When is 'optional' really optional? - Tim Ward

When is ‘optional’ really optional?
http://www.paremus.com
info@paremus.com

Tim Ward

Tuesday, 29 October 13

Copyright © 2005 - 2013 Paremus Ltd.
May not be reproduced by any means without express permission. All rights reserved.

Oct 2013

Who is Tim Ward?
@TimothyWard

• Senior Consulting Engineer and Architect at Paremus
• 5 years at IBM developing WebSphere Application Server
• Container Implementation experience with Java EE and OSGi, including Blueprint, JPA, EJB
and JTA

• OSGi Speciﬁcation lead for JPA and Bytecode Weaving
• PMC member of the Apache Aries project
• Previous speaker at EclipseCon, Devoxx, Jazoon, JAX London, OSGi
Community Event...

• Author of Manning’s Enterprise OSGi in Action
• http://www.manning.com/cummins



Oct 2013

Optional Services




Oct 2013

Optional Services

• OSGi bundles communicate and collaborate using loosely coupled services
• Making a service dependency “optional” is pretty easy
OSGi API
ServiceReference<Foo> ref = ctx
.getServiceReference(Foo.class);

Declarative Services API
private Foo fooService;
@Reference(cardinality = OPTIONAL)
public synchronized void setFoo(Foo service) {
fooService = service;
}

if(ref != null) {
Foo service = ctx.getService(ref);
if(service != null) {
try {
...
public synchronized void unsetFoo(Foo service) {
} finally {
fooService = (fooService == service) ? null :
ctx.ungetService(ref);
service;
}
}
}
Oct 2013
} When is ‘optional’ really optional?

Difficulties with Optional Services

• The OSGi API and DS need null checks, which is messy
• Blueprint injects a proxy to the service - No more nulls!
Blueprint code

Blueprint XML

private Foo fooService;

<blueprint>

public void setFoo(Foo service) {
fooService = service;
}

<reference interface=com.paremus.Foo
id=”foo” availability=”optional”/>
<bean id=”bar” class=”com.paremus.Bar>
<property name=”foo” ref=”foo”>
</bean>
</blueprint>




Oct 2013

Blueprint is the winner! Or is it?




Oct 2013


• At ﬁrst inspection Blueprint looks much simpler




Oct 2013


• But what happens when there is no backing service?




Oct 2013


• Calling an unbound blueprint service proxy blocks up to the “timeout”. If no
service arrives then you get a ServiceUnavailableException




Oct 2013



• The default timeout is 5 minutes!!!




Oct 2013



• You can decrease the timeout (but not to zero)




Oct 2013



• You can use a ReferenceListener (this is horrible code, and risks deadlock)



Oct 2013



• You can use a ReferenceListener (this is horrible code, and risks deadlock)
• You can inject a ReferenceList (like DS, but with the wrong cardinality)



Oct 2013

Blueprint 1.1 to the rescue




Oct 2013


• Despite aiming to be “simple” Blueprint 1.0 sucked at optionality...




Oct 2013


• Blueprint 1.1 proposes several ﬁxes:




Oct 2013


• Allow users to have an immediate timeout (no service, no waiting)




Oct 2013


• Still have to cope with a ServiceUnavailableException




Oct 2013


• Allow users to specify a “default” service implementation




Oct 2013


• This can be a nice way to avoid branches in your code




Oct 2013


• Allows users to use a “null proxy” as the default implementation




Oct 2013


• Allows users to use a “null proxy” as the default implementation
• Avoids making you an API provider



Oct 2013

Is Service Optionality really ‘optional’




Oct 2013


• The optionality so far has made an implicit assumption




Oct 2013


• The API was not optional!




Oct 2013


• OSGi does a great job of managing dependencies




Oct 2013


• Automatic dependency provisioning




Oct 2013


• Prevent resolution unless dependencies are satisﬁed




Oct 2013


• Prevent resolution unless dependencies are satisﬁed
• Optional dependencies don’t have the same guarantees!



Oct 2013

Optional Packages




Oct 2013

Handling Package Optionality




Oct 2013


• Having optional package imports makes OSGi more like a normal classpath




Oct 2013


• You need to be defensive - ClassNotFoundException could happen!




Oct 2013


• Unusually for OSGi the factory pattern can be helpful




Oct 2013


• Select the implementation with or without the dependency as appropriate




Oct 2013


• Blueprint can use factories to create managed beans




Oct 2013


• Blueprint can use factories to create managed beans
• It can be useful to isolate dependent code in a handler class



Oct 2013

Handling Package Optionality (2)




Oct 2013


• Optional packages can be very hard to manage




Oct 2013


• Having multiple optional dependencies can explode complexity




Oct 2013


• Eﬀectively you’re back to the complexity of a Java Classpath!




Oct 2013


• Possible approaches to consider:




Oct 2013


• Don’t make the API optional, have two bundles, one with the dependency
and the other one without




Oct 2013



• If all you care about is ease of use then repackage the API



Oct 2013



• If all you care about is ease of use then repackage the API
• Keep a careful internal dependency structure



Oct 2013

Thanks!

• For more about OSGi...
• Speciﬁcations at http://www.osgi.org
• Enterprise OSGi in Action
• http://www.manning.com/cummins

Questions?

http://www.paremus.com
info@paremus.com



Oct 2013

When is 'optional' really optional? - Tim Ward

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