- Reactive programming - Spring 5 - MT Examples etc.

2. BAD MT EXAMPLES
try(Transaction t = db.startTransaction()){
List<User> all = db.select10000Users();
all.forEach(u->{
String text = template.formatEmail(u);
smtp.send(u.email, text);
});
}
Future<?> future = executorService.submit(() -> {
String threadName = Thread.currentThread().getName();
System.out.println("Hello " + threadName);
});
future.get();
• NO MT CODE
private Lock synchro = new ReentrantLock();
public void wrong2(Map<String, String> sharedResource)
{
synchro.lock();
try{
sharedResource.put("Key1", "Value1");
}finally {
synchro.unlock();
}
}
• ERROR IN PRIMITIVE USE
• ERROR IN RESPONSIBILITY AND TRANSACTION SCOPE
class Singleton //doublecheck
{
static volatile Singleton instance;
static Object accessor…
static Singleton get()
{
if(instance == null){
lock(accessor){
if(instance == null)
instance = new;
}
}
return instance;
}
}
• BROKEN PATTERN

3. WHERE IS MY CPU
• BAD CPU UTILIZATION
• NO LOAD RESERVE

4. EVERYBODY LIKES SPRING ( )
ReactiveX
Reactor IO
• NETTY, JETTY, TOMCAT, UNDERTOW (SERVLET 3.1 )
• JDK 9 AS JAVA.UTIL.CONCURRENT.FLOW
• > JAVA.NIO.FILE.*(AsynchronousFileChannel)
• HTTP | WEBSOCKET | TCP | UDP
Streams

5. THINK REACTIVE
event Iterable (pull) Observable (push)
retrieve data T next() onNext(T)
discover error throws Exception onError(Exception)
complete !hasNext() onCompleted()

6. BASIC PLAYERS (FROM REACTOR.IO)
• SUBSCRIBER
• PUBLISHER
• FLUX -> 0:N (∞)
• MONO -> 0:1
public void subscribe(Subscriber<? super T> s);
// Factory for Subscription interface
void onSubscribe(Subscription s);
void onNext(T t);
void onError(Throwable t);
void onComplete();

7. LET’S RUN SIMPLE
Flux.just("red", "white", "blue")
.log()
.map(value -> value.toUpperCase())
.subscribe(System.out::println);
Flux.just("red", "white", "blue")
.subscribeOn(Schedulers.parallel()) // !!!!
.log()
.map(value -> value.toUpperCase())
.subscribe(System.out::println);
01:37:18.678 [main] INFO reactor.core.publisher.FluxLog -
ON_SUBSCRIBE(reactor.core.publisher.FluxArray$ArraySubscription@35bbe5e8)
01:37:18.681 [main] INFO reactor.core.publisher.FluxLog - REQUEST(unbounded)
01:37:18.682 [main] INFO reactor.core.publisher.FluxLog - ON_NEXT(red)
RED
01:37:18.682 [main] INFO reactor.core.publisher.FluxLog - ON_NEXT(white)
WHITE
01:37:18.682 [main] INFO reactor.core.publisher.FluxLog - ON_NEXT(blue)
BLUE
01:37:18.682 [main] INFO reactor.core.publisher.FluxLog - ON_COMPLETE()
...
17:26:15.184 [main] INFO reactor.Flux.SubscribeOn.1 - request(unbounded)
17:26:15.189 [parallel-1] INFO reactor.Flux.SubscribeOn.1 - onNext(red)
RED
17:26:15.189 [parallel-1] INFO reactor.Flux.SubscribeOn.1 - onNext(white)
WHITE
17:26:15.189 [parallel-1] INFO reactor.Flux.SubscribeOn.1 - onNext(blue)
BLUE
17:26:15.190 [parallel-1] INFO reactor.Flux.SubscribeOn.1 - onComplete()

8. COMPLICATED EXAMPLE@Document
class Person
{
@Id
String id;
String name;
}
interface PersonRepository
extends MongoRepository<Person, String>
{
@Query("{}")
Stream<Person> all();
CompletableFuture<Person> findById(String id);
}
@Controller
class PersonController
{
@Autowired
PersonRepository personRepository;
@GetMapping("/person")
public Flux<Person> all(){
return Flux.fromStream( personRepository.all() );
}
@GetMapping("/person/{id}")
public Mono<Person> byId(@PathVariable String userId){
return Optional
.ofNullable(userId)
.map(id -> personRepository.findById(id))
.map(person-> Mono.fromFuture(person))
.orElseThrow(() -> new IllegalStateException("error - null id"))
;
}
}

9. UNDER THE HOOD
Thread
HTTP
Request
Thread
HTTP
Request
100
. . .
@GetMapping("/person")
List<Person> get(){
List<Person> result =
db.longOperation();
return
Response.ok(result);
}
HTTP
HTTP
HTTPHTTP
HTTP
< 100
@GetMapping("/person")
public Flux<Person> all(){
return Flux.fromStream( personRepository.all() );
}

10. COMMON INTERVIEW MISTAKE
• ASYNC REQUEST AS AN ANSWER FOR LONG
REQUESTS (WRONG)
JavaScript queries
process
Long Running
server
Display results
• PULLING OR PUSHING (RIGHT)
Client query
Pull status of Job
Display Result
Server
Render Job ID
Track Status of Job

11. DO IT IN REACTIVE WAY
@RequestMapping(value = "/report/{userId}", method = RequestMethod.PUT)
@ResponseStatus(HttpStatus.OK)
public void longReport(@PathVariable String userId){
Mono.just(userId)
.subscribeOn(Schedulers.parallel()) // MOST IMPORTANT LINE
.log()
.subscribe(
personRepository::longOperation,
websocketRegistry::notifyError, //use WebSockets to report error
websocketRegistry::notifySuccess //use WebSockets to report success
);
;
//here we return to client status OK
}

12. {FINALIZE}
• REACTIVE REVOLUTIONIZE MULTITHREADING BY
EXPOSING GOOD DESIGN PATTERNS;
• SPRING PUTS ALL TOGETHER;
• CHANGE YOUR MIND TO BE REACTIVE.
• ANY QUESTIONS?