Package org.apache.kafka.streams.kstream

Interface Transformer<K,V,R>

Type Parameters:

K - key type

V - value type

R - KeyValue return type (both key and value type can be set arbitrarily)

public interface Transformer<K,V,R>

The Transformer interface is for stateful mapping of an input record to zero, one, or multiple new output records (both key and value type can be altered arbitrarily). This is a stateful record-by-record operation, i.e, transform(Object, Object) is invoked individually for each record of a stream and can access and modify a state that is available beyond a single call of transform(Object, Object) (cf. KeyValueMapper for stateless record transformation). Additionally, this Transformer can schedule a method to be called periodically with the provided context.

Use TransformerSupplier to provide new instances of Transformer to Kafka Stream's runtime.

If only a record's value should be modified ValueTransformer can be used.

See Also:

TransformerSupplier, KStream.transform(TransformerSupplier, String...), ValueTransformer, KStream.map(KeyValueMapper), KStream.flatMap(KeyValueMapper)

Method Summary

Modifier and Type	Method	Description
void	close()	Close this transformer and clean up any resources.
void	init(ProcessorContext context)	Initialize this transformer.
R	transform(K key, V value)	Transform the record with the given key and value.

Method Details

init

void init(ProcessorContext context)

Initialize this transformer. This is called once per instance when the topology gets initialized. When the framework is done with the transformer, close() will be called on it; the framework may later re-use the transformer by calling init(ProcessorContext) again.

The provided context can be used to access topology and record meta data, to schedule a method to be called periodically and to access attached StateStores.

Note, that ProcessorContext is updated in the background with the current record's meta data. Thus, it only contains valid record meta data when accessed within transform(Object, Object).

Parameters:

context - the context

transform

R transform(K key,
V value)

Transform the record with the given key and value. Additionally, any state that is attached to this operator can be accessed and modified arbitrarily (cf. ProcessorContext.getStateStore(String)).

If only one record should be forward downstream, transform can return a new KeyValue. If more than one output record should be forwarded downstream, ProcessorContext.forward(Object, Object) and ProcessorContext.forward(Object, Object, To) can be used. If no record should be forwarded downstream, transform can return null. Note that returning a new KeyValue is merely for convenience. The same can be achieved by using ProcessorContext.forward(Object, Object) and returning null.

Parameters:

key - the key for the record

value - the value for the record

Returns:

new KeyValue pair—if null no key-value pair will be forwarded to down stream

close

void close()

Close this transformer and clean up any resources. The framework may later re-use this transformer by calling init(ProcessorContext) on it again.

To generate new KeyValue pairs ProcessorContext.forward(Object, Object) and ProcessorContext.forward(Object, Object, To) can be used.