Current state: Under Discussion
Discussion thread: here
JIRA:
-
KAFKA-5363Getting issue details...
STATUS
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
Currently, when restoring a state store in a Kafka Streams application, we put one key-value pair at a time into the store.
This proposal aims to make this recovery more efficient for certain persistent stores that can be optimized for bulk writes by adding a new interface with "restoreAll" functionality.
Additionally this proposal will add an interface used as an event listener to do the following:
- Notification when the restoration (bulk or not) process starts.
- Intermediate notification as batches are restored with number of records and last offset restored.
- Notification when the restoration (again bulk or not) process ends.
The proposed listener interface will be available for two use cases:
- External or user notification of state restoration progress for monitoring purposes when the application is fully online. This will require adding a setter method on the
KafkaStreams
instance described in the next section. - Internal or per state store notification so the state store can perform any required resource management at the beginning or end of the restoration. Closing and re-opening a RocksDB database to use bulk loading configurations is one intended result of providing this listener.
We'll outline these use cases in more detail below.
NOTE: This work is building off ideas originally proposed in - KAFKA-4322Getting issue details... STATUS and will be incorporated into this KIP.
Public Interfaces
This KIP will introduce the following interfaces:
- The
BatchingStateRestoreCallback interface
. - The
StateRestoreListener
interface.
public interface BatchingStateRestoreCallback extends StateRestoreCallback { void restoreAll(Collection<KeyValue<byte[], byte []>> records); }
This interface will allow for state stores to implement a bulk loading approach during the restore phase. The StateRestoreCallback
interface is kept as is for backwards compatibility
public interface StateRestoreListener { void onRestoreStart(StateStore storeName, long startingOffset, long endOffset); void onBatchRestored(String storeName, long batchEndOffset, long numRestored); void onRestoreEnd(String storeName, long totalRestored); }
The onBatchRestored
method is called after records retrieved from each poll()
call have been restored. This is to give users a sense of progress being made in the restore process.
The number of times onBatchRestored
is called is (Total records in change log / MAX_POLL_RECORDS)
.
The changes also include adding a setter
method on the KafkaStreams
object:
public void setStateRestoreListener(final StateRestoreListener stateRestoreListener)
StateRestoreListener Use Cases
The first use case is user updates of restore progress - In this case users of a Kafka Streams program want to get notification of the progress, reporting status back to UI for example. The StateRestoreListener
set via the KafkaStreams.setStateRestoreListener
method will be a global listener. This means for reporting purposes there will only be one listener reporting on the status for all stores.
The second use case is internal state store management, closing and re-opening a RocksDB instance for bulk loading with different configuration settings for example. In this case implementors of a custom store want notification of restoration start, progress and ending for state manage purposes. In this case the StateRestoreListener
implementation is used internally by the given state store. In this use case users can specify a StateStoreListener
per store, but the intent here is not for reporting but for internal state management.
To use the listener functionality users will implement the StateRestoreListener
interface in addition to the StateRestoreCallback
or BatchingStateRestoreCallback
interfaces when constructing their callbacks. Providing the callback is still done via the ProcessorContext.register
method.
Compatibility, Deprecation, and Migration Plan
- Since the
BatchingStateResoreCallback
extends theStateRestoreCallback
there should be no impact to classes already implementing this interface. - The
StateRestoreContext
interface is an addition to the code base so no impact is expected. - The addition of a setter method on the
KafkaStreams
object adds no impact to existing code. - Abstract classes implementing the different callback approaches and the
StateRestoreListener
interface with no-op methods are provided.
- Since the
Rejected Alternatives
N/A