Status

Current state: Under Accepted

Discussion thread: here

Discussion Voting thread: here

JIRA:

Pull request: PR-9107

Motivation

KStream#branch method uses varargs to supply predicates and returns array of streams ('Each stream in the result array corresponds position-wise (index) to the predicate in the supplied predicates').

This is poor API design that makes building branches very inconvenient because of 'impedance mismatch' between arrays and generics in Java language.

...

• as(String name) -- sets the name of the branch (auto-generated by default, when split operation is named, then the names are index-suffixed).
• withwithFunction(Function<? super KStream<? super K, ? super KStream<K, V>, ? extends KStream<? extends K, ? extends KStream<K, V>> chain) -- — sets an operation with a given branch. By default, it is an s->s identity function. Can be complex, like s->s.mapValues...., a composition of functions etc.
• withwithConsumer(Function<Consumer<? super KStream<K, V>> chain) — sets a consumer for a given branch.
• withFunction(Function< KStream<? super KKStream<K, ? super V>, ? extends KStream<? extends K, ? extends KStream<K, V>> chain, String name) — sets both an operation and a name.
• withConsumer(Consumer<? super KStream<K, V>> chain, String name) -- — sets both operation a consumer and a name.

The Map returned by defaultBranch/noDefaultBranch allows us to collect all the KStream branch objects in a single scope. The branches collected are the results of transformations defined by `chain` functions. If a function returns `null`, its result is omitted.

How the resulting Map is formed

The keys of the Map entries are defined by the following rules:

• If Named parameter was provided for split , its value is used as a prefix for each key. By default, no prefix is used
• If a name is provided for the branch, its value is appended to the prefix to form the Map key
• If a name is not provided for the branch, then the key defaults to prefix + position of the branch as a decimal number, starting from "1"
• If a name is not provided for the defaultBranch call, then the key defaults to prefix + "0"

The values of the Map entries are formed as following:

• If no chain function or consumer is provided, then the value is the branch itself (which is equivalent to ks→ks identity chain function)
• If a chain function is provided and returns a non-null value for a given branch, then the value is the result returned by this function
• If a chain function returns null for a given branch, then the respective entry is not put to the map
• If a consumer is provided for a given branch, then the the respective entry is not put to the map

For example:

var result = 
   source.split(Named.as("foo-"))
  .branch(predicate1, Branched.as("bar"))            // "foo-bar"
  .branch(predicate2, Branched.with(ks->ks.to("A"))  // no entry: a Consumer is provided
  .branch(predicate3, Branched.with(ks->null))       // no entry: chain function returns null
  .branch(predicate4)                                // "foo-4": name defaults to the branch position
  .defaultBranch()                                   // "foo-0": "0" is the default name for the default branch

Usage Examples

The following section demonstrates some standard use cases for the proposed API

Simple Example: Direct Branch Consuming

...

source.split()
.branch((key, value) -> value.contains("A"), Branched.with(ks->{ks>ks.to("A"); return ks;}))
.branch((key, value) -> value.contains("B"), Branched.with(ks->{ks>ks.to("B"); return ks;}))
.defaultBranch(Branched.withJavaConsumerwith(ks->{ks>ks.to("C"); return ks;}));

More Complex Example: Merging Branches

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class Branched<K, V> implements Named<Branched<K,V>> {
    static Branched<K, V> as(String name);
    static Branched<K, V> withwithFunction(Function<? super KStream<? super KKStream<K, V>, ? superextends V>, ? extends KStream<? extends K, ? extendsKStream<K, V>> chain);
    static Branched<K, V> withConsumer(Consumer<? super KStream<K, V>> chain);
    static Branched<K, V> withwithFunction(Function<? super KStream<K, V>, KStream<? superextends KKStream<K, V>> ?chain, super V>, ? extends KStream<? extends K, ? extendsString name);
    static Branched<K, V> withConsumer(Consumer<? super KStream<K, V>> chain, String name);
}

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