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From Brian Pontarelli <>
Subject Re: [s2] A thought - next generation OSGi-based?
Date Fri, 25 Apr 2008 17:45:40 GMT

Ian Roughley wrote:
> Maybe I'm overstepping my knowledge from fact, but I thought the 
> benefit of OSGi was that you could have completely different 
> implementations (i.e. PackageConfig) running on the same JVM, with 
> OSGi providing the classloading magic so that the implementations 
> don't collide.
It actually is a compile time issue first and then later is a method 
invocation issue and has nothing to do with OSGi. Think of it this way, 
you have an interface:


It has two versions 1.0 and 2.0. They are very different and look like:

public void method1();
public String method2();
public Object method3(String);
public Object method4(String, int);

public void method1();
public int method2();
public Object method4(int, String);
public Object method5(long);

As you can see, method1 is the same, but the rest of it is changed 
drastically. The issue is now that the interface is still in the same 
package and has the same name. How do you compile against both versions? 
Furthermore, how would OSGi be able to resolve the method invocations at 
runtime to the two different versions?

Very difficult problems that I've done some work on. There isn't any 
good solution. The only way to make it work is to follow the Java 
Virtual Machine specifications for compatibility or implement dynamic 
method translation at runtime. I've done that for fun to see if it works 
and it does, but requires translators for each interface and then you 
have loads of more code to manage and TONS of edge cases. But, it was a 
cool weekend project I whipped up to see if it was possible using CGLIB 
and some other tools.

> My question with Don's proposal is with item #4.  Is it new features 
> of the same webapp that use new APIs or new webapps that use the new 
> APIs?  Option one is going to be hard, and may not be possible even 
> with OSGi.  For option 2, I'm wondering if OSGi is overkill (i.e. 
> would webapps be separated enough via the container).  A better 
> question might be is it the webapps that need to change the APIs they 
> are using, or is it the plugins?
Yeah, my thoughts were along the same lines. Not sure OSGi really helps 
that much. Here's the main case I want to cover and OSGi doesn't really 
provide a solution:

    You are writing something that uses Struts2 (plugin, extension, 
fork, webapp, whatever) and want to upgrade from 2.0 to 2.1. Since 
traditionally these are "minor" versions, the changes should be minor, 
meaning they should be compatible. Therefore, you should be able to 
"just drop the JAR in the classpath" and everything should work. This is 
the ultimate definition of compatibility and something I feel Struts 
needs to start ensuring.


> /Ian
> Brian Pontarelli wrote:
>> Yours and mine are the same because you still have implementations 
>> inside Struts for some of the API interfaces. For example, 
>> org.apache.struts2.config.PackageProvider (yeah, I did move this from 
>> XWork to Struts for my example ;), will be an API interface that 
>> applications can implement. Struts2 also implements this API.
>> Another case is a non-interface POJO like PackageConfig. This class 
>> can be instantiated by app1 and then passed back to struts. Likewise, 
>> app4 might instantiate a different version and pass it back. You have 
>> to separate these into namespaces in order to compile Struts if the 
>> versions of PackageConfig are fundamentally different.
>> If we take the concrete example of 2.0.* and 2.1 for PackageConfig, 
>> one is mutable, the other is immutable with the Builder pattern. 
>> However, they have the same package and classname. The issue is that 
>> at compile time Struts needs to import and use PackageConfig. The 
>> only way I know how to pull this off is extract a common interface 
>> (Java interface that is - public interface IPackageConfig) and make 
>> adapters OR to separate them into namespaces.
>>    org.apache.struts.config.PackageConfig_2_0
>>    org.apache.struts.config.PackageConfig_2_1
>> Then you can see the ensuing code inside Struts to deal with these:
>>    org.apache.struts.config.ConfigurationManager_2_0
>>    org.apache.struts.config.ConfigurationManager_2_1
>> And then you need some method of determining the version of the API 
>> that the application is using and then select a configuration manager 
>> based on that.
>> Now, there might be some type of tricks and things that OSGi can 
>> provide to make this happen, but I'm not familiar enough with 
>> advanced bundling and classloading to know how it would work. From 
>> what I understand of OSGi, any bundle can only use a version of a 
>> dependent bundle that is compatible with the version of that 
>> dependent bundle it compiled against.
>> For example, if I compiled against log4j 1.0, I can use 1.3 because 
>> they are compatible at runtime and compile time. However, I can't use 
>> log4j 2.0 because the interfaces changed and things will blow up 
>> nicely at runtime. What OSGi does provide is the ability to have one 
>> bundle using log4j 1.x and another bundle using log4j 2.x in the same 
>> VM. I'm not aware that they have gone any further than that.
>> Ian Roughley wrote:
>>> Is this the case, or was the thinking more like:
>>>                                      - app 1
>>>                 - api 1.0 --{
>>>                /                     - app 2
>>> Struts ---{
>>>                \                     - app 4
>>>                 - api 2.0 --{
>>>                                      - app 3
>>> I think this is how I perceived it.  I also agree with Brian that 
>>> there will be a burden on s2 to provide the necessary features for 
>>> all APIs, but I think it's less of a burden in this layout.
>>> /Ian
>>> Brian Pontarelli wrote:
>>>> Here's a few things I think about when considering API versioning:
>>>> 1. How many implementors are there? It sounds like there will be 
>>>> one - Struts2
>>>> 2. Do you want to allow implementors to implement multiple APIs? 
>>>> Sounds like yes.
>>>> 3. How much is shared between APIs? Probably a lot.
>>>> From what it sounds like, and correct me if I'm wrong, you are 
>>>> looking to do something like this:
>>>> API 1.0----------\
>>>>                 |----------- Struts2
>>>> API 2.0----------/
>>>> If this is the case, it will require some interesting coding 
>>>> tactics. Sun and IBM have some white papers on these types of 
>>>> cases. OSGi will shield the two APIs from each other so there 
>>>> aren't any conflicts, however, the implementor will have the 
>>>> unfortunate task of implementing both. This becomes difficult 
>>>> without proper structure at compile time because struts2 will need 
>>>> to implement multiple interfaces from both versions and these 
>>>> interfaces might overlap.
>>>> I've done some of this type of work before and in order to truly 
>>>> break compatibility between 1.0 and 2.0, you need namespaces in 
>>>> order to allow Struts2 to implement both. Otherwise you get naming 
>>>> conflicts that cannot be resolved by the compiler. I've do things 
>>>> like this before:
>>>> org.apache.struts.api1.SomeInterface
>>>> org.apache.struts.api2.SomeInterface
>>>> This is the same interface, but breaks compatibility between the 
>>>> API versions. Only by separating the namespaces will you be able to 
>>>> implement both at compile time. I've also worked with other 
>>>> situations like this:
>>>> org.apache.struts.api.SomeInterface_1_0
>>>> org.apache.struts.api.SomeInterface_2_0
>>>> What it comes down to is that if you are going to break 
>>>> compatibility at the API level you need to actually create a brand 
>>>> new API. When you look at it from this perspective, OSGi really 
>>>> isn't needed, just nice to have. Since the two API versions are in 
>>>> different namespaces, there aren't any collisions at compile-time 
>>>> or runtime, eliminating the need for bundle separation.
>>>> Having done some of these types of solutions before, I can attest 
>>>> to the pain that they can cause. They can also become complex to 
>>>> manage. Which sorta leads back to my original statements about 
>>>> compatibility. I'd much rather see something like this:
>>>> 1. The APIs locked down
>>>> 2. These APIs called Struts3
>>>> 3. No APIs break compatibility until Struts4
>>>> Therefore, 3.0, 3.1, 3.2, etc are all compatible. Then when Struts4 
>>>> start wanting to break compatibility, you branch Struts3, and start 
>>>> breaking away on Trunk.
>>>> -bp
>>>> Don Brown wrote:
>>>>> As I learn more and more about OSGi, I wonder if it might be the
>>>>> solution to several big problems we seem to have at the moment: poor
>>>>> reloadability and the lack of a solid API.  With OSGi, you can drop
>>>>> bundles in and out of the system at runtime, even running multiple
>>>>> versions of the same bundle side-by-side, but the feature I'm most
>>>>> interested in right now is how it would allow us to put in a proper
>>>>> API while maintaining full backwards-compatibility.
>>>>> Evolving a web framework is hard because apps tend to be written on a
>>>>> specific version, and to migrate them to new versions has two
>>>>> problems: development may not be continuously funded and the upgrade
>>>>> may require too many changes to the application.  On the other hand,
>>>>> if you don't evolve your web framework, you quickly go out-of-date 
>>>>> and
>>>>> lose interest from new developers.  In our case, despite being a
>>>>> relatively new framework, we have legacy code around from 2004 
>>>>> that we
>>>>> can't just remove, yet we want to provide an attractive, modern, 
>>>>> clean
>>>>> framework for new development.
>>>>> The specific issue it hand that I've been thinking about is how to 
>>>>> get
>>>>> a proper API into Struts 2 yet keep backwards compatibility, and I
>>>>> think OSGi might provide a solution.  What about this:
>>>>>  1. Struts 2 and its plugins remain the way they are now - 100%
>>>>> backwards-compatibility
>>>>>  2. An OSGi plugin provides the platform for the next generation 
>>>>> of Struts 2
>>>>>  3. A new API bundle is created, implemented by the underlying Struts
>>>>> 2 framework
>>>>>  4. Old apps can continue to write and deploy code against Struts 2,
>>>>> yet new development can start to use the new API
>>>>>  5. Later, when we want to write API version 2, we create a new 
>>>>> bundle
>>>>> that runs side-by-side the old bundle, both implemented by Struts 2
>>>>> Basically, OSGi would allow us to write a clean layer on top of a
>>>>> framework, much like how Grails builds on Spring, but we get, as a
>>>>> side benefit, all the architectural advantages of OSGi for free.
>>>>> Furthermore, if we do it right, users don't have to know or care that
>>>>> OSGi is under the hood - all they know is they write a jar, drop 
>>>>> it in
>>>>> a directory or upload it via a form and they just installed part of
>>>>> their application at runtime.
>>>>> Don
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