Grails offers a number of options for implementing asynchronous and event-driven applications, from the low-level Servlet 3.0 async support to the Promise API abstraction, which supports GPars, RxJava 1 & 2. At the GORM level, there is also the async namespace, and RxGORM. Finally, a brand new events API shipped with Grails 3.3, which moves off Reactor, will help users to implement event-driven applications. In this session, the speaker will walk the audience through such options.

1. Async and event-driven
Grails applications
Álvaro Sánchez-Mariscal
@alvaro_sanchez

2. About me
— Coming from Madrid
!
— Developer since 2001 (Java / Spring stack).
— Grails fanboy since v0.4.
— Working @ OCI since 2015: Groovy, Grails & Micronaut!
— Father since 2017!
"
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3. Introduction
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4. Grails Async Framework
— https://async.grails.org
— Introduced in Grails 2.3 with the Promise API. Spun-
off in Grails 3.3
— Supports different async libraries: GPars, RxJava and
Reactor.
— Application events, Spring events, GORM events.
— Async GORM.
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5. Grails Async Framework
— Server Sent Events.
— RxGORM.
— Async request/response processing.
— Servlet 3.0 async support.
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6. Ge"ing started
— Add the dependency in build.gradle:
runtime "org.grails.plugins:async"
— Check the Grails plugin page for more information:
http://plugins.grails.org/plugin/grails/async
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7. Servlet 3.0 API
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8. Servlet 3.0
— Spec'ed in 2009!
!
— Allows offloading of blocking operations to a different
thread.
— Implement grails.async.web.AsyncController to get
started.
— Call startAsync() to get the javax.servlet.AsyncContext
— When done, call complete() or dispatch()
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9. Demo
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10. Advanced usage
ctx.addListener(new AsyncListener() {
void onStartAsync(AsyncEvent event) throws IOException { }
void onComplete(AsyncEvent event) throws IOException { }
void onTimeout(AsyncEvent event) throws IOException { }
void onError(AsyncEvent event) throws IOException { }
})
ctx.timeout = 5_000
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11. Promise API
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12. Grails Promise API
— Builds on top of java.util.concurrent.Future.
— grails.async.Promises helps you to create Promise<T>
instances.
— In controllers, better use grails.async.web.WebPromises
(Servlet 3.0 Async under the covers)
— PromiseFactory allows pluggable implementations.
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13. PromiseFactory API
— CachedThreadPoolPromiseFactory (default): unbound
thread pool.
— GparsPromiseFactory: if org:grails:grails-async-gpars
dependecy is on the classpath.
— RxPromiseFactory: if either org.grails:grails-async-
rxjava or org.grails:grails-async-rxjava2 are on the
classpath.
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14. PromiseFactory API
— SynchronousPromiseFactory: useful for unit testing.
import org.grails.async.factory.*
import grails.async.*
Promises.promiseFactory = new SynchronousPromiseFactory()
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15. Demo
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16. PromiseList
import grails.async.*
PromiseList<Integer> list = new PromiseList<>()
list << { 2 * 2 }
list << { 4 * 4 }
list << { 8 * 8 }
list.onComplete { List<Integer> results ->
assert [4,16,64] == results
}
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17. PromiseMap
import static grails.async.Promises.*
PromiseMap promiseMap = tasks one: { 2 * 2 },
two: { 4 * 4},
three:{ 8 * 8 }
assert [one:4,two:16,three:64] == promiseMap.get()
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18. @DelegateAsync
import grails.async.*
class BookService {
List<Book> findBooks(String title) { ... }
}
class AsyncBookService {
@DelegateAsync BookService bookService
}
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19. @DelegateAsync
import grails.async.*
class BookService {
List<Book> findBooks(String title) { ... }
}
class AsyncBookService { //Equivalent
Promise<List<Book>> findBooks(String title) {
task {
bookService.findBooks(title)
}
}
}
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20. Using the service
@Autowired AsyncBookService asyncBookService
def findBooks(String title) {
asyncBookService.findBooks(title)
.onComplete { List<Book> results ->
render "Books = ${results}"
}
}
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21. Events
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22. Grails Events abstraction
— Add the dependency in build.gradle:
runtime "org.grails.plugins:events"
— By default Grails creates an EventBus based off of the
currently active PromiseFactory.
— Or you can use:
— org.grails:grails-events-gpars
— org.grails:grails-events-rxjava
— org.grails:grails-events-rxjava2
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23. Publishing events
— With the @Publisher annotation:
class SumService {
@Publisher
int sum(int a, int b) {
a + b
}
}
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24. Publishing events
— With the EventPublisher API:
class SumService implements EventPublisher {
int sum(int a, int b) {
int result = a + b
notify("sum", result)
return result
}
}
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25. Subscribing to events
— With the @Subscriber annotation:
class TotalService {
AtomicInteger total = new AtomicInteger(0)
@Subscriber
void onSum(int num) {
total.addAndGet(num)
}
}
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26. Subscribing to events
— With the EventBusAware API:
class TotalService implements EventBusAware {
AtomicInteger total = new AtomicInteger(0)
@PostConstruct
void init() {
eventBus.subscribe("sum") { int num ->
total.addAndGet(num)
}
}
}
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27. GORM events
— DatastoreInitializedEvent
— PostDeleteEvent
— PostInsertEvent
— PostLoadEvent
— PostUpdateEvent
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28. GORM events
— PreDeleteEvent
— PreInsertEvent
— PreLoadEvent
— PreUpdateEvent
— SaveOrUpdateEvent
— ValidationEvent
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29. Asynchronous subscription
— They cannot cancel or manipulate the persistence
operations.
@Subscriber
void beforeInsert(PreInsertEvent event) {
//do stuff
}
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30. Synchronous subscription
@Listener
void tagFunnyBooks(PreInsertEvent event) {
String title = event.getEntityAccess()
.getPropertyValue("title")
if(title?.contains("funny")) {
event.getEntityAccess()
.setProperty("title", "Humor - ${title}".toString())
}
}
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31. Spring events
— Disabled by default, for performance reasons.
— Enable them by setting grails.events.spring to true.
@Events(namespace="spring")
class MyService {
@Subscriber
void applicationStarted(ApplicationStartedEvent event) {
// fired when the application starts
}
@Subscriber
void servletRequestHandled(RequestHandledEvent event) {
// fired each time a request is handled
}
}
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32. Async GORM
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33. Get started
— NOTE: drivers are still blocking. This just offloads it to
a different thread pool.
— Include compile "org.grails:grails-datastore-gorm-
async" in your build.
— Implement the AsyncEntity trait:
class Book implements AsyncEntity<Book> {
...
}
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34. The async namespace
Promise<Person> p1 = Person.async.get(1L)
Promise<Person> p2 = Person.async.get(2L)
List<Person> people = waitAll(p1, p2) //Blocks
Person.async.list().onComplete { List<Person> results ->
println "Got people = ${results}"
}
Promise<Person> person = Person.where {
lastName == "Simpson"
}.async.list()
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35. Async and the Hibernate session
— The Hibernate session is not concurrency safe.
— Objects returned from asynchronous queries will be
detached entities.
— This will fail:
Person p = Person.async.findByFirstName("Homer").get()
p.firstName = "Bart"
p.save()
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36. Async and the Hibernate session
— Entities need to be merged first with the session
bound to the calling thread
Person p = Person.async.findByFirstName("Homer").get()
p.merge()
p.firstName = "Bart"
p.save()
— Also, be aware that association lazy loading will not
work. Use eager queries / fetch joins / etc.
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37. Multiple async GORM calls
Promise<Person> promise = Person.async.task {
withTransaction {
Person person = findByFirstName("Homer")
person.firstName = "Bart"
person.save(flush:true)
}
}
Person updatedPerson = promise.get()
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38. Async models
import static grails.async.WebPromises.*
def index() {
tasks books: Book.async.list(),
totalBooks: Book.async.count(),
otherValue: {
// do hard work
}
}
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39. RxJava Support
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40. RxJava support
— Add org.grails.plugins:rxjava to your dependencies.
— You can then return rx.Observable's from controllers,
and Grails will:
1. Create a new asynchronous request
2. Spawn a new thread that subscribes to the
observable
3. When the observable emits a result, process the
result using the respond method.
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41. Demo
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42. Q & A
Álvaro Sánchez-Mariscal
@alvaro_sanchez