Table of Contents |
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Status
Current state: Under Accepted
Discussion thread: here
Discussion Voting thread: here
JIRA: Jira server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key KAFKA-5488
Jira server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution
Motivation
serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key KAFKA-8296
Pull request: PR-9107
Motivation
KStream#branch
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.
...
1. The split(Named named)
operation returns BranchedKStream
BranchedKStream<K,V>
. Named parameter is needed so one can name the branch operator itself, and then all the branches might get index-suffixed names built from the branch operator name.
...
2. BranchedKStream
has the following methods:
BranchedKStream BranchedKStream<K,V> branch(Predicate<K,Predicate<? super K, ? super V> predicate, Branched<K,V> branched)
-- creates a branch for messages that match the predicate and returnsthis
in order to facilitate method chaining.Map<String, KStream> KStream<K,V>> defaultBranch(Branched<K,V> branched)
-- creates a default branch (for messages not intercepted by other branches) and returns the dictionary of named KStreams.Map<String, KStream<K, KStream> V>> noDefaultBranch()
-- returns the dictionary of named KStreams.
Both branch
and defaultBranch
operations also have overloaded parameterless alternatives without the Branched
parameter.
3. Branched
parameter extends NamedOperation
and has the following builder methods:static methods:
as(String name)
-- sets the name of the branch (auto-generated by default, whensplit
operation is named, then the names are index-suffixed).withFunction(Function<? super KStream<K, V>, ? extends withChain(Function<KStream<K, V>, KStream<K, V>> chain)
-- a chain of operations — sets an operation with a given branch. By default, it is ans->s
identity function. Can be complex, likes->s.mapValues....
, some a composition of functions etc.withName(String name)
-- sets the name of the branch (auto-generated by default, whensplit
operation is named, then the names are index-suffixed).withConsumer(Consumer<? super KStream<K, V>> chain)
— sets a consumer for a given branch.withFunction(Function<? super KStream<K, V>, ? extends KStream<K, V>> chain, String name)
— sets both an operation and a name.withConsumerwithJavaConsumer(Consumer<? super KStream<K, V>> consumer)
-- directs the final result of chain to a given java.util.function.Consumer. (The method is called withJavaConsumer in order not to confuse it with Kafka Consumer interface). By default, a NO-OP consumer is providedchain, String name)
— sets both 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 `withChain` functions.
...
How the resulting Map is formed
The keys of the Map entries are defined by the following rules:
- If
Named
parameter was provided forsplit
, its value is used as a prefix for each key. By default, no prefix is used - If a
name
is provided for thebranch
, 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 thedefaultBranch
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:
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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
In many cases we do not need to have a single scope for all the branches, each branch being processed completely independently from others. Then we can use 'consuming' lambdas or method references in Branched
parameter:
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source.split() .branch((key, value) -> value.contains("A"), Branched.withJavaConsumerwith(ks->ks.to("A"))) .branch((key, value) -> value.contains("B"), Branched.withJavaConsumerwith(ks->ks.to("B"))) .defaultBranch(Branched.withJavaConsumerwith(ks->ks.to("C"))); |
More Complex Example: Merging Branches
In other cases we want to combine branches again after splitting. The map returned by defaultBranch
/noDefaultBranch
methods provides access to the branches in the same scope:
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Map<String, KStream<String, String>> branches = source.split() .branch((key, value) -> value == null, Branched.withName("null").withChain(with(s->s.mapValues(v->"NULL"), "null") .defaultBranch( Branched.withNameas("non-null")); branches.get("non-null") .merge(branches.get("null")); |
...
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BranchedKStream branched = stream.split(); for (RecordType recordType : RecordType.values()) branched.branch((k, v) -> v.getRecType() == recordType, Branched.withJavaConsumerwith(recordType::processRecords)); |
...
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RecordType[] recordTypes = RecordType.values(); if (recordTypes.length != 0) { BranchedKStream branched = stream. branch((k, v) -> v.getRecType() == recordTypes[0], Branched.withJavaConsumerwith(recordType::processRecords)); for (int i = 1; i < recordTypes.length; i++) branched.branch((k, v) -> v.getRecType() == recordTyperecordTypes[i], Branched.withJavaConsumerwith(recordType::processRecords)); } |
Proposed Changes
- Add the following methods to
KStream
:
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BranchedKStream<K,V> split();
BranchedKStream<K,V> split(Named n); |
2. Deprecate the existing KStream#branch
method.
3. Add and implement the following Branched class:
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class Branched<K, V> implements Named<Branched<K,V>> {
static Branched<K, V> as(String name);
static Branched<K, V> withFunction(Function<? super KStream<K, V>, ? extends KStream<K, V>> chain);
static Branched<K, V> withConsumer(Consumer<? super KStream<K, V>> chain);
static Branched<K, V> withFunction(Function<? super KStream<K, V>, ? extends KStream<K, V>> chain, String name);
static Branched<K, V> withConsumer(Consumer<? super KStream<K, V>> chain, String name);
} |
Add and implement the following BranchedKStream interface:
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interface BranchedKStream<K, V> {
BranchedKStream<K, V> branch(Predicate<? super K, ? super V> predicate);
BranchedKStream<K, V> branch(Predicate<? super K, ? super V> predicate, Branched<K, V> branched);
Map<String, KStream<K, V>> defaultBranch(Branched<K, V> branched);
Map<String, KStream<K, V>> defaultBranch();
Map<String, KStream<K, V>> noDefaultBranch();
} |
(See new BranchedKStream class and branch() method for KStream (see https://github.com/apache/kafka/pull/6512for a very rough draft).
Compatibility, Deprecation, and Migration Plan
The proposed change is backwards compatible.
The old KStreams#branch method should be deprecated.
Rejected Alternatives
- A KStreamsBrancher class that works the same way, but does not require KStream interface modification:
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new KafkaStreamsBrancher<String, String>() .branch((key, value) -> value.contains("A"), ks->ks.to("A")) .branch((key, value) -> value.contains("B"), ks->ks.to("B")) //default branch should not necessarily be defined in the end! .defaultBranch(ks->ks.to("C")) .onTopOf(builder.stream("source")); |
Rejected because of validation violation of method-chaining (new auxiliary object is needed).
2.
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source .split() .branch((key, value) -> value.contains("A"), ks->ks.to("A")) .branch((key, value) -> value.contains("B"), ks->ks.to("B")) .defaultBranch(ks->ks.to("C")); |
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