Asynchronous OSGi – Promises for the Masses - T Ward

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Asynchronous OSGi – Promises for the masses Jun 2014
Asynchronous OSGi:
Promises for the masses
Tim Ward
http://www.paremus.com
info@paremus.com

•Senior Consulting Engineer, Trainer and Architect at Paremus

•5 years at IBM developing WebSphere Application Server

• Container Implementations for Java EE and OSGi, including Blueprint, JPA, EJB and JTA

•OSGi Speciﬁcation lead for JPA, Weaving Hooks and Asynchronous Services

•PMC member of the Apache Aries project

•Previous speaker at Java One, EclipseCon, Devoxx, Jazoon, JAX London,
OSGi Community Event...

•Author of Manning’s Enterprise OSGi in Action

•http://www.manning.com/cummins
Who is Tim Ward?
@TimothyWard

What we’re going to cover
•Why is the World Going Asynchronous?

!
•Async Programming Primitives

!
•Making Synchronous Services Asynchronous

!
•Optimising Calls into Asynchronous Services

!
•Demo

New OSGi Specifications
Last week the OSGi Core Release 6 Specification received final approval!
!
It’s available now at http://www.osgi.org/Specifications/HomePage!
!
!
The OSGi Board also approved a public draft of OSGi Enterprise Release 6!
!
Available at http://www.osgi.org/Specifications/Drafts!
!
This draft contains the specifications we’re going to be talking about!
NEW

Why is the World Going Asynchronous?

Origins of Asynchronous Programming
Asynchronous Programming concepts have been with us for a long time!
User Interface EventHandler callbacks
“must not block the UI thread!”

The terms “Promise” and “Future” originate
from 1970s research papers

Proposed as mechanisms to handle
large-scale parallel execution

Many peopler think of Async Programming as “recently popular”…

•Asynchronous systems oﬀer better performance

•This does assume that there isn’t a single bottleneck!

!
•Asynchronous Distributed systems typically scale better

•Less time spent waiting for high-latency calls

•More reasonable failure characteristics

!
•Parallelism is easier to exploit

!
•Network Infrastructure is intrinsically asynchronous

•Network Engineers have been working this way for decades
Advantages of Asynchronous Programming
100% 100%

Problems with Asynchronous Programming
Human brains aren’t really designed to cope with asynchronous logic

•Thought processes tend to be “do A”, then “do B” then “the result is C”

•Asynchronous Programming is only useful if the beneﬁts of using it
outweigh the extra cognitive overhead

!
Synchronous calls oﬀer a natural “brake” in RPC

•Asynchronous “Event Storms” can overwhelm remote systems, even using
a single client thread.

!
Deadlocks are still possible, and can be hard to diagnose

So what’s changed?
The latest asynchronous revolution seems to be sticking!
JavaScript has been a major
driver in the last 5-10 years, in a
single threaded environment.
Async is a necessity!

Multi-core is pervasive now,
request-level parallelism isn’t always
enough to max out hardware any
more
Commonly used OO languages are
becoming more “functional”

Java and C++ have added lambdas

Async Programming Primitives

Async Basics
The Promise is the primitive of Asynchronous Programming
Strictly, the OSGi (and most JavaScript) Promises are “Future” types as
they have no methods for resolving themselves

In both cases a Deferred type can create and resolve a default
Promise implementation
A “Promise” represents a delayed value that will be “resolved” in the future
“Resolving” a promise sets its value, or its failure

OSGi Promises are based on JavaScript
Promise concepts, but also signiﬁcantly
inﬂuenced by Scala

The OSGi Promise
The Promise Contract (based on JavaScript Promises)

•The Promise is resolved or failed with a single value, at most once

•Listener callbacks are called at most once

•Promises remember their state (including their resolution / failure value)

•Promises behave the same way regardless of whether they are already
resolved or resolved in the future.

!
Promises can be treated as a shareable value type

•Eﬀectively immutable (unless you created it!)

•Thread safe

•No need to unwrap it…

The Deferred
OSGi provides a default Promise implementation for clients to use

The Deferred is held by the provider of the Promise, and can be used to set
its value or failure

!
! ! final Deferred<Long> deferred = new Deferred<Long>();!
!
! ! new Thread() {!
! ! ! public void run() {!
! ! ! ! try {!
! ! ! ! ! Long total = service.calculateDifficultSum();!
! ! ! ! ! deferred.resolve(total);!
! ! ! ! } catch (Exception e) {!
! ! ! ! ! deferred.fail(e);!
! ! ! ! }!
! ! ! }!
! ! }.start();!
!
! ! Promise<Long> promise = deferred.getPromise();

Promise Callbacks
Promises do support “synchronous” usage similar to Java’s Future

•isDone(), getValue(), getFailure()

•The getXxx() methods block

!
It’s better to use Promises with callbacks (never block, remember?)

•then(Success) or then(Success, Failure)!
!
Success and Failure are both SAM interfaces, suitable for Lambda usage

then((p) -> success(p.getValue()), !
! (p) -> fail(p.getFailure()));!
!
Callbacks should be fast - Success can return a Promise for longer tasks

Chaining Promises
Basic chaining uses then(…), a new Promise is created that resolves when:

•the success callback returns null; or

•the success callback throws an error; or

•the Promise returned by the success callback resolves

!
Chaining allows you to compose sequences of events

•Promises are Monads

•Complex behaviours can be achieved without if/else branching

!
More complex functional chains are natively supported

•Filtering/Mapping values, recovering failures…

The Promises utility class
The Promises type provides number of common functions

•Creating Promises that are already resolved or failed

•Very useful when mapping / recovering promises

!
Another useful utility is Promises.all(Promise…)

•It creates a “latch” Promise that resolves when all others are complete

•Good for ﬁnal clean up, or for notifying users about aggregate tasks

!
Remember that chains and latches work even if a Promise is already resolved!

A Chaining Example
Download an XML ﬁle, preferably from a mirror URL, and parse it using JAXB
! Promise<Library> promise = getMirror(libraryURL)!
! ! ! ! .fallbackTo(Promises.resolved(libraryURL))!
! ! ! ! .then((p) -> downloadFile(p.getValue()))!
! ! ! ! .map((p) -> (Library) JAXB.unmarshal(p, Library.class)));!
!
!
! public Promise<URL> getMirror(URL url)!
! ! throws Exception {!
! ! . . .!
! }!
! !
! public Promise<File> downloadFile(URL url) !
! ! throws Exception {!
! ! . . .!
! }!

Making Synchronous Services
Asynchronous

The OSGi service model
The OSGi service model is useful and widely used

!
!
!
!
Service objects are obtained from the Service Registry and invoked

•The call semantics depend upon the Service API

•Most APIs are synchronous in operation
!
Consumer
!
Provider

Question?
As a service client, how do I make an asynchronous call to a service?

•I can’t / don’t want to rewrite the service

•I’d prefer not to start threads or manage a thread pool

•I’d also like my code to stay as clean as possible

!
Ad-hoc solutions are messy, but have been the only choice up to now

!

The Async Service - Requirements
•Allow arbitrary OSGi services to be called asynchronously

•The call must be transparent to the backing service, no opt-in required

!
•Must support methods that have a return value, as well as void

!
•Must not rely on generated source, client-local interfaces or reﬂection

•Source gets out of sync, and is hard to maintain

•Reﬂection is just ugly!

!
•Must allow services that are already asynchronous to “hook-in” to the model

•This does require opt-in

Unit testing Mocks - Requirements
•Allow arbitrary Objects to be called, remembering the calls that are made

•The mocking must be transparent, no opt-in required

!
•Mocks must allow methods to return a speciﬁc value

!
•Must not rely on generated source, client-local interfaces or reﬂection

•The test code is using the real interfaces/objects

!
!
!
Notice any similarities?

How to use the Async Service
To make asynchronous calls on a service you need an Async Mediator

! MyService mediated = async.mediate(myService);!
!
Services can be mediated from a ServiceReference or the service object

•ServiceReference is better because the mediator can track availability

!
Clients then use the mediated object to make a normal method call

•This records the call, just like a mock would in a test

!
To begin the asynchronous task you pass the return value to async.call(T)

! Promise<Long> p = async.call(mediated.pianosInNYC());!

How to use the Async Service (2)
Void methods can be slightly trickier

•There’s a no-args version of call() which returns Promise<?>

•Promises for void methods resolve with null when they complete

!
If you don’t need a Promise, then consider fire-and-forget

•The execute() method doesn’t create a Promise

•Fire-and-forget can be much more efficient*

!
The Async Service is Thread Safe, but mediators may not be

•Never share the Async object between Bundles, always get your own

•Failure to do the above can lead to class space errors and security flaws!
* More about this later!

Async Service Example
Perform two long running calls in parallel, then aggregate the result
! public void setAsync(Async async) {!
! ! this.async = async;!
! }!
! !
! public void setMyService(ServiceReference<MyService> myService) {!
! ! this.myService = myService;!
! }!
!
! public Long getCombinedTotal() throws Exception {!
! ! MyService mediated = async.mediate(myService);!
! ! !
! ! Promise<Long> bars = async.call(mediated.barsInNYC());!
!
! ! return async.call(mediated.pianosInNYC())!
! ! ! ! .flatMap((pVal) -> bars.map((bVal) -> pVal + bVal))!
! ! ! ! .getValue();!
! }

Optimising Calls into Asynchronous
Services

What if we can do better?
Sometimes there’s more to a service than meets the eye…

!
Some service implementations are asynchronous under the covers

• Blocking a thread from the Async Service to call these services is a waste!

!
We need to allow these services to provide a Promise to the Async Service

• Enter the AsyncDelegate interface!

!
Services that implement AsyncDelegate are given the opportunity to provide
their own Promise, bypassing the Async Service

null can be returned if the AsyncDelegate can’t optimise a particular call

Isn’t this a bit niche?
Implementing AsyncDelegate may seem like a micro-optimisation

• Short lived objects are cheap

• How many services does this really apply to?

!
Remote Services allows most OSGi services to be transparently remoted

• The details of the distribution mechanism are hidden

• If the distribution provider supports asynchronous communications then it
can optimise calls that it knows to be asynchronous

Distribution
Provider
Distribution
Provider

Further optimising our calls
Earlier we talked about fire-and-forget calls…

!
Fire and forget ignores return values, and offers no callback on completion

• This is often less useful, but can be exactly what you need

• It can also be very heavily optimised by remote services!

!
!
!
!
Removing the return flow decreases work at both the local and remote ends

• It also significantly reduces the load on the network
Distribution
Provider
Distribution
Provider

Implementing your own AsyncDelegate
The AsyncDelegate interface deﬁnes two methods:

• Promise<?> async(Method, Object[]) throws Exception;

• boolean execute(Method, Object[]) throws Exception;

!
It’s only worth implementing if:

• You can optimise the asynchronous execution of one or more methods

• Your service is likely to be called asynchronously

!
Return null or false if the call can’t be optimised

Exceptions should only be thrown if the call is impossible (e.g. too few args)

Demonstration: using the Async Service

•For more about OSGi...

• Speciﬁcations at http://www.osgi.org

• Enterprise OSGi in Action

• http://www.manning.com/cummins

!
•For Service Fabric examples and docs

• http://docs.paremus.com/

!
•For more about the demo (including source code)

• http://bit.ly/paremus-async

Questions?
Thanks!
44% off OSGi
books using
code osgi14cf at
manning.com
http://www.paremus.com
info@paremus.com

Asynchronous OSGi – Promises for the Masses - T Ward

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