Page History

...

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 (immutable) 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. The serialization of the [int, bye[]] header set will on the wire using a strict format
7. Each headers value will be custom serializable by the interceptors/plugins that use the header.
8. 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

...

Wire protocol change - use attribute bit4 as flag for if headers present. and add (optional) header size

...

and field to the message format

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

A key to this is to ensure the headers cause as little no overhead or none if not present as possible.

• The attributes flag bit is used to keep the message size the same as before if no headers are usedHeadersLength is a variable length encoded int saving bytes where headers are small in size/numbersee: https://github.com/msgpack/msgpack/blob/master/spec.md#formats-int
  
• OR
see http://grepcode.com/file/repository.cloudera.com/content/repositories/releases/com.cloudera.hadoop/hadoop-core/0.20.2-320/org/apache/hadoop/io/file/tfile/Utils.java

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 => variable int32 <------------------ NEW [optional] size of the byte[] of the serialized headers if headers
    (optional) Headers => bytes <--------------------------------- NEW [optional] serialized form of the headers Map<int, byte[]>
    ValueLength => int32
    Value => bytes

...

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

...

.

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

...

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

...

local (In-house) custom per

...

organisation

...