Package org.apache.kafka.streams.processor

Interface StateStore

All Known Subinterfaces:

KeyValueStore<K,V>, SessionStore<K,AGG>, TimestampedKeyValueStore<K,V>, TimestampedWindowStore<K,V>, WindowStore<K,V>

public interface StateStore

A storage engine for managing state maintained by a stream processor.

If the store is implemented as a persistent store, it must use the store name as directory name and write all data into this store directory. The store directory must be created with the state directory. The state directory can be obtained via #stateDir() using the ProcessorContext provided via init(...).

Using nested store directories within the state directory isolates different state stores. If a state store would write into the state directory directly, it might conflict with others state stores and thus, data might get corrupted and/or Streams might fail with an error. Furthermore, Kafka Streams relies on using the store name as store directory name to perform internal cleanup tasks.

This interface does not specify any query capabilities, which, of course, would be query engine specific. Instead it just specifies the minimum functionality required to reload a storage engine from its changelog as well as basic lifecycle management.

Method Summary

Modifier and Type	Method	Description
void	close()	Close the storage engine.
void	flush()	Flush any cached data
void	init(ProcessorContext context, StateStore root)	Deprecated. Since 2.7.0.
default void	init(StateStoreContext context, StateStore root)	Initializes this state store.
boolean	isOpen()	Is this store open for reading and writing
String	name()	The name of this store.
boolean	persistent()	Return if the storage is persistent or not.

Method Detail

name

String name()

The name of this store.

Returns:

the storage name

init

@Deprecated
void init(ProcessorContext context,
          StateStore root)

Deprecated.

Since 2.7.0. Callers should invoke init(StateStoreContext, StateStore) instead. Implementers may choose to implement this method for backward compatibility or to throw an informative exception instead.

Initializes this state store.

The implementation of this function must register the root store in the context via the ProcessorContext.register(StateStore, StateRestoreCallback) function, where the first StateStore parameter should always be the passed-in root object, and the second parameter should be an object of user's implementation of the StateRestoreCallback interface used for restoring the state store from the changelog.

Note that if the state store engine itself supports bulk writes, users can implement another interface BatchingStateRestoreCallback which extends StateRestoreCallback to let users implement bulk-load restoration logic instead of restoring one record at a time.

This method is not called if init(StateStoreContext, StateStore) is implemented.

Throws:

IllegalStateException - If store gets registered after initialized is already finished

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

init

default void init(StateStoreContext context,
                  StateStore root)

Initializes this state store.

The implementation of this function must register the root store in the context via the StateStoreContext.register(StateStore, StateRestoreCallback) function, where the first StateStore parameter should always be the passed-in root object, and the second parameter should be an object of user's implementation of the StateRestoreCallback interface used for restoring the state store from the changelog.

Note that if the state store engine itself supports bulk writes, users can implement another interface BatchingStateRestoreCallback which extends StateRestoreCallback to let users implement bulk-load restoration logic instead of restoring one record at a time.

Throws:

IllegalStateException - If store gets registered after initialized is already finished

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

flush

void flush()

Flush any cached data

close

void close()

Close the storage engine. Note that this function needs to be idempotent since it may be called several times on the same state store.

Users only need to implement this function but should NEVER need to call this api explicitly as it will be called by the library automatically when necessary

persistent

boolean persistent()

Return if the storage is persistent or not.

Returns:

true if the storage is persistent—false otherwise

isOpen

boolean isOpen()

Is this store open for reading and writing

Returns:

true if the store is open