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  • KIP-466: Add support for List<T> serialization and deserialization
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Status

Current stateAccepted

Discussion thread: Link

JIRA KAFKA-8326 - Getting issue details... STATUS

Motivation

I believe there are many use cases where List Serde could be useful.

Ex. https://stackoverflow.com/questions/41427174/aggregate-java-objects-in-a-list-with-kafka-streams-dsl-windowshttps://stackoverflow.com/questions/46365884/issue-with-arraylist-serde-in-kafka-streams-api 

For instance, aggregate grouped (by key) values together in a list to do other subsequent operations on the collection.

Public Interfaces

  • New class org.apache.kafka.common.serialization.ListSerializer which implements the Serializer<List<T>>interface
  • New class org.apache.kafka.common.serialization.ListDeserializer which implements Deserializer<List<T>> interface
  • New subclass ListSerde<T> in org.apache.kafka.common.serialization.Serdes which creates new serde based on ListSerializer and ListDeserializer classes
  • New method public static <T> Serde<List<T>> ListSerde(Class listClass, Serde<T> innerSerde) in org.apache.kafka.common.serialization.Serdes class

Proposed Changes

This KIP proposes adding new ListSerializer and ListDeserializer classes as well as support for the new ListSerde nested class inside the Serdes class. This will allow using List<T> Serde directly from Consumers, Producers and Streams.

List<T> serialization and deserialization will be done through repeatedly calling a serializer/deserializer for each entry provided by passed generic T's Serde. For example, if you want to create List of Strings serde, then serializer/deserializer of Serdes.StringSerde will be used to serialize/deserialize each entry in List<String>.

Proposed Configurations

List serde is an unusual type of serde because we need to consider two things here: the implementation of List interface(i.e. ArrayList, LinkedList, etc) as well as its enclosed elements' type.

First, we need to specify that we are going to use a list serde:

default.key/value.serde = org.apache.kafka.common.serialization.Serdes$ListSerde

Then, we need to introduce two brand new configurations and here I'm proposing these four extra properties:

CommonClientConfigs.class (duplicated in StreamsConfig.class): DEFAULT_LIST_KEY/TYPE_SERDE_TYPE_CLASS = "default.list.key.serde.type"

Ex. default.list.key/value.serde.type = java.util.ArrayList

CommonClientConfigs.class (duplicated in StreamsConfig.class): DEFAULT_LIST_KEY/TYPE_SERDE_INNER_CLASS = "default.list.key.serde.inner"

Ex. default.list.key/value.serde.inner = org.apache.kafka.common.serialization.Serdes$IntegerSerde

Other proposed properties:

ConsumerConfig.classLIST_KEY_DESERIALIZER_TYPE_CLASS_CONFIG = "list.key.deserializer.type"

ConsumerConfig.classLIST_KEY_DEERIALIZER_INNER_CLASS_CONFIG = "list.key.deserializer.inner"

ProducerConfig.classLIST_KEY_SERIALIZER_INNER_CLASS_CONFIG = "list.key.serializer.inner"

P.S. We do not need a type class config for the serializer since we do not care about the type of the list class during serialization.

P.P.S. Properties default.list.key/value.* will be ignored as long as default.key/value.serde is not set to org.apache.kafka.common.serialization.Serdes$ListSerde

Serialization Strategy

For the performance purposes the following serialization strategy was put in place. Depending on the type of an inner serde (a list's element type) the serialization will be performed in the following ways:

  1. If an inner serde has one of the following serializers (LongSerializer.class, IntegerSerializer.class, ShortSerializer.class, FloatSerializer.class, DoubleSerializer.class), then the final payload will not contain each element's size encoded since the size of presented types is static (8 bytes, 4 bytes, 2 bytes, etc)
  2. If the inner serde doesn't have one of the serializers listed above, then a size of each element will be encoded in the final payload (see below)
                                                                        
                Case 1                             Case 2               
                                                                        
         +------------------+               +------------------+        
         |                  |               |                  |        
     Int |   Size of list   |           Int |   Size of list   |        
         |                  |               |                  |        
         |------------------|               |------------------|        
         |                  |               |                  |        
         |     Entry 1      |           Int |  Size of entry 1 |        
         |                  |               |                  |        
         |------------------|               |------------------|        
         |                  |               |                  |        
         |     Entry 2      |               |     Entry 1      |        
         |                  |               |                  |        
         |------------------|               |------------------|        
         |                  |               |                  |        
         |                  |           Int |  Size of entry 2 |        
         |                  |               |                  |        
         |                  |               |------------------|        
         |                  |               |                  |        
         |                  |               |     Entry 2      |        
         |       ...        |               |                  |        
         |                  |               |------------------|        
         |                  |               |                  |        
         |                  |               |                  |        
         |                  |               |       ...        |        
         |                  |               |                  |        
         |                  |               |                  |        
         +------------------+               +------------------+        


Compatibility, Deprecation, and Migration Plan

Does not apply

Rejected Alternatives

Not known

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