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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. Add a headers (Map<int, byte[]>) field to ProducerRecord and ConsumerRecord. A producer/interceptors will be able to set headers on a ProducerRecord. A consumer/interceptors will see the message headers when it sees the messages.

3. Add accessor methods on the Producer/ConsumerRecord void setHeader(int, byte[]) and a byte[] getHeader(int)
4. Add ProduceRequest/ProduceResponse V3 which uses the new message format.
5. Add FetchRequest/FetchResponse V3 which uses the new message format.
6. 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

7. 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> {    /** * 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 add(Header header) throws IllegalStateException;   /** * Adds a header by key and value, returning if the operation succeeded. * If headers is in read-only, will always fail the operation with throwing IllegalStateException. */ Headers add(String key, byte[] value) throws IllegalStateException;   /** * Removes ALL HEADERS for the given key returning if the operation succeeded. * If headers is in read-only, will always fail the operation with throwing IllegalStateException. */ Headers remove(String key) throws IllegalStateException; /** * Returns JUST ONE (the very last) header for the given key, if present. */  Header lastHeader(String key)   /** * Returns ALL headers for the given key, if present. */ Iterable<Header> headers(String key);   }

   3. Implementation Detail

      1. Add accessor methods on the Headers class 

         1. Headers add(Header header)
         2. Headers add(String key, byte[] value)
         3. Headers remove(Header header)
         4. Header lastHeader(String key)
         5. 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)
         6. implement Iterable<Header>
      2. interceptors and k,v serialisers are expected to add headers during the produce intercept stage.
8. Add a headers field to ProducerRecord and ConsumerRecord. 
9. Add constructor(s) of Producer/ConsumerRecord to allow passing in of Iterable<Header> 
   1. use case is MirrorMakers able to copy headers.
10. 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(); ... }

11. Changes needed, will piggyback onto V3 of ProduceRequest and V5 of FetchRequest which were introduced in KIP-98
12. The serialisation of the [String, byte[]] header array The serialization of the [int, bye[]] header set will on the wire using a strict format
13. Each headers value will be custom serializable serialisable by the interceptors/plugins/serdes that use the header.
14. As int key based headers for compactness ranges will be reserved for different usages:
    1. client headers adopted by the kafka open source (not necessary the kafka open source owns it)
    2. broker headers adopted by the kafka open source (not necessary the kafka open source owns it)
    3. commercial vendor space
    4. custom inhouse

15. 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.

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

There are four options proposed before this proposal. This details our proposed solution of Option 1 described here. The other options Option 2, Option 3 and Option 4 are in the Rejected Alternatives section.

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

The below is for the core Message wire protocol change needed to fit headers into the message.

• A header key can occur multiple times, clients should expect to

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• handle this, this can be represented as a multi map, or an array of values per key in clients.

This is basing off KIP-98 Message protocol proposals.

MessageAndOffsetMessage => Offset MessageSize Message
  Offset => int64  
  MessageSizeLength => int32varint
  
  Message => Crc MagicByte Attributes Timestamp=> KeyLengthint8
 Key HeadersLength Headers ValueLength Value
    CrcTimestampDelta => int32varlong
    MagicByte => int8  <---------------------- Bump up magic byte to 2
OffsetDelta => varint
        AttributesKeyLen => int8 <---------------------- Use Bit 4 as boolean flag for if headers presentvarint
        Key => data
    Timestamp => int64
    KeyLengthValueLen => int32varint 
       Key Value => bytesdata
       (optional) HeadersLengthHeaders => variable int32[Header] <------------------ NEW [optional]Added sizeArray of theheaders
 byte[] of the serialized headers if headers 
Header =>
		Key => string (optionalutf8) Headers => bytes <--------------------------------- NEW [optional]UTF8 serializedencoded formstring of(uses the headers Map<int, byte[]>varint length)
    ValueLength => int32
    Value => bytes

Wire protocol of the headers bytes (if present not above mentioned attributes bit flag)

Headers (bytes) => Count Set(Key, ValueLength, Value)
  Count => variable length encoded int32  
  Set =>
	Key => variable length encoded int32
    ValueLength => variable length encoded int32
    Value => bytes
 

Key Allocation

As mentioned above ranges of keys will be reserved for different usages, we use variable encode int keys to reduce the key size overhead.

Whilst the open space of headers may bring to fruition many possible keys, the likely hood of a cluster/broker using 100's is unlikely so we should assign/reserve key space for the most used areas.

With this where ints are in the below ranges we get the benefits of less bytes for an int than standard 4 byte allocation and best benefit.

As such we propose that:

+ve ints 0->255 are reserved for the apache kafka open registered headers as these are more likely to be more commonly used as such saves space

-ve ints -1 -> -128 are reserved for in-house registered headers as these are the next most likely to be heavily used

-ve ints -129 and below can be used as a scratch space either where more in-house header space is required or devleopment

+ve ints 256 and above can be used for other vendor released or as a spill over for open source headers.

Sample register that would end up having in the open source space.

To assit and help ensure ease of use and uniformity a constants class should be kept and updated with the above (similar to how java sql codes work) e.g.

package org.apache.kafka.common.config;

public class KafkaHeaderKeys
{

   public static final int CLIENT_ID_KEY = 1;
   public static final int CLUSTER_ID_KEY = 2;
   public static final int CORRELATION_ID_KEY = 3;
   
}

Sample register that would end up having in-house custom per user/company.

 <------------------------------------ NEW header value as data (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 with empty headersand as no headers on send would have none.
  • older consumers simply would consume a message oblivious to there being any headersthey 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.

• 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

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.

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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.The advantages of this proposal are:

• Benefits

Value Message Wrapper - Message<H, P>

This concept is the current defacto way many users are having to temporally deal with the situation, but has some core key issues that it does not resolve.

• Benefits
  • This will cause no broker side changes, and or message format changes
• Disadvantages
  • This would not work with compaction where headers are needed to be sent on delete record which then would not deliver on many of the requirements.
  • Couple Serialization and Deserialization of the value for both the header and payload.

ProducerRecord<K, H, V>, ConsumerRecord<K, H, V>

The proposed change is that headers are Map<String, String> Map<int, byte[]> only, this alternative is that headers can be of any type denoted by H

• Benefits
  • Complete customization customisation of what a header is.
• Disadvatages
  • As generics don't allow for default type, this would cause breaking client interface compatibility if done on Producer/ConsumerRecord.
    • Possible work-around would be to have HeadersProducer/ConsumerRecord<K, H, V> that then Producer/ConsumerRecord extend where H is object, this though becomes ugly fast if kept for a time or would require a deprecation / refactor v2 period.

Common Value Message Wrapper - Message<V>

This builds on the status quo and addresses some core issues, but fails to address some more advanced and future use cases and also has some compatibility issues for upgrade/clients not supporting.

please see: Headers Value Message Wrapper

Status Quo - Keep Custom Value Message Wrapper - Message<H, P>

This concept is the current defacto way many users are having to temporally deal with the situation, but has some core key issues that it does not resolve.

• Benefits
  • This will cause no broker side changes to handle the message
• Disadvantages
  • This would not work with compaction where headers are needed to be sent on delete record which then would not deliver on many of the requirements.
  • Every organization has custom solution
    • No ecosystem of tooling can evolve
    • Cost of Third party vendors integration high, as custom for every organization they integrate for.
  • Not able to make use of headers server side
  • Couple Serialisation and Deserialisation of the value for both the header and payload.