Adventures with Reactive Streams @NiteshKant Nitesh Kant @jovoordeckers Jo Voordeckers 

Image source: https://interestingengineering.com/12-of-the-worlds-most-fascinating-dams 

Image source: https://en.wikipedia.org/wiki/Oroville_Dam_crisis 

@NiteshKant Nitesh Kant @jovoordeckers Jo Voordeckers 

Find my friends GET /friends?id=3 HTTP/1.1 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … 

HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() Find my friends getFriendsCsv() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … Producer Consumer HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() 

HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … Producer Consumer Control response.send() 

I am interested in consuming data! Control 

I am interested in consuming data! Give me 10 items! Control 

I am interested in consuming data! Give me 10 items! I am not interested any more. Control 

Java Message Service https://docs.oracle.com/javaee/5/tutorial/doc/bncdx.html // Synchronous Message m = consumer.receive(); Message m = consumer.receive(1000); // time out 1000ms // Asynchronous MessageListener myListener = new MyListener(); // void onMessage(Message message) consumer.setMessageListener(myListener); request(1) 

Java Message Service https://docs.oracle.com/javaee/5/tutorial/doc/bncdx.html // Synchronous Message m = consumer.receive(); Message m = consumer.receive(1000); // time out 1000ms // Asynchronous MessageListener myListener = new MyListener(); // void onMessage(Message message) consumer.setMessageListener(myListener); flow control depends on broker 

Apache Kafka https://kafka.apache.org/10/javadoc/?org/apache/kafka/clients/consumer/KafkaConsumer.html KafkaConsumer consumer = new KafkaConsumer<>(props); consumer.subscribe(Arrays.asList("foo", "bar")); while (true) { ConsumerRecords records = consumer.poll(100); // time out for (ConsumerRecord record : records) { // iterate over batch } consumer.commitSync(); } // Flow control public void pause(java.util.Collection partitions) public void resume(java.util.Collection partitions) request(n) 

Apache Kafka https://kafka.apache.org/10/javadoc/?org/apache/kafka/clients/consumer/KafkaConsumer.html KafkaConsumer consumer = new KafkaConsumer<>(props); consumer.subscribe(Arrays.asList("foo", "bar")); while (true) { ConsumerRecords records = consumer.poll(100); // time out for (ConsumerRecord record : records) { // iterate over batch } consumer.commitSync(); } // Flow control public void pause(java.util.Collection partitions) public void resume(java.util.Collection partitions) additional flow control by topic/partition 

Apache Kafka JMS // Synchronous Message m = consumer.receive(); Message m = consumer.receive(1000); // Asynchronous // void onMessage(Message message) MessageListener myListener = new MyListener(); consumer.setMessageListener(myListener); KafkaConsumer consumer = new KafkaConsumer<>(props); consumer.subscribe(Arrays.asList("foo", "bar")); while (true) { ConsumerRecords records = consumer.poll(100); for (ConsumerRecord record : records) { // iterate over batch } consumer.commitSync(); } // Flow control public void pause(java.util.Collection partitions) public void resume(java.util.Collection partitions) 

Apache Kafka JMS // Synchronous Message m = consumer.receive(); Message m = consumer.receive(1000); // Asynchronous // void onMessage(Message message) MessageListener myListener = new MyListener(); consumer.setMessageListener(myListener); KafkaConsumer consumer = new KafkaConsumer<>(props); consumer.subscribe(Arrays.asList("foo", "bar")); while (true) { ConsumerRecords records = consumer.poll(100); for (ConsumerRecord record : records) { // iterate over batch } consumer.commitSync(); } // Flow control public void pause(java.util.Collection partitions) public void resume(java.util.Collection partitions) how do we bridge these APIs? 

Service A Service B 

Service A Service B cache ? 

Service A Service B cache Service C ? 

Service A Service B cache Service C DB ? 

Service A Service B cache Service C DB ? define contract for interoperability 

Service A Service B cache Service C DB write adapters for all the things \o/ ? 

Service A Service B cache Service C DB write adapters for all the things \o/ ? ✓ cancelation ✓ buffering 

https://github.com/reactive-streams/reactive-streams-jvm 

https://github.com/reactive-streams/reactive-streams-jvm 

Reactive Streams public interface Publisher { public void subscribe(Subscriber s); } 

Reactive Streams public interface Publisher { public void subscribe(Subscriber s); } I am interested in consuming data! 

Reactive Streams public interface Subscriber { public void onSubscribe(Subscription s); public void onNext(T t); public void onError(Throwable t); public void onComplete(); } 

Reactive Streams public interface Subscriber { public void onSubscribe(Subscription s); public void onNext(T t); public void onError(Throwable t); public void onComplete(); } Control exchange 

Reactive Streams public interface Subscription { public void request(long n); public void cancel(); } 

Reactive Streams public interface Subscription { public void request(long n); public void cancel(); } Give me “n” items! 

Reactive Streams public interface Subscription { public void request(long n); public void cancel(); } I am not interested any more. 

under-specification Image source: https://newrepublic.com/article/118416/what-dhaka-bangladesh-traffic-capital-world-can-teach-us 

under-specification risk of chaos between actors of the interaction rules Image source: https://newrepublic.com/article/118416/what-dhaka-bangladesh-traffic-capital-world-can-teach-us 

under-specification risk of chaos between actors target least common denominator of the interaction rules Image source: https://newrepublic.com/article/118416/what-dhaka-bangladesh-traffic-capital-world-can-teach-us 

under-specification risk of chaos between actors target least common denominator systemic inefficiency of the interaction rules Image source: https://newrepublic.com/article/118416/what-dhaka-bangladesh-traffic-capital-world-can-teach-us 

subscribe() Publisher Subscriber 

onSubscribe() subscribe() Publisher Subscription onSubscribe() Subscriber 

request() onSubscribe() subscribe() Publisher Subscription onSubscribe() Subscriber 

onNext() onComplete() onError() onSubscribe() subscribe() Publisher Subscription onNext() onComplete() onError() onSubscribe() Subscriber request() 

sequence onSubscribe() onComplete() subscribe() request() onSubscribe() subscribe() onError() request() onSubscribe() subscribe() onNext() onError() request() onSubscribe() subscribe() onNext() onNext() onComplete() request() request() cancel() onSubscribe() subscribe() request() cancel() onNext() onComplete() onSubscribe() subscribe() 

lifecycle management onNext() X subscribe() cancel() request() subscribe() onSubscribe() onNext() request() X onNext() onSubscribe() onNext() request() onComplete() X onError() subscribe() X 

How difficult could it be to implement? 

Single item, asynchronously produced and delivered 

Single item, asynchronously produced and delivered (Attempt #1) class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } * This is a hot/eager source, i.e. the work has started before a Subscriber arrives 

Single item, asynchronously produced and delivered (Attempt #1) class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } * This is a hot/eager source, i.e. the work has started before a Subscriber arrives emit() subscribe() request() cancel() 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Asynchrony emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Asynchrony emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Asynchrony emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Asynchrony emit() subscribe() request() cancel() 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() X class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() https://github.com/reactive-streams/reactive-streams-jvm#1.3 class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); s.onComplete(); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #1) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() https://github.com/reactive-streams/reactive-streams-jvm#2.7 class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); s.onComplete(); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #2) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); s.onComplete(); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Single item, asynchronously produced and delivered (Attempt #2) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives Concurrency emit() subscribe() request() cancel() class SingleValue implements Publisher { private T value; private boolean subscriberReady; private Subscriber subscriber; @Override public synchronized void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { if (value != null) { s.onNext(value); s.onComplete(); } else { subscriberReady = true; } } @Override public void cancel() { subscriberReady = false; } }); } public synchronized void emit(T value) { if (subscriber != null && subscriberReady) { subscriber.onNext(value); } else { this.value = value; } } } 

Cost of synchronized? Image source: https://www.shutterstock.com/image-photo/iceberg-above-underwater-view-taken-greenland-1127373485 

s.onNext(value); public synchronized void emit(T value) ¯\_(ツ)_/¯ 

Single item, asynchronously produced and delivered (Attempt #3) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives https://github.com/reactive-streams/reactive-streams-jvm#3.4 

class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } Single item, asynchronously produced and delivered (Attempt #4) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives State Machine IDLE EMITTED REQUESTED 

Single item, asynchronously produced and delivered (Attempt #4) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives State Machine IDLE class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } EMITTED REQUESTED 

Single item, asynchronously produced and delivered (Attempt #5) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives State Machine IDLE class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int CANCELLED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(CANCELLED); } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } EMITTED REQUESTED CANCELLED 

class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int CANCELLED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(CANCELLED); } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } Single item, asynchronously produced and delivered (Attempt #5) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives State Machine IDLE EMITTED REQUESTED CANCELLED 

Single item, asynchronously produced and delivered (Attempt #5) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives subscription.cancel(); subscription.request(1) class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int CANCELLED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(CANCELLED); } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } 

Single item, asynchronously produced and delivered (Attempt #6) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives State Machine IDLE class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { subscriber = s; s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndSet(REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { } }); } public void emit(T value) { this.value = value; if (state.getAndSet(EMITTED) == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } EMITTED REQUESTED https://github.com/reactive-streams/reactive-streams-jvm#3.6 

Single item, asynchronously produced and delivered (Attempt #6) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int CANCELLED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndUpdate(val -> val == CANCELLED ? CANCELLED : REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(CANCELLED); } }); subscriber = s; } public void emit(T value) { this.value = value; int old = state.getAndUpdate(val -> val == CANCELLED ? CANCELLED : EMITTED); if (old == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } 

class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int CANCELLED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { s.onSubscribe(new Subscription() { @Override public void request(long n) { int old = state.getAndUpdate(val -> val == CANCELLED ? CANCELLED : REQUESTED); if (old == EMITTED) { s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(CANCELLED); } }); subscriber = s; } public void emit(T value) { this.value = value; int old = state.getAndUpdate(val -> val == CANCELLED ? CANCELLED : EMITTED); if (old == REQUESTED) { subscriber.onNext(value); subscriber.onComplete(); } } } Single item, asynchronously produced and delivered (Attempt #6) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives 

Single item, asynchronously produced and delivered (Attempt #6) * This is a hot/eager source, i.e. the work has started before a Subscriber arrives class SingleValue implements Publisher { private static final int IDLE = 0; private static final int REQUESTED = 1; private static final int EMITTED = 2; private static final int TERMINATED = 3; private T value; private Subscriber subscriber; private final AtomicInteger state = new AtomicInteger(IDLE); @Override public void subscribe(Subscriber s) { s.onSubscribe(new Subscription() { @Override public void request(long n) { if (n <= 0 && state.getAndSet(TERMINATED) != TERMINATED) { s.onError(new IllegalArgumentException("Non positive request signals are illegal.")); return; } int old = state.getAndUpdate(val -> val == TERMINATED ? TERMINATED : REQUESTED); if (old == EMITTED) { state.set(TERMINATED); s.onNext(value); s.onComplete(); } } @Override public void cancel() { state.set(TERMINATED); } }); subscriber = s; } public void emit(T value) { this.value = value; int old = state.getAndUpdate(val -> val == TERMINATED ? TERMINATED : EMITTED); if (old == REQUESTED) { state.set(TERMINATED); subscriber.onNext(value); subscriber.onComplete(); } } } Want to see how multiple item emission will look like? 

Let’s get real … ✋ 

response.send() Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 011101100 101110011 101100101 110011100 101110111 011001101 HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher Control 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher subscription.request(1) 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher subscription.request(1) Demand is required to compose through multiple components 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher subscription.cancel() Cancellation is required to compose through multiple components 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher Publisher client.get("friends?id=1") response.send(Publisher resp) 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher response.send( client.get("friends?id=1") ); 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher List transformToJson(List csvs) { List friends; for (String csv : csvs) { friends.add(toJson(csv)); } return friends; } 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher response.send( client.get("friends?id=1") ); 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher response.send( client.get("friends?id=1") .map(csv -> toJson(csv)) ); 

Find my friends GET /friends?id=3 HTTP/1.1 getFriendsCsv() transformToJson() HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … response.send() HTTP/1.1 200 OK Nitesh, Kant, SF Jo, Voordeckers, SF … HTTP/1.1 200 OK {First: Nitesh, Last: Kant, Location: SF} {First: Jo, Last: Voordeckers, Location: SF} … Socket Socket Subscriber Publisher response.send( client.get("friends?id=1") .map(csv -> toJson(csv)) ); Function composition. 

Reactive Streams is not sufficient as an end-user API. 

Function Composition Lack of standards Project Reactor RxJava JDK Streams Akka Streams Kafka Streams 

Practically, most applications have blocking code. 

https://github.com/reactive-streams/reactive-streams-jvm#2.2 

https://github.com/reactive-streams/reactive-streams-jvm#2.2 Embracing this reality is necessary… 

Not everything is a stream Publisher getUser(String id); 

Not everything is a stream Publisher getUser(String id); Publisher fireAndForget(String log); 

Not everything is a stream Publisher getUser(String id); Publisher fireAndForget(String log); Publisher request(HttpRequest request); 

Reactive Streams HTTP Kafka Cassandra RPC Redis () Function Composition Not everything is a stream Blocking safety 

Image source: http://sciencemeetsfood.org/team-work/ 

Thank You! @NiteshKant Nitesh Kant @jovoordeckers Jo Voordeckers +