Table of Contents |
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Status
Current state: Under Accepted
Discussion thread: here
Discussion Voting thread: TBD 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.
- In general, the code have poor cohesion: we need to define predicates in one place, and respective stream processors in another place of code. In case of change we must remember to edit two pieces of code.
- If the number of predicates is predefined, this method forces us to use 'magic numbers' to extract the right branch from the result (see examples here).
- If we need to build branches dynamically (e. g. one branch per enum value) we inevitably have to deal with 'generic arrays' and 'unchecked typecasts'.
Public Interfaces
In accordance with KStreams DSL Grammar, we introduce the following new elements:
split
DSLOperationBranchedKStream
DSLObject with following DSLOperations:branch
defaultBranch
noDefaultBranch
Branched
DSLParameter.
The proposed class `KafkaStreamBrancher` introduces new standard way to build branches on top of KStream.
Description
1. The split(Named named)
operation returns 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.
The overloaded parameterless alternative split()
is also available.
2. BranchedKStream
has the following methods:
BranchedKStream<K,V> branch(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<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,V>> noDefaultBranch()
-- returns the dictionary of named KStreams.
Both branch
and defaultBranch
operations also have overloaded alternatives without the Branched
parameter.
3. Branched
parameter extends NamedOperation
and has the following 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 KStream<K, V>> chain)
— sets an operation with a given branch. By default, it is ans->s
identity function. Can be complex, likes->s.mapValues....
, a composition of functions etc.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.withConsumer(Consumer<? super KStream<K, V>> chain, 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.
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:
Code Block | ||
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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:
Code Block | ||
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| ||
source.split()
.branch(( |
...
Code Block | ||
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KStream<String, String> source_o365_user_activity = builder.stream("source"); KStream<String, String>[] branches = source_o365_user_activity.branch( (key, value) -> value.contains("A"), Branched.with(ks->ks.to("A"))) .branch((key, value) -> value.contains("B"), Branched.with(ks->ks.to("B"))) .defaultBranch(Branched.with(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:
Code Block | ||
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Map<String, KStream<String, String>> branches = source.split() .branch((key, value) -> value.contains("B"), (key, value) -> true ); branches[0].to("A"); branches[1].to("B"); branches[2].to("C"); |
...
== null,
Branched.with(s->s.mapValues(v->"NULL"), "null")
.defaultBranch(
Branched.as("non-null"));
branches.get("non-null")
.merge(branches.get("null")); |
Dynamic Branching
There is also a case when one might need to create branches dynamically, e. g. one per enum value. This can be implemented the following way:
Code Block | ||
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BranchedKStream branched = stream.split();
for (RecordType recordType : RecordType.values())
branched.branch((k, v) -> v.getRecType() == recordType,
Branched.with(recordType::processRecords)); |
This is why 'starting' split()
operation is necessary and it is better to have it rather than add new branch
method to KStream
directly.
Otherwise we should treat the first iteration separately, and the code for dynamic branching becomes cluttered:
Code Block | ||
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| ||
RecordType[] recordTypes = RecordType.values();
if (recordTypes.length != 0) {
BranchedKStream branched = stream.
branch((k, v) -> v.getRecType() == recordTypes[0],
Branched.with(recordType::processRecords));
for (int i = 1; i < recordTypes.length; i++)
branched.branch((k, v) -> v.getRecType() == recordTypes[i],
Branched.with(recordType::processRecords));
} |
Proposed Changes
- Add the following methods to
KStream
:
Code Block | ||
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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:
Code Block | ||
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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:
Code Block | ||
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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 https://github.com/apache/kafka/pull/6512 for 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:
Code Block | ||
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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")); |
(onTopOf method returns the provided stream so we can continue with method chaining and do something more with the original stream)
Public Interfaces
Add the new org.apache.kafka.streams.kstream.KafkaStreamsBrancher class (see https://github.com/apache/kafka/pull/6164).
Proposed Changes
Add the new org.apache.kafka.streams.kstream.KafkaStreamsBrancher class (see https://github.com/apache/kafka/pull/6164).
Compatibility, Deprecation, and Migration Plan
The proposed change has no impact on existing code and is backwards compatible. All the old code that uses branch method will continue to work, we will just get the new way to perform branching.
Rejected Alternatives
Rejected because of violation of method-chaining (new auxiliary object is needed).
2.
Code Block | ||
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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")); |
Here the new KStream#branch() method returns KBranchedStream<K, V> object, which, in turn, contains `branch` and `defaultBranch` methods. This is critical that KStream consumers in .branch methods should be invoked immediately during the `branch` methods invocation. This is necessary for the case when we need to gather the streams that were defined in separate scopes back into one scope using auxiliary object:
Code Block | ||
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@Setter
class CouponIssuer{
private KStream<....> coffePurchases;
private KStream<....> electronicsPurchases;
KStream<...> coupons(){
return coffePurchases.join(electronicsPurchases...)...
}
}
CouponIssuer couponIssuer = new CouponIssuer();
transactionStream.branch()
.branch(predicate1, couponIssuer::setCoffePurchases)
.branch(predicate2, couponIssuer::setElectronicsPurchases);
KStream<..> coupons = couponIssuer.coupons();
|
This was rejected because of the difficulty of having branches in the same scope.No