## Introduction Kafka Streams is a Java library for building stream processing applications with Apache Kafka. It can be used or embedded within Java applications to process streaming data in Kafka topics. It's a standalone library that only depends on Kafka and uses it as the foundation for high availability and reliability. Kafka Streams has two types of APIs: 1. Streams DSL - A high-level API 2. Processor - A low-level API This guide covers the high-level Streams DSL API, which provides a functional programming model to write stream processing topologies concisely with a few lines of code. The Streams DSL API offers many abstractions such as `KStreams`, `KTable` etc. One way of breaking it down is to categorize the functionality offered by these APIs as follows: 1. Stateless operations 2. Stateful operations This guide will include code examples to demonstrate stateless operations such as `map` and `filter`, and stateful computations like `aggregate` and `count`. ## Kafka Streams Stateless functions The following `KStream` methods have been covered in this section: * map * filter * groupBy and groupByKey * through and to * print and peek * merge ### map `map` can transform individual records into a `KStream` by applying a function. It can be used to transform both key and value. If you only want to transform the value, use `mapValues` method. The `flatMap` method can return multiple records (`KeyValue`). Let's look at a few examples. `map` can be used to convert the key and value of each `KStream` record to lowercase `String`: KStream words = builder.stream("words"); words.map(new KeyValueMapper>() { @Override public KeyValue apply(String key, String val) { return new KeyValue<>(key.toLowerCase(), val.toLowerCase()); } }); Or, just use `mapValues` to work with values only: words.mapValues(new ValueMapper() { @Override public String apply(String val) { return val.toLowerCase(); } }); Using `flatMapValues`, you can further break down a value into a collection of values. stream.flatMap(new KeyValueMapper>>() { @Override public Iterable> apply(String key, String val) { String[] values = val.split(","); return Arrays.asList(values) .stream() .map(value -> new KeyValue<>(key, value)) .collect(Collectors.toList()); } }) ### filter Generally, the `filter` method only includes records in a `KStream` that fulfill a specific criterion. It is complemented by the `filterNot` method that is used to exclude records. In both cases, the filtration criterion is defined using a `Predicate` object. For example, to only process transactions of a specific credit card type: KStream transactions = builder.stream("user-transactions"); transactions.filter(new Predicate() { @Override public boolean test(String userID, Transaction tx) { return tx.cardType().equals("VISA"); } }) To ignore/exclude all users who haven't set their password: KStream users = builder.stream("users"); users.filterNot(new Predicate() { @Override public boolean test(String userID, User user) { return user.isPwdSet(); } }) ### group Grouping operations are often used to convert the contents of a `KStream` to a `KGroupedStream` to perform stateful computations (covered later in this guide). This can be achieved using `groupByKey` or a more generic `group` method. While using `groupByKey` is straightforward, note that a `KeyValueMapper` can be used with `groupBy` to use a different key. For example, you can use it to group user transactions based on card type: KStream transactions = builder.stream("transactions"); KGroupedStream grouped = transactions.groupBy(new KeyValueMapper() { @Override public String apply(String txID, User user) { return user.getCardType(); } }); ### to and through The `to` method is different from some of the other operations you've encountered so far. It's quite simple, yet very powerful since it lets us materialize (store) `KStream` records to another topic in Kafka - just with a simple method call! It returns a `void` result instead of a `KStream` (or `KTable`) - such types of operations are also known as *Terminal* methods. In this example, all the lowercase words are sent to a topic named `lowercase-words` after being transformed using `mapValues` operation (which was covered earlier). KStream words = builder.stream("words"); words.mapValues(new ValueMapper() { @Override public String apply(String val) { return val.toLowerCase(); } }) .to("lowercase-words"); `through` is another simple yet powerful operation that is often used to complement the `to` method while building streaming pipelines. To continue the above example - Say, after storing all the lowercase words in a topic, you need to remove all the words that have a specific character (e.g. a hyphen `-`) and store the final results in another Kafka topic. Instead of using `to` method and creating a new `KStream` from the topic `lowercase-words`, it's possible to simplify the code like this: KStream words = builder.stream("words"); words.mapValues(new ValueMapper() { @Override public String apply(String val) { return val.toLowerCase(); } }) .through("lowercase-words") .filter(new Predicate() { @Override public boolean test(String k, String v) { return v.contains("-"); } }) .to("processed-words"); ### print and peek If you want to log the `KStream` records (for debugging purposes), `print` is a handy method (it's also a terminal operation, just like `to`). It's also possible to configure the behavior of this method using a `Printed` object (that `print` accepts). For example, to log the values of a `KStream` to the standard out terminal: KStream words = builder.stream("words"); words.mapValues(new ValueMapper() { @Override public String apply(String val) { return val.toLowerCase(); } }) .print(Printed.withLabel("demo").toSysOut()); The `peek` method is similar to `print` in terms of functionality, but it's *not* a terminal operation. Instead, it allows the caller to use a `ForeachAction` to define the specific action and returns the same `KStream` instance. In this example, we simply log the key and value to standard out: KStream words = builder.stream("words"); words.mapValues(new ValueMapper() { @Override public String apply(String val) { return val.toLowerCase(); } }) .peek(new ForeachAction() { @Override public void apply(String k, String v) { System.out.println("key is "+k+", value is "+v); } }) .to("lowercase-words"); ### merge If you have two streams and need to combine them, use `merge`. KStream fte = builder.stream("fte"); KStream contractor = builder.stream("contractors"); fte.merge(contractor).to("all-employees"); ## Kafka Streams Stateful functions This section will cover aggregation operations (`aggregate`, `count` and `reduce`) along with an overview of Windowing in Kafka Streams. A side-effect of all these operations is "state" (hence the name *Stateful* operations), and it's important to understand where it's stored and how it's managed. The state associated with these operations is stored in local "state stores" - either in-memory or on disk. The "data locality" makes the processing much more efficient. You can also configure your application such that this state store data is also sent Kafka topics. This is important for high availability and fault tolerance since the data can be restored from Kafka in case of application issues or crashes. Let's go over some of these stateful operations. ### count `KGroupedStream` supports this operation. It makes it convenient to count the number of records of a specific key by using this operation using a single method. Continuing with the `groupBy` example presented earlier. Once we group the transactions by card type, we can simply use `count` to get the number of transactions for each card type. KStream transactions = builder.stream("transactions"); KGroupedStream grouped = transactions.groupBy(new KeyValueMapper() { @Override public String apply(String txID, User user) { return user.getCardType(); } }); KTable txPerCardType = grouped.count(); In order to store this state (count) locally, count accepts an instance of `Materialized`, which can be used as follows: KTable txPerCardType = grouped.count(Materialized.as("tx-per-card-type")); ### aggregate `aggregate` comes in handy when executing calculations like moving averages over a streaming data set. This requires the state to be handled and has to take into account the current value and the computed aggregate's current value. A good way to understand `aggregate` is to actually use it to implement `count` operation. When the first record is received, the `Initializer` is used to initialize the state (in this example, the count is set to zero) and invoked with the first record. After that, the `Aggregator` takes over - In this example, whenever a record is received, the current count is incremented by one. KStream stream = builder.stream("transactions"); KTable aggregate = stream.groupByKey() .aggregate(new Initializer() { @Override public Result apply() { return new Result("", 0); } }, new Aggregator() { @Override public Result apply(String k, String v, Result count) { Integer currentCount = count.getCount(); return new Result(k, currentCount + 1); } }); ### reduce `reduce` operation can be used to combine streams of values and implement `sum`, `min`, `max` etc. You can think of `aggregate` operation as a generic version of `reduce`. ## Windowing with Kafka Streams For example, a common requirement for website analytics is to have metrics about the number of unique page views per hour, clicks per minute, etc. `Windowing` lets you confine the stream processing operations to execute within a time range. Supported time windows include: sliding, tumbling, hopping, and session-based time windows. For counting unique page views per hour, you can use a *tumbling* time window of 60 minutes. Thus, page views for a product from 1 PM to 2 PM will be aggregated and a fresh time block will start after that. Here is an example of how you might achieve this: KStream views = builder.stream("product-views"); views.groupByKey() .windowedBy(SessionWindows.with(Duration.ofMinutes(60))) .toStream() .to("views-per-hour"); ## Conclusion This guide provided an introduction to Kafka Streams and the type of APIs. This was followed by coverage of commonly used Stateless and Stateful operations, along with examples. You can refer to the [Kafka Streams Javadocs](https://kafka.apache.org/24/javadoc/index.html?org/apache/kafka/streams/) and [Kafka documentation](https://kafka.apache.org/documentation/streams/) for further reading.

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