
   • The order of headers must be retained throughout a record's end-to-end lifetime: from producer to consumer.

   Create a Headers Interface and Implementation to encapsulate headers protocol.

   • See above public interfaces section for sample interfaces.

   Add a headers field Headers to both ProducerRecord and ConsumerRecord

   • Accessor methods of Headers headers() added to interface of the ProducerRecord/ConsumerRecord.
   • Add constructor(s) of Producer/ConsumerRecord to allow passing in of an existing/pre-constructed headers via Iterable<Header> 
     1. use case is MirrorMakers able to copy headers.

    

   Add new method to make headers accessible during de/serialization

   • Add new default method to Serialization/Deserialzation class, with Header parameter
     • Due to KIP-118 not being implemented, a new extended interface ExtendedSerializer ExtendedDeserializer with new extra methods is introduced instead
       • Existing De/Serialization will be wrapped and work as before.

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

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   Code Block
   language java
   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
   • Given the mutability of the headers and the fact that we close them at some point, it means that users should not resend the same record if they use headers and interceptors.
   • • They should either configure the producer so that automatic retries are attempted (preferred although there are some known limitations regarding records that are timed out before a send isactually attempted)
     • or 
     • They need to create a new record while being careful about what headers they include (if they rely fully on interceptors for headers, they could just not include any headers in the new headers).

   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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