Dfyjkt

I have been readying the doc about state store but it is still not clear to me if it can fit my purpose. I would like to use some Distributed Graph Database as as a state store that other external application can consume from. Is that possible, what effort does that involve and can anyone point me to the class/code that will need to be extended for that functionality to happen.

You can implement custom state store using Processor API as described here :
https://docs.confluent.io/current/streams/developer-guide/interactive-queries.html#querying-local-custom-state-stores

• Your custom state store must implement StateStore.


• You must have an interface to represent the operations available on the store.


• You must provide an implementation of StoreBuilder for creating instances of your store.


• It is recommended that you provide an interface that restricts access to read-only operations. This prevents users of this API from mutating the state of your running Kafka Streams application out-of-band.

Implementation will look something like this :

public class MyCustomStore<K,V> implements StateStore, MyWriteableCustomStore<K,V> 
 // implementation of the actual store


// Read-write interface for MyCustomStore
public interface MyWriteableCustomStore<K,V> extends MyReadableCustomStore<K,V> 
 void write(K Key, V value);


// Read-only interface for MyCustomStore
public interface MyReadableCustomStore<K,V> 
 V read(K key);


public class MyCustomStoreBuilder implements StoreBuilder<MyCustomStore<K,V>> 
 // implementation of the supplier for MyCustomStore

In order to make it queryable;

• Provide an implementation of QueryableStoreType.


• Provide a wrapper class that has access to all of the underlying instances of the store and is used for querying.

Example :

public class MyCustomStoreType<K,V> implements QueryableStoreType<MyReadableCustomStore<K,V>> 

 // Only accept StateStores that are of type MyCustomStore
 public boolean accepts(final StateStore stateStore) 
 return stateStore instanceOf MyCustomStore;
 

 public MyReadableCustomStore<K,V> create(final StateStoreProvider storeProvider, final String storeName) 
 return new MyCustomStoreTypeWrapper(storeProvider, storeName, this);