Interactive queries in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka Streams interactive query?

Consider a Kafka Streams application with a state store named user-store. The following code snippet queries the store for a user ID and prints the result.

ReadOnlyKeyValueStore<String, String> store = streams.store(StoreQueryParameters.fromNameAndType("user-store", QueryableStoreTypes.keyValueStore()));
String user = store.get("user123");
System.out.println(user);

Assuming the store contains the entry {"user123": "Alice"}, what will be printed?

Kafka

ReadOnlyKeyValueStore<String, String> store = streams.store(StoreQueryParameters.fromNameAndType("user-store", QueryableStoreTypes.keyValueStore()));
String user = store.get("user123");
System.out.println(user);

A Alice

B null

C user123

D Throws a NullPointerException

💡 Hint

The store.get() method returns the value associated with the key or null if not found.

The key "user123" exists in the store with value "Alice", so store.get("user123") returns "Alice" which is printed.

🧠 Conceptual

intermediate

Which statement about Kafka Streams interactive queries is true?

Select the correct statement about interactive queries in Kafka Streams.

A Interactive queries require stopping the Kafka Streams application to access state.

B Interactive queries allow querying the local state store of a Kafka Streams instance.

C Interactive queries can only be performed on remote Kafka brokers, not local stores.

D Interactive queries automatically replicate state stores to all instances.

💡 Hint

Think about where the state store lives and how queries access it.

Interactive queries let you query the local state store directly without stopping the app or querying Kafka brokers.

🔧 Debug

advanced

Why does this interactive query code throw an exception?

Given the following code snippet:

ReadOnlyKeyValueStore<String, String> store = streams.store("user-store", QueryableStoreTypes.keyValueStore());
String user = store.get("user123");
System.out.println(user);

When running, it throws a IllegalArgumentException. What is the cause?

A The key "user123" does not exist in the store, causing the exception.

B The QueryableStoreTypes.keyValueStore() is not supported for interactive queries.

C The store name must be passed using StoreQueryParameters, not as separate arguments.

D The streams object is not started before querying the store.

💡 Hint

Check the method signature for streams.store() in recent Kafka versions.

The streams.store() method requires a StoreQueryParameters object since Kafka 2.4. Passing name and type separately causes IllegalArgumentException.

📝 Syntax

advanced

Which code snippet correctly performs an interactive query to get all entries from a key-value store?

You want to iterate over all entries in a local key-value store named user-store. Which snippet is correct?

A

ReadOnlyKeyValueStore<String, String> store = streams.store(StoreQueryParameters.fromNameAndType("user-store", QueryableStoreTypes.keyValueStore()));
KeyValueIterator<String, String> iter = store.all();
while (iter.hasNext()) {
  KeyValue<String, String> kv = iter.next();
  System.out.println(kv.key + ": " + kv.value);
}
iter.close();

B

ReadOnlyKeyValueStore<String, String> store = streams.store("user-store", QueryableStoreTypes.keyValueStore());
for (KeyValue<String, String> kv : store) {
  System.out.println(kv.key + ": " + kv.value);
}

C

KeyValueIterator<String, String> iter = streams.store(StoreQueryParameters.fromNameAndType("user-store", QueryableStoreTypes.keyValueStore())).all();
for (KeyValue<String, String> kv : iter) {
  System.out.println(kv.key + ": " + kv.value);
}

D

ReadOnlyKeyValueStore<String, String> store = streams.store(StoreQueryParameters.fromNameAndType("user-store", QueryableStoreTypes.keyValueStore()));
Iterator<KeyValue<String, String>> iter = store.iterator();
while (iter.hasNext()) {
  KeyValue<String, String> kv = iter.next();
  System.out.println(kv.key + ": " + kv.value);
}

💡 Hint

Check the correct method to get all entries and how to iterate the KeyValueIterator.

The store.all() method returns a KeyValueIterator which must be iterated with hasNext()/next() and closed after use. Option A follows this pattern correctly.

🚀 Application

expert

How to implement a fault-tolerant interactive query across multiple Kafka Streams instances?

You have a Kafka Streams application running on 3 instances, each with a local state store user-store. You want to query the state for a user ID, but the key may be on any instance. Which approach ensures fault-tolerant interactive queries?

A Store all keys redundantly on all instances to query any local store directly.

B Query the local store on each instance sequentially until the key is found or all checked.

C Broadcast the query to all instances simultaneously and merge results client-side.

D Use the Kafka Streams Metadata API to find the instance hosting the key, then query that instance's store via a REST call.

💡 Hint

Think about how Kafka Streams partitions state and how to locate the correct instance.

Kafka Streams partitions state stores by key. The Metadata API tells which instance hosts a key. Querying that instance via REST is fault-tolerant and efficient.