THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Status
Current state: Adopted
Discussion thread: Link
JIRA:
Motivation
I believe there are many use cases where List Serde could be useful:
- https://stackoverflow.com/questions/41427174/aggregate-java-objects-in-a-list-with-kafka-streams-dsl-windows
- https://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<Inner> implements Serializer<List<Inner>> interface
- New class org.apache.kafka.common.serialization.ListDeserializer<Inner> implements Deserializer<List<Inner>> interface
- New static subclass class ListSerde<Inner> extends WrapperSerde<List<Inner>> in org.apache.kafka.common.serialization.Serdes
- New method public static <L extends List, Inner> Serde<List<Inner>> ListSerde(Class<L> listClass, Serde<Inner> 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 Serde<List<Inner>> directly from Consumers, Producers and Streams.
Serde<List<Inner>> serialization and deserialization will be done through repeatedly calling a serializer/deserializer for each entry provided by passed generic Inner'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$ListSerde
Then, we need to introduce two brand new configurations and here I'm proposing these four extra properties:
CommonClientConfigs.class: DEFAULT_LIST_KEY/VALUE_SERDE_TYPE_CLASS = "default.list.key.serde.type"Ex. default.list.key/value.serde.type = java.util.ArrayList
CommonClientConfigs.class: DEFAULT_LIST_KEY/VALUE_SERDE_INNER_CLASS = "default.list.key.serde.inner"Ex. default.list.key/value.serde.inner = org.apache.kafka.common.serialization.Serdes$IntegerSerde
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 strategies were put in place:
enum SerializationStrategy {
CONSTANT_SIZE,
VARIABLE_SIZE;
}
Depending on the type of an inner serde (a list's element type) the serialization will be performed in the following ways:
- For SerializationStrategy.CONSTANT_SIZE, if an inner serde has one of the following serializers (ShortSerializer.class, IntegerSerializer.class, FloatSerializer.class, LongSerializer.class, DoubleSerializer.class, UUIDSerializer.class), then the final payload will not contain each element's size encoded since sizes of presented types are static (2 bytes, 4 bytes, 8 bytes, etc.)
- For SerializationStrategy.VARIABLE_SIZE, 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)
Additionally, there are two different ways of serializing NULL values within the payload:
- For SerializationStrategy.CONSTANT_SIZE, the list serializer will generate a null index list that contains indexes of all null entries within the payload
- For SerializationStrategy.VARIABLE_SIZE, the list serializer instead will write Serdes.ListSerde.NULL_ENTRY_VALUE (-1 by default) for the size of a null entry
CONSTANT_SIZE VARIABLE_SIZE
+----------------------------+ +----------------------------+
| SerializationStrategy | | SerializationStrategy |
| Flag | | Flag |
|----------------------------| |----------------------------|
| NullIndexList.size() | | PayloadList.size() |
|----------------------------| |----------------------------|
| Null index 1 | | Size of entry 1 |
|----------------------------| |----------------------------|
| Null index 2 | | |
|----------------------------| | Entry 1 |
| ... | | |
|----------------------------| |----------------------------|
| PayloadList.size() | | Size of entry 2 |
|----------------------------| |----------------------------|
| | | |
| Entry 1 | | Entry 1 |
| | | | -
|----------------------------| |----------------------------|
| | | |
| Entry 2 | | |
| | | |
|----------------------------| | |
| | | |
| | | ... |
| | | |
| | | |
| ... | | |
| | | |
| | | |
| | | |
| | | |
| | | |
+----------------------------+ +----------------------------+ Compatibility, Deprecation, and Migration Plan
Does not apply
Rejected Alternatives
Not known