Order

This article briefly introduces the join operation of kafka streams.

join

A join operation merges two streams based on the keys of their data records, and yields a new stream. A join over record streams usually needs to be performed on a windowing basis because otherwise the number of records that mu st be maintained for performing the join may grow indefinitely.

Example

        KStreamBuilder builder = new KStreamBuilder();
        KStream<String, String> left = builder.stream("intpu-left");
        KStream<String, String> right = builder.stream("intpu-right");

        KStream<String, String> all = left.selectKey((key, value) -> value.split(",")[1])
                .join(right.selectKey((key, value) -> value.split(",")[0]), new ValueJoiner<String, String, String>() {
            @Override
            public String apply(String value1, String value2) {
                return value1 + "--" + value2;
            }
        }, JoinWindows.of(30000));

        all.print();

Since the join operation is based on the key, it is usually necessary to map the key again.

test

sh bin/kafka-topics.sh --create --topic intpu-left --replication-factor 1 --partitions 3 --zookeeper localhost:2181

sh bin/kafka-topics.sh --create --topic intpu-right --replication-factor 1 --partitions 3 --zookeeper localhost:2181


sh bin/kafka-console-producer.sh --broker-list localhost:9092 --topic intpu-left
sh bin/kafka-console-producer.sh --broker-list localhost:9092 --topic intpu-right

Left input such as

1,a
2,b
3,c
3,c
4,d
1,a
2,b
3,c
1,a
2,b
3,c
4,e
5,h
6,f
7,g

Right input such as

a,hello
b,world
c,hehehe
c,aaa
d,eee
a,cccc
b,aaaaaa
c,332435
a,dddd
b,2324
c,ddddd
e,23453
h,2222222
f,0o0o0o0
g,ssss

Output instance

[KSTREAM-MERGE-0000000014]: a , 1,a--a,dddd
[KSTREAM-MERGE-0000000014]: b , 2,b--b,2324
2017-10-17 22:17:34.578  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing all tasks because the commit interval 30000ms has elapsed
2017-10-17 22:17:34.578  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 0_0
2017-10-17 22:17:34.585  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 0_1

Join category

We use inner join, left join and outer join here. If there is no matching record in the time window, you can use outer join to store it additionally, and then filter it again according to the matching record.

Output instance

[KSTREAM-MERGE-0000000014]: f , null--f,ddddddd
[KSTREAM-MERGE-0000000014]: f , 4,f--f,ddddddd
2017-10-17 22:31:12.530  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing all tasks because the commit interval 30000ms has elapsed
2017-10-17 22:31:12.530  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 0_0
2017-10-17 22:31:12.531  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 0_1
2017-10-17 22:31:12.531  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 1_0
2017-10-17 22:31:12.531  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 0_2
2017-10-17 22:31:12.533  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 1_1
2017-10-17 22:31:12.533  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 2_0
2017-10-17 22:31:12.539  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 1_2
2017-10-17 22:31:12.540  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 2_1
2017-10-17 22:31:12.541  INFO   --- [ StreamThread-1] o.a.k.s.p.internals.StreamThread         : stream-thread [StreamThread-1] Committing task StreamTask 2_2
[KSTREAM-MERGE-0000000014]: g , 5,g--null
[KSTREAM-MERGE-0000000014]: h , 6,h--null
[KSTREAM-MERGE-0000000014]: h , 6,h--h,ddddddd

Summary

Kafka streams’ join operation is very suitable for real-time matching operation of different data sources.