Stream topology in Kafka - Step-by-Step Execution

Process Flow - Stream topology

Define Source Topic

↓

Create Stream from Source

↓

Apply Transformations

↓

Send to Sink Topic

↓

Stream Processing Continues

A stream topology defines how data flows from source topics through processing steps to sink topics in Kafka Streams.

Execution Sample

Kafka

StreamsBuilder builder = new StreamsBuilder();
KStream<String, String> source = builder.stream("source-topic");
KStream<String, String> transformed = source.mapValues(value -> value.toUpperCase());
transformed.to("sink-topic");

This code creates a stream from 'source-topic', converts each message value to uppercase, and writes results to 'sink-topic'.

Process Table

Step	Action	Input Message	Transformation	Output Message	Topic
1	Read message	hello	none	hello	source-topic
2	Apply mapValues	hello	toUpperCase	HELLO	in-memory stream
3	Write message	HELLO	none	HELLO	sink-topic
4	Read message	world	none	world	source-topic
5	Apply mapValues	world	toUpperCase	WORLD	in-memory stream
6	Write message	WORLD	none	WORLD	sink-topic
7	No more messages	-	-	-	-

💡 No more messages in source-topic, stream processing ends.

Status Tracker

Variable	Start	After 1	After 2	Final
source message	-	hello	world	-
transformed message	-	HELLO	WORLD	-

Key Moments - 2 Insights

Why does the transformed message appear after the source message in the execution table?

Because the stream reads the source message first (rows 1 and 4), then applies the transformation (rows 2 and 5), and finally writes the transformed message to the sink topic (rows 3 and 6). This order shows the flow of data through the topology.

What happens if there are no messages in the source topic?

The execution stops at step 7 as shown in the exit note, because the stream topology processes messages only when they arrive from the source topic.

Visual Quiz - 3 Questions

Look at the execution table, what is the output message at step 3?

A hello

B HELLO

C world

D WORLD

💡 Hint

Check the 'Output Message' column at step 3 in the execution_table.

At which step does the transformation to uppercase happen for the message 'world'?

A Step 6

B Step 4

C Step 5

D Step 7

💡 Hint

Look for the 'Transformation' column where 'toUpperCase' is applied for 'world' in the execution_table.

If the source topic had no messages, what would the execution table show?

A Only step 7 indicating no messages

B Steps 1 to 6 with empty messages

C Only steps 1 and 2

D All steps repeated infinitely

💡 Hint

Refer to the exit_note and step 7 in the execution_table.

Concept Snapshot

Stream topology in Kafka Streams:
- Define source topic stream
- Apply transformations (map, filter, etc.)
- Write results to sink topic
- Data flows step-by-step through topology
- Processing happens as messages arrive

Full Transcript

This visual execution shows how a Kafka Streams topology processes messages. First, messages are read from the source topic. Then, each message is transformed, here by converting the value to uppercase. Finally, the transformed message is written to the sink topic. The execution table traces each step for two example messages, 'hello' and 'world'. Variables track the message values before and after transformation. Key moments clarify why the order of steps matters and what happens if no messages arrive. The quiz tests understanding of the flow and output at each step.