Map

As explained in Apache Kafka in Action

map is a stream-processing function that transforms each input record into a new record or value.

A map processor transforms every input message into a new message. Its function receives the original key and value and returns a new key and value, making it suitable for reshaping raw stream data or changing record identity. Use mapValues() instead when only the value changes; preserving the key avoids the repartitioning and performance cost that often follows key changes. section 12.2.1

Choose the processor according to the part of the record that must change:

• Use map() to transform both fields, such as turning an unkeyed value {id: "Apple", price: 1.50} into key Apple and value 1.50. section 12.2.1
• Use mapValues() to convert, mask, or transform values while retaining the key, such as converting prices from dollars to cents. section 12.2.1
• Place transformed records into another Kafka topic with to(topicName), forming part of a larger processor topology. section 12.2.1 section 12.7

Creating a key from a raw price record

Starting with a value such as {id: "Apple", price: 1.50} and no key, map((key, value) -> (value.id, value.price)) produces a record keyed by Apple with value 1.50. If the key already identifies the record and only the price needs conversion, mapValues((key, value) -> value.price * 100) changes the value without triggering a key change. section 12.2.1

Figure 12.2 The filter and map processors

Use the processor diagram to compare filter(), which selects records, with map(), which transforms their key-value representation. section 12.2.1

How Apache Kafka in Action covers it

How deep it goes

Defines input/output signatures, contrasts mapValues(), and explains repartitioning-related performance loss without detailing implementation internals.

Examples

Demonstrates dollar-to-cent conversion with mapValues() and key creation from an unkeyed Apple price record.

What you can do after

Apply it via Kafka Streams to transform records, create keys, and write the resulting stream to a topic.

How it connects

Connects to filter(), mapValues(), processor topologies, repartitioning, Kafka Streams, and Flink.

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Also explained in

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  Spring in Action, Sixth Editionmap is a Project Reactor operator that transforms each value emitted by a reactive publisher into another value while preserving the publisher type.
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  Learn Haskell by Examplemap is a standard Haskell function that applies a function independently to every element of a list and returns the resulting list.
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  Clojure: The Essential Referencemap is a Clojure function that applies a function to corresponding elements of one or more collections and returns a lazy sequence of results.
• 

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  D3.js in Action, Third EditionThe JavaScript map() method creates a new array by applying a callback to every element of an input array.
• 

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  Secrets of the JavaScript Ninja, Second EditionAn Array method that creates a new array by applying a callback to each element.
• 

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  F# in Action: A Revised Edition of Get Programming with F#A map operation applies a function to a value inside a computational effect and rewraps the result in that effect.
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  Fabulous Adventures in Data Structures and AlgorithmsA monadic map operation applies a function to values inside a monadic computational context while preserving that context's structure.

Compare how 8 books explain map →

Related concepts

• arrayproduces · An array is an indexed, ordered collection of elements arranged in one or more dimensions, with elements accessed by position.
• JavaScriptused by · JavaScript is a high-level, dynamically typed, multi-paradigm programming language standardized as ECMAScript and used for web and general-purpose programming.
• functional, stateless philosophyuses · Functional programming is a programming paradigm that expresses computation through the evaluation and composition of functions, typically favoring immutable data and minimizing mutable state and side effects.
• pipe methodused by · The RxJS pipe method composes operators to transform or otherwise process values emitted by an Observable.
• map-indexedalternative to · map-indexed is a Clojure function that applies a function to each collection element together with its index and returns the results as a lazy sequence or transducer output.
• transducerproduces · A transducer is a composable transformation that processes inputs independently of the context in which the transformation is consumed.

This explainer was generated by Manning's knowledge graph from the text of Apache Kafka in Action. The section links point to the parts of the book it draws on.