Filter and map operations in Kafka - Step-by-Step Execution

Process Flow - Filter and map operations

Start with stream of messages

↓

Apply filter condition

↓

Keep message

↓

Apply map function

↓

Output transformed messages

Messages flow through a filter to keep only those matching a condition, then a map transforms each kept message.

Execution Sample

Kafka

stream.filter((key, value) -> value.toLowerCase().contains("error"))
      .mapValues(value -> value.toUpperCase())
      .foreach((key, value) -> System.out.println(value));

This code filters messages containing 'error' and converts their values to uppercase before printing.

Process Table

Step	Message Value	Filter Condition (contains 'error')	Filter Result	Map Operation (toUpperCase)	Output
1	System started	false	Discard	-	-
2	Error detected	true	Keep	ERROR DETECTED	ERROR DETECTED
3	User login	false	Discard	-	-
4	Critical error	true	Keep	CRITICAL ERROR	CRITICAL ERROR
5	Shutdown	false	Discard	-	-

💡 All messages processed; only those with 'error' passed filter and were mapped.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Message	-	System started	Error detected	User login	Critical error	Shutdown	-
Filter Result	-	false	true	false	true	false	-
Mapped Value	-	-	ERROR DETECTED	-	CRITICAL ERROR	-	-
Output	-	-	ERROR DETECTED	-	CRITICAL ERROR	-	-

Key Moments - 2 Insights

Why are some messages not transformed or printed?

Messages failing the filter condition are discarded early (see rows 1,3,5 in execution_table), so map and output steps do not run for them.

Does the map operation change the original message?

No, map creates a new transformed value (uppercase string) without modifying the original message, as shown in the 'Mapped Value' column.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 4?

A Critical error

B CRITICAL ERROR

C critical error

D -

💡 Hint

Check the 'Output' column at step 4 in the execution_table.

At which step does the filter condition become false for the first time?

A Step 3

B Step 2

C Step 1

D Step 5

💡 Hint

Look at the 'Filter Result' column in execution_table for the earliest 'false'.

If the filter condition changed to check for 'warning' instead of 'error', which message would be kept at step 2?

A -

B System started

C User login

D Error detected

💡 Hint

Refer to the original message values and see if 'warning' appears in step 2 message.

Concept Snapshot

Filter and map in Kafka streams:
- filter(condition): keeps messages matching condition
- map(function): transforms each kept message
- Order matters: filter first, then map
- Messages failing filter are dropped
- Map creates new transformed output

Full Transcript

In Kafka streams, messages flow through operations like filter and map. First, filter checks each message's value to keep only those containing 'error'. Messages not matching are discarded immediately. Then, map transforms each kept message by converting its value to uppercase. The output shows only transformed messages that passed the filter. This step-by-step trace shows how messages are processed, filtered, transformed, and output. Understanding this flow helps beginners see why some messages are dropped and how transformations apply only after filtering.