Interactive queries in Kafka - Step-by-Step Execution

Process Flow - Interactive queries

Start Kafka Streams App

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Process Streams & Update Local State

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Expose Local State Store

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Receive Query Request

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Check Local Store for Data

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Return Data

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Respond to Client with Data

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End

The app processes streams and updates local state. When a query comes, it checks local data. If found, returns it; otherwise, asks other instances.

Execution Sample

Kafka

StreamsBuilder builder = new StreamsBuilder();
KTable<String, Long> counts = builder.table("input-topic", Materialized.<String, Long>as("counts-store"));
KafkaStreams streams = new KafkaStreams(builder.build(), props);
streams.start();
InteractiveQueryService iqService = streams.interactiveQueryService();
ReadOnlyKeyValueStore<String, Long> store = iqService.getQueryableStore("counts-store", QueryableStoreTypes.keyValueStore());
Long value = store.get("key1");

This code builds a stream table, gets the interactive query service, accesses the local state store, and queries a value by key.

Process Table

Step	Action	Evaluation	Result
1	Start Kafka Streams app	App initializes and starts processing	Streams app running
2	Process input topic and update local state store	Records processed and counts updated	Local store 'counts-store' updated
3	Receive query for key 'key1'	Check local store for 'key1'	Key found locally? Yes
4	Return value for 'key1' from local store	Value retrieved	Value = 42
5	Respond to client with value	Send response	Client receives value 42
6	Query for key 'keyX' not in local store	Check local store for 'keyX'	Key found locally? No
7	Forward query to remote instances	Remote query sent	Remote instance returns value 100
8	Respond to client with remote value	Send response	Client receives value 100
9	End	All queries handled	App continues running

💡 Queries stop when data is found locally or remotely; app keeps running.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 7	Final
streams	null	initialized	running	running	running
counts-store	empty	updated with keys	contains key1=42	contains key1=42, keyX unknown	contains key1=42, keyX unknown
query key	none	none	key1	keyX	none
query result	none	none	42	100 (from remote)	none

Key Moments - 3 Insights

Why does the app check the local store first before asking other instances?

Because checking local store is faster and reduces network calls. See execution_table rows 3 and 6 where local check decides next step.

What happens if the key is not found locally or remotely?

The query returns null or empty result. The app tries remote queries after local miss (row 7), but if no instance has data, client gets no data.

How does the app keep the local state updated?

By continuously processing the input topic and updating the local store (row 2). This keeps data fresh for queries.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value returned for key 'key1' at step 4?

A 100

B 42

C null

D key1

💡 Hint

Check row 4 in execution_table where local store returns value 42.

At which step does the app forward the query to remote instances?

A Step 3

B Step 6

C Step 7

D Step 5

💡 Hint

Look at execution_table row 7 where remote query is sent after local miss.

If the local store had the key 'keyX', how would the execution_table change?

A Step 6 would return value locally, skipping steps 7 and 8

B Step 7 would still forward query to remote

C Step 4 would return null

D Step 3 would forward query to remote

💡 Hint

Refer to variable_tracker and execution_table rows 3-8 about local vs remote query handling.

Concept Snapshot

Interactive Queries in Kafka Streams:
- Streams app maintains local state stores.
- Queries first check local store for data.
- If data missing locally, query forwards to remote instances.
- Enables fast, scalable state access in distributed apps.
- Use InteractiveQueryService to access stores programmatically.

Full Transcript

This visual execution trace shows how Kafka Streams interactive queries work. The app starts and processes streams, updating a local state store. When a query arrives, it first checks the local store. If the key is found, it returns the value immediately. If not found, it forwards the query to other instances to find the data remotely. The app responds to the client with the found value. Variables like the streams app state, local store contents, query keys, and results change step-by-step. Key moments clarify why local checks happen first, what if data is missing, and how state updates. The quiz tests understanding of values returned, query forwarding steps, and how local data presence affects flow. The snapshot summarizes the concept for quick recall.