Package org.apache.kafka.streams.kstream

Interface ValueTransformerWithKey<K,V,VR>

Type Parameters:

K - key type

V - value type

VR - transformed value type

public interface ValueTransformerWithKey<K,V,VR>

The ValueTransformerWithKey interface for stateful mapping of a value to a new value (with possible new type). 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. ValueMapper for stateless value transformation). Additionally, this ValueTransformerWithKey can schedule a method to be called periodically with the provided context. Note that the key is read-only and should not be modified, as this can lead to corrupt partitioning. If ValueTransformerWithKey is applied to a KeyValue pair record the record's key is preserved.

Use ValueTransformerWithKeySupplier to provide new instances of ValueTransformerWithKey to Kafka Stream's runtime.

If a record's key and value should be modified Transformer can be used.

See Also:

• ValueTransformer
• ValueTransformerWithKeySupplier
• KStream.transformValues(ValueTransformerSupplier, String...)
• KStream.transformValues(ValueTransformerWithKeySupplier, String...)
• Transformer

Method Summary

Modifier and Type	Method	Description
void	close()	Close this processor and clean up any resources.
void	init(ProcessorContext context)	Initialize this transformer.
VR	transform(K readOnlyKey, V value)	Transform the given [key and] value to a new value.

Method Details

init

void init(ProcessorContext context)

Initialize this transformer. This is called once per instance when the topology gets initialized.

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).

Note that using ProcessorContext.forward(Object, Object) or ProcessorContext.forward(Object, Object, To) is not allowed within any method of ValueTransformerWithKey and will result in an exception.

Parameters:

context - the context

Throws:

IllegalStateException - If store gets registered after initialization is already finished

StreamsException - if the store's change log does not contain the partition

transform

VR transform(K readOnlyKey,
 V value)

Transform the given [key and] value to a new value. Additionally, any StateStore that is attached to this operator can be accessed and modified arbitrarily (cf. ProcessorContext.getStateStore(String)).

Note that using ProcessorContext.forward(Object, Object) or ProcessorContext.forward(Object, Object, To) is not allowed within transform and will result in an exception.

Note that if a ValueTransformerWithKey is used in a KTable.transformValues(ValueTransformerWithKeySupplier, String...) (or any other overload of KTable#transformValues(...)) operation, then the provided ProcessorContext from init(ProcessorContext) does not guarantee that all context information will be available when transform() is executed, as it might be executed "out-of-band" due to some internal optimizations applied by the Kafka Streams DSL.

Parameters:

readOnlyKey - the read-only key

value - the value to be transformed

Returns:

the new value

close

void close()

Close this processor and clean up any resources.

It is not possible to return any new output records within close(). Using ProcessorContext.forward(Object, Object) or ProcessorContext.forward(Object, Object, To), will result in an exception.