org.apache.kafka.streams.kstream

Interface ValueTransformer<V,VR>

Type Parameters:

V - value type

VR - transformed value type

public interface ValueTransformer<V,VR>

The ValueTransformer 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) 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) (cf. ValueMapper for stateless value transformation). Additionally, this ValueTransformer can schedule a method to be called periodically with the provided context. If ValueTransformer is applied to a KeyValue pair record the record's key is preserved.

Use ValueTransformerSupplier to provide new instances of ValueTransformer to Kafka Stream's runtime.

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

See Also:

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

Method Summary

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

Method Detail

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).

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

Parameters:

context - the context

Throws:

java.lang.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(V value)

Transform the given 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.

Parameters:

value - the value to be transformed

Returns:

the new value

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.

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.