While Reactive Programming is very different from the usual Imperative way, there’s no denying it fits “the Cloud”, as every bit of resource is used to its fullest. Let’s see how to migrate from the latter to the former using a Spring Boot web app as an example.

1. @nicolas_frankel
From Imperative to
Reactive to Coroutines
Nicolas Fränkel

2. @nicolas_frankel
Me, myself and I
 Former developer
 Developer Advocate
 Not a Reactive guru

3. @nicolas_frankel
Hazelcast
HAZELCAST IMDG is an operational,
in-memory, distributed computing
platform that manages data using
in-memory storage and performs
execution for breakthrough
and scale.
HAZELCAST JET is the ultra
fast, application embeddable,
3rd generation stream
processing engine for low
latency batch and stream
processing.

4. @nicolas_frankel
2014: The Reactive Manifesto
 Responsive: “The system responds in a
timely manner if at all possible”
 Resilient: “The system stays responsive
in the face of failure”
 Elastic: “The system stays responsive
under varying workload”
 Message Driven: “Reactive Systems rely
on asynchronous message-passing”

5. @nicolas_frankel
Actor Model

6. @nicolas_frankel
Reactive Streams
“The purpose of Reactive Streams is to
provide a standard for asynchronous
stream processing with non-blocking
backpressure”
-- https://www.reactive-streams.org/

7. @nicolas_frankel
The Reactive Loop
Requests
Event
Event queue
Event loop
Event
handler

8. @nicolas_frankel
Why Reactive?
 Scalability
• One thread per request doesn’t scale
 “Cloud-friendly”
• Optimized resource usage

9. @nicolas_frankel
Downsides of Reactive
 More complex mental model
 Harder to debug
 Specific APIs
 One single blocking call in the call chain
and it’s not reactive anymore!

10. @nicolas_frankel
Reactive Streams API

11. @nicolas_frankel
Project Reactor
 Reactive Streams implementation
 Lead by Pivotal
• No dependency on Spring
• (The other way around)

12. @nicolas_frankel
JDK 9 Flow
 “There will be a migratory period, while
libraries move to adopt the new types in
the JDK, however this period is expected
to be short” 😂
• JDK 9 2017
 org.reactivestreams.FlowAdapters

13. @nicolas_frankel
Spring Framework Web
 v5 supports Reactive Types
 Spring WebFlux
• org.springframework.web.reactive

14. @nicolas_frankel
Spring MVC vs. Spring WebFlux
Spring MVC Spring
WebFlux
Reactive? ✘ ✓
Annotations? ✓ ✓
API ✓ ✓

15. @nicolas_frankel
Talk is cheap, show me the code!

16. @nicolas_frankel
Summary
 Migrate to Functional APIs first
 Everything needs to be reactive
• Blockhound for the win
 More work but nothing impossible
 Coroutines make non-blocking easier
 Don’t use Reactive because others do!

17. @nicolas_frankel
Thanks for your time!
 https://blog.frankel.ch/
 @nicolas_frankel
 https://hazelcast.org/blog/migrating-from-
imperative-to-reactive/
 http://bit.ly/to-reactive
 https://slack.hazelcast.com/