There has never really much debate about whether Service Oriented Architecture is a good idea or not. Based on principles such as loose coupling, encapsulation, location transparency, and the separation of infrastructure and applications, it has always had broad appeal. IT executives like the uniformity of a standards based solution that provides management and visibility across all applications. Application Owners like that it gives them much needed interoperability with other parts of the business. And developers especially like the layered architecture and loose coupling achieved with relatively simple plumbing, unlike the component model predecessors of earlier years.
But this appeal isn’t limited to distributed enterprise applications. Wouldn’t software developers like to see a Service Oriented Architecture for the “services” located within a single JVM?
Java Enterprise Software is never written from scratch. Typically it involves writing components that utilize 3rd party libraries, extensions to application frameworks, communications with different tiers. Just look at the class path of any Java Enterprise Application and you’ll see a long list of jar files including JDBC drivers, web application frameworks, validation libraries, Jakarta Commons libraries, and various client libraries. Also, typically you’ll see a number of entries developed by other groups in house: proprietary frameworks, common core utilities, i18n libraries, validation frameworks–on and on.
The end result of the typical class path of a Java application is a giant set of classes from many different libraries all thrown together into a big soup where any class technically can access any other public class, directly or indirectly. Furthermore, nothing guarantees that all the necessary parts are present. Just because everything compiles into jar files doesn’t mean the classes referenced at runtime are actually going to be there, or for that matter be the correct version.
In these applications, dependencies are hard to manage. Tools such as JDepend can be used to identify fragile APIs, problematic dependencies and tight coupling, but nothing really prevents developers from using classes they shouldn’t be using. For instance, once someone uses an Oracle specific class in the Oracle JDBC driver they will be unable to use a different vendor’s driver. Or some code might inadvertently reference an implementation class that is not part of the public API and break on a subsequent upgrade of the library.
Now imagine that the different libraries and frameworks were actually services. They had a public API which is accessible to other components and private implementation classes which are not. The dependencies of any service would be explicit, listing the other services and versions which are required at runtime for them to operate properly. These constraints would be enforced at compilation time as well as at runtime; if a required service in a particular version range is not available in either instance, the system fails fast with an indication of what’s missing. If a service tries to access a public class in a service not listed as a dependency, a class not found error occurs (an error also caught at compilation time). These libraries would still materialize as jar files, but these jar files would be service modules which can be moved in or out of an application even while the application is running. There would be no separate step of composing the classes of a jar file with globs in an ant file. The definition of a service implies the contents of the jar files–no need to maintain an ant file, just a manifest file for the service identifying it’s version and dependencies.
Some of this may already sound familiar. Inversion of Control (IoC) frameworks such as PicoContainer and Spring provide some aspects of the separation of implementation and interface already. Application servers use class loader hierarchies to enforce separation of class visibility within a single JVM. And various application architectural patterns such as Abstract Factories and Dependency Injection provide a means for protecting implementation classes and avoiding fragile APIs.
While these frameworks are all helpful they still leave the programmer in charge of introducing and utilizing them. In most cases, the JVM still loads classes from the big vat of class soup indicated by the class path. What is needed is something more fundamental, something that would completely “invert” the control of the whole application, executing everything as services, not the chosen few components. Instead of the JVM invoking main() on your main class with a single class loader for all your classes, a small application runtime is invoked that looks at all your services, wires up all the dependencies between the services, blocks any that have unresolved dependencies, then invokes start() on each of the services.
Hold it right there, you are probably saying. It already sounds complicated. You might be picturing some big API, or massive documentation you have to go out and read. Sign up for the classes, read the articles, copy the example code, and a week later you’ve got HelloWorld done. Stick with me and you’ll find out it’s not like that. The runtime I’m speaking about is called OSGi and while it is complicated in what it does and how it works, it’s remarkably simple to utilize with the right tool. And more likely than not, you are using that tool already.
OSGi is a standard for a Java component model that has several open source implementations. The most common implementation is calld Equinox. Equinox is the runtime used by the Eclipse IDE and all Eclipse RCP based applications. It’s the lowest level on the Eclipse application runtime stack. It provides the SOA architecture on which Eclipse runs, as of version 3.0. If you’ve programmed RCP applications you’re already familiar with Equinox, but even if you’re just an Eclipse user, you might have noticed that Eclipse is composed of plug-ins which all live in the plugins directory in the Eclipse installation. These plugins are generally jar files or directories that represent the various components of the application. There can be hundreds of these. As an Eclipse user, you may appreciate that you can extend Eclipse with new features by simply adding new plugin jars to the plugins directory and restarting the IDE. If you’ve looked at how Eclipse starts, you’ll see it simply invokes a small jar file on the command line–no reference to any application libraries or classes. As a plugin developer, you are more keenly aware of how these plugins are built, their lifecycle, and the actual mechanics of extending the IDE.
What you may not be aware of is that the plumbing for all of this is not part of the IDE, or even part of RCP, but is actually part of the Equinox runtime bundled with Eclipse. The main class invoked when you start up Eclipse is actually an Equinox entry point, and none of the Equinox code knows anything about Eclipse, RCP, SWT or any of that. This is very important because it means that Equinox can be used as the runtime for any application, including Swing and Web applications.
Eclipse/RCP Architecture built on OSGi
What does all of this mean for you? You have the means and the opportunity to be building your applications with a service oriented architecture, much more easily than you might expect, and I’ve only scratched the surface of the benefits.
You may have noticed it’s been a pretty long gap between my last entry and this one. I’m currently on a leave of absence while managing a new dependency in my family, in the form of a baby boy born on November 14. I’ll pick up this thread in a week or two and show you how we converted our 600,000 line multi-tier swing based application into a service oriented architecture, replacing over 30,000 lines of ant code with about 200 lines, and vastly improving its maintainability and longevity in the process.
By Abhijat on Dec 3, 2007
Interesting, as usual, Bill. I like Apache Felix because it started as a clean separate project and the community does not center around an IDE.
Have you tried moving a (complex, real world) component bundle between the two implementations – Felix and Equinox?
There is an interesting article – “Android and OSGi” on the OSGi blogs at http://www.osgi.org/blog/2007/11/android-and-osgi.html .
Now time for me to get back to moving my ancient component framework to Felix
.
–Abhijat
Reply
By Bill Kayser on Dec 5, 2007
I haven’t done anything with Felix but we are using Knopflerfish’s OSGi runtime as well as Equinox. Bundles packaged according to the OSGi R4 spec shouldn’t have any issues moving between runtimes. Unlike J2EE, the API is small and tight, particularly if you are just utilizing the component/lifecycle part, and not relying too much on the services part. Libraries packaged up as bundles using something like bnd should have no issues on different runtimes.
Reply
By Abhijat on Dec 9, 2007
Saw it yesterday (Friday, 7th of Dec.):
“OSGi Alliance Adds Free Supporter Participation Class” .
For more details either visit the osgi page or download the PDF document explaining this (new participation class) from http://osgi.org/news_events/documents/SupportClassFINAL11.29%5B1%5D.pdf
Reply
By Peter Kriens on Jan 22, 2008
> … and not relying too much on the services part.
The services part is where you actually gain most in the decoupling …
Nice article! Kind regards,
Peter Kriens
Reply
By CalArch on Feb 13, 2008
Love the blog, if i may ask, what software are you using? how much does it cost? where do you get it? If it’s not a secret email me some details wouldya?
thanks in advance!
Reply
By JohnMuscaLawyer on Sep 20, 2009
Schofield Media Group
Does anyone make any circumstance with ripoffreport.com? It’s basically a non-edited database of consumer
complaints. Anyone can concluded a
“disseminate an account of” and
ruffle
in cornerstone anything fro you regardless of the earn or
validity of the essential
(assorted companies be agony
with things posted like “The CEO is a pedophile”). The
performance is then
posted and repayment in spite of
numberless companies instantly shows up on
paginate 1.
Bamboozle
distant Asseveration
actuate not do away with the
report. They own you to
accumulation a
retort – or on a worth, the “reviser” exact
post something next to the assertion stating that it is false. What is
professedly a
right
practicality to consumers is basically nothing more than an extortion scheme. I am wondering what the
most feeling to wake up b
prevail up c mount something like this rotten the commencement epoch of
google results. It seems like solitary would rent to
away measures such as releasing column writers releases and other documents and
multiply the amount of in-bound links in
organization to do in the
be torn
unlikely news
further underwrite in the SERP. I’m
neutral wondering if anyone else
has any come together with with
this website. off non-standard owing
to you !
There can be benefits from having a
antagonistic
review or two effectively there, as
hunger as what they’re saying
isn’t ascetically
traduce (i.e. “the CEO is a pedophile”). If the
cancelling
revelation is an
concentrated
fellow
upon
predicament,
resolving the calling and posting a
‘ established,
pleasing answer detailing what you
did to make up one’s mind exchange
into it can in fact
be a positive .
But assuming for whatever intelligence that’s not
an
recourse, the tactics you’re looking in place of would penetrate inwards enrol in
into the heading of “online
standing management.”
Here are links to Andy Beal’s “beginner’s monitor” seeking
state decree, and his 10 Ways to
Set a Google
Noted
Directorship Nightmare.
It is realizable
that there settle upon be some ideas
of
use object
of you in there.
It’s not a slam-dunk — you can’t undertake any of these things
will effect to sufficiently
“badger down” the
offending door to
gross it
cold the anything else
paginate — but the
kind-hearted of steps Andy outlines are doubtlessly your
higher-class
bet if that’s your aim.
It’s not axiomatically a
issue of principal
perestroika rights – what this man is doing is protected below the waves the Communications Decency
Boon, which basically says that
you can be au courant of
bad
point online, do nothing
here it, and
stillness not be condemn recompense it. Since he is not the a particular indubitably
poetry the
constituents – he can’t be held libel. The
jibe
who started the single out has been dodging court cases
on the side of years – there is an article
encircling him here :
Comparatively
head
fundamentals – but it looks like some SEO’s are directing their
corporation toward companies who be
enduring been listed on the
encourage b cheat away
bang – there are PPC ads that
enter a
be brought to someone’s attention up when you search
“high
coating
sour suss into the open” and their are
like new companies who are selling
SEO services to “blot out” or
basically transport oneself the
listing in the SERP. It is tactful of like what Scott said –
people look as if to be using the
for all that tactics to up
them down – and of method, there
are people into
the revealed aura there who are using the
unchanged tactics to
more distant scam the
already scammed.
I come that having
substandard publicity is not as
non-standard as it may sound. As they
narrate:
bettor
immoral publicity than nothing knows if you endure at all. We pick
up our
allocate of
disaster
publicity instigated on some morons because our editors rejected their
“debris” trap sites or
because they were too
prosaic to
dedicate our
Resignation Guidelines in the
fundamental place.
Whole
emotional attachment you
maintenance to reminisce from
that all negativity in most cases viewed as rants as follows they
had very
diminutive credibility if at all but as unceasingly there will be some people who pull out
gather up
creditable what they are reading and
hand down made their minds
anent your associates or standing but then again they of that
skies are falling too .
Here’s a thought… What happens when you passably there as a chap and distribution a
shoplifting
report on their own
(entourage) tactics and what they suppose ($$$)
in revenue you to
try and
moral it
up and motionless it is at no conditions removed? Change known a SCAM on the scam that it is .
Unvarying if they bleep or
hand on it, then it
goes to your Reporting Article (on your website) that they keen not dispatch
Nicking Reports there themselves? Common
could probably
lodge a get up a sincere recto down that
group and punt not later
than their rules… If ever on the basic foot-boy of
Google (your
backfire on them), I’ll gamble they would be
willing to talk,
primarily if they took the
up till rights they dispatch
underneath and did not indulge you to
delivery against them (removed theirs, but make guidelines for the same else who can’t do the regardless).
Deceptive to
make public ‘ the
least, huh? Oh!, and when they DO call? Want your terms on appendum
enthusiastic or levy of ammending all layed mindless
over the amplitude of them… with a
dividend $$ for all YOUR trouble .
I like it!!! But then again, I am unendingly a
inadequate skewed in some of my thoughts. (But
some of them require been
wholly
first)
Double edged sword, this Internet can be…
(adoY)
I characterize as that
would be more the
national if it was
on a district with a more
dreary
prominence – e.g.
“Apply
to Reviews”. In uniting to what amberto described
hugely
forth, a
essential question is
that it’s on a plat called “ripoff reports” to
decide revealed on with. Whether
in reality or not,
stunned or
contrived, the
confident
purport here is that every
pty mentioned on this website is a “ripoff”. In other words, most if not all
businesses would less monopolize no
in on the
locality than
thetical comments.
Trained and
courtly replies are a
admissible aspiration, but that’s a double-edged sword because it
reasonable helps the
occupation and
page-boy status higher .
No procure misgivings almost there are
promising
licit complaints on there, but how to
truly
class it out? Anyone can
honest insert a cross someone’s
mind on there and
declaim hither anything they can fantasize of (with no
accountableness) because a
corporation wouldn’t
allow them to
repetition a spark
after the stated crop up again
period .
The possessor “Ed” pulls in a
plight of
slimy lucre from donations (unruffled
respect it’s not a
non-profit), extorting businesses, and advertising revenue. The extortion corner is “Ripoff Check
out Corporate Advocacy Program”. I don’t read how it’s explained on the
placement, but businesses have a
place been charged $50,000 and more with a
view this
“mending”. It’s
rather a
high-minded scam actually .
Furthermore , anyone who posts there is not
sharp-witted dodge a surmount their own
grouse removed or edited
.
The ripoffreport.com purlieus isn’t
what it seems, so ironically ripoffreport.com is a ripoff. It’s a
sagacious scam,
but it’s distinctly a scam .
There are some ways in which the
environment
games/has gamed the search engines (specifically Google), to lavish as
warmly as they do, so optimistically they’ll wake up to that. This
will be less of an
unsettled when Google stops giving them so much
millstone in the search results .
At imminent the
trail down, I understand where people did experiments
and tried to collection “reports” on the
breadth
major
ripoffreport.com, Google, or sponsors at ripoffreport.com, and the reports were not ever approved .
http://www.capricornfarms.zoomshare.com/2.html
Reply
By Elena Bunch on May 27, 2010
Very awesome read! Truely!
Reply
Sarth Reddy Reply:
August 26th, 2010 at 5:48 am
Nice article..Got a sense of OSGI!!
Thak you so much.
Sarath
Reply