It is wonderful to not be completely shut down this time. A few years ago
making the kind of comments I am making now and I would have been
condescendingly looked at, while you saw people thinking that I probably tried
to hide a deficiency in OSGi. :-)
This is a fundamental computing problem. I love OO since 1981, I came to like
types, but they both have severe limitations once you go distributed (which
includes persisting data). And very few interesting programs do not have one of
those problems.
Kind regards,
Peter Kriens
> On 20 Feb 2017, at 18:32, Matt Sicker <boa...@gmail.com> wrote:
>
> I was thinking about how ActiveMQ added extra security around serialization
> by forcing you to configure which packages it allows for serialization. That
> struck me as similar to import-/export-package and made me think that if
> you're using serialization like this, you should at the very least be
> required to mark your DTOs as publicly accessible. On the import side for
> your service, though, things get out of whack like Peter mentioned with
> different versions of the same bundle running on separate instances (or even
> the same instance).
>
> On 20 February 2017 at 08:24, Peter Kriens <peter.kri...@aqute.biz
> <mailto:peter.kri...@aqute.biz>> wrote:
> The problem is that in all distributed I’ve been involved in there were
> rolling updates. This implies that you cannot guarantee that each server runs
> the same bundle. And clusters that do not do rolling updates seem kind of
> useless because they introduce a huge failure point. So I have a hard time
> understanding how you can guarantee that invariant? Yes, it works most of the
> time but when I learned to work with computers that tended not be good enough.
>
> Now I do understand ( and sympathise) that you cannot change the app. I’ve
> been there.
>
> You indicate you have the same bundle on both sides. So the class graph on
> both sides is equal. This implies that if you ensure that the API interface
> is in the same bundle as the implementation classes then you’re safe as long
> as you use the interface’s class loader as the root loader for classes you
> find. This is of course highly not modular but then you’re not anyway under
> the covers :-) You could also make the API bundle import the implementation
> bundles, that should work as well.
>
> As long as you make sure OSGi has the proper dependency information things
> tend to work out of the box.
>
> Kind regards,
>
> Peter Kriens
>
>
>
>> On 20 Feb 2017, at 11:47, Peter <j...@zeus.net.au <mailto:j...@zeus.net.au>>
>> wrote:
>>
>> Thanks Pete, good to hear from you again, I must admit it's been too long.
>> We last spoke when I was refactoring a class dependency tool to use ASM
>> instead of the jdk's tools.jar. You once asked, how do you find a
>> dependency for calls to Class.forName?
>>
>> The reasons you've stated are also why I've chosen to support a very narrow
>> use case, in which you may have already noted that the serialized connection
>> is between two identical bundles, in separate jvm's with compatible package
>> imports.
>>
>> There’s no intent to support transferring any classes outside of the Service
>> Now API and that includes overriding classes. What you describe about data
>> hiding reminds me of Entry's, which have public fields.
>>
>>
>> I'll be the first to admit there are significant issues with the design of
>> Java's Serialization's extralinguistic api. Ironically though the wire
>> protocol is reasonably well thought out, with regards to evolution.
>>
>> As an exercise to fix security issues, I have reimplemented Java
>> serialization with input validation using a public api, it has backward
>> compatible serial form, but only supports a subset of Java serialization, it
>> doesn't support circular object graphs for example as this would compromise
>> security. It performs input validation, sets resource limits and expects
>> periodic stream resets to avoid DOS and gadget attacks. The problem is
>> there's a lot of existing software that utilises java Serialization, that's
>> going to need support for some time. Things like Serialization and Remote
>> method invocation are damaged by attempts to implement too much
>> functionality, when a more rigid subset would avoid a number of issues. But
>> I guess no one was thinking of modularity and versioning when they created
>> these frameworks either.
>>
>> James Gosling said something once about why Generics weren't included in
>> Java from the outset, which was because at the time they didn't know how to
>> do it properly, it's better leaving it out until you do.
>>
>> JBoss has a nice web page with some graphics that illustrate some major
>> issues with implementation hiding you've mentioned with Serialization and
>> modular frameworks here:
>> https://developer.jboss.org/wiki/ModularSerialization
>> <https://developer.jboss.org/wiki/ModularSerialization>
>>
>> It's worth noting Service API of the smart proxy bundle doesn't need to be
>> Serializable, instead it's relegated to a communication means between two
>> identical bundles in different JVM's. It's also important to recognise
>> that it doesn't need to be the communcation mechanism either. These
>> bundles have an identical class namespace, although there may be variances
>> in package import versions.
>>
>> Yes we are also looking at moving away from java serialization.
>>
>> Also over time, because this is a service interface, at some point down the
>> track, serialization can be replaced, without impacting the public api. So
>> yes the underlying protocols can be stripped out to data and message passing
>> if that's more satisfactory.
>>
>> So yes java serialization is an existing part of our application and it has
>> it's warts.
>>
>> But we've also had a number of users over the years who have requested
>> support for OSGi.
>>
>> This is not a greenfields project, I'm hoping that I'm not going to be told
>> that no, the chasm is too wide you can't cross over to OSGi, rewrite or
>> start again, there's just too many LOC.
>>
>> So you have raised some important questions. Some of our users have had a
>> lot of success with Maven (recognising there a pro's and cons's with module
>> based versioning and transitive dependencies), where versioning on a module
>> level allows codebase annotations to be utilised in remote invocation,
>> avoiding class visibility issues by mapping module ClassLoader's directly to
>> a URI based identity. However with OSGi there's a mismatch between
>> different jvm's and how bundles and packages imports will be resolved will
>> end up being wired, so we can't rely on codebase annotations for OSGi.
>>
>> Jvm's using OSGi frameworks are quite likely to have different dependency
>> graphs (wires) between bundles and their package imports.
>>
>> While I don't expect to solve the worlds problems or boil the ocean, I'm
>> looking for the most workable compromise, one that doesn't promise the world
>> and is easier to explain what users can and can't expect to do. I'm
>> relatively pragmatic. To me it would seem logical that a subset where two
>> identical bundles (that should have resolved similar package import
>> versions) should be a good place to start.
>>
>> Hence my post on this list, as I realise many of you have already spent a
>> lot of time bumping into these issues.
>>
>> Cheers,
>>
>> Peter.
>>
>> On 20/02/2017 6:38 PM, Peter Kriens wrote:
>>> After working in this area for too many years I’ve come to the conclusion
>>> that objects cannot be really transferred to other systems in a reliable
>>> way, only self typed data can. JPA, RMI, and many other systems promise
>>> heaven to the programmer that they can use their objects local and remote
>>> transparently. The consequence of this dream is a huge amount of complexity
>>> that far outweighs any gains in programmer friendliness. Few things have
>>> caused so much trauma in the software world as ORM. (Persistence is
>>> communications to a future process.)
>>>
>>> The reason objects are so complex to use in communications is that it is in
>>> direct violation of the goal of OO to hide your data. However, once you
>>> expose the internal data on the wire you have effectively made it public
>>> but too many people they can still have the advantages of abstract data
>>> types. OSGi is a bitch in this case because it tells you that you’re trying
>>> to do something wrong by refusing to cooperate. In this case, it balks at
>>> you because you create an invisible dependency between the sender and the
>>> receiver. Though this is a good thing too often the receivers of this
>>> message blame the messenger.
>>>
>>> You can handle this dependency but you’ll find out is that it is a hugely
>>> complex task that introduces a lot of frailty in the overall system. Having
>>> tried this several times I can assure you that any gains in programmer
>>> friendliness are dwarfed by the complexity of creating this facade.
>>>
>>> The best solution I found is to give up on data hiding. The fact your
>>> objects is on the wire means that that wire format is public. I therefore
>>> use Data Transfer Objects, in my case objects with public fields. On both
>>> sides I have my own objects to provide behavior to this data with methods
>>> and classes but this data record is at the core of my code. Since this data
>>> is public because it goes over the wire it is better to wrap you code
>>> around that ‘standardized public’ object than to try you internal object
>>> data.
>>>
>>> If you look at the OSGi specifications of the past 5 year then you will
>>> notice that all applicable APIs have been designed to be useful with
>>> Distributed OSGi. Calls do not pass objects but they pass DTOs back and
>>> forth. They do not rely that the receiver and sender have exactly the same
>>> type and version. In this model it is easy to replace an endpoint using
>>> another language, which is a really good sign.
>>>
>>> For Java developers this is often an unpleasant message, and quite often
>>> OSGi get the blame. However, the fact OSGi gives you these problems means
>>> that you’re trying to do something that has hidden dependencies.
>>>
>>> Distributed computing has 7 well known fallacies[1] but I strongly believe
>>> that there is an eighth: ’One can communicate objects over a network’.
>>>
>>> Now your question. Yes, you could run a resolve and load the proper bundles
>>> but you introduce a huge amount of error cases and a large amount of
>>> complexity and you won’t solve the fundamental problem.
>>>
>>> Kind regards,
>>>
>>> Peter Kriens
>>>
>>> [1]: https://en.wikipedia.org/wiki/Fallacies_of_distributed_computing
>>> <https://en.wikipedia.org/wiki/Fallacies_of_distributed_computing>
>>>
>>>
>>>> On 20 Feb 2017, at 05:13, Peter <j...@zeus.net.au
>>>> <mailto:j...@zeus.net.au> <mailto:j...@zeus.net.au
>>>> <mailto:j...@zeus.net.au>>> wrote:
>>>>
>>>> Hello,
>>>>
>>>> I'm currently working on converting an existing application to OSGi.
>>>>
>>>> This application has a network service architecture based on java
>>>> interfaces. I've broken the application into modules, using a Maven
>>>> build, which uses bnd and bndtools to create bundle manifests. Some of
>>>> these modules are ServiceLoader provider's, so I've used annotations to
>>>> ensure these are loaded into the OSGi service registry using the Service
>>>> Loader Mediator.
>>>>
>>>> The main issue that I face is this application is a networked application
>>>> and has it's own Remote Invocation protocols (which currently utilise Java
>>>> Serialization, but not Java RMI). As you'll appreciate, class visiblity
>>>> is a little different in OSGi. :)
>>>>
>>>> The services mentioned above are remote services, these remote services
>>>> have a proxy which implements the service interface, these services are
>>>> discovered and installed at the client. There are two types of proxy's,
>>>> one, called a smart proxy, requires a codebase from which to retrieve a
>>>> jar or jar files that are downloaded and installed at the cleint
>>>> (traditionally during deserialization), the other type of proxy is called
>>>> a dynamic proxy (it's basically just an instance of
>>>> java.lang.reflect.Proxy), which is dynamically generated at the client.
>>>>
>>>> The Service implementation is broken up into three components:
>>>>
>>>> 1. The service api
>>>> 2. The smart proxy (resolved and provisioned into in client jvm).
>>>> 3. The server
>>>>
>>>> The server bundle imports packages from the smart proxy bundle, while the
>>>> smart proxy imports packages from the service api as well as exporting
>>>> it's own packages, as required by the server bundle.
>>>>
>>>> The server that provides the remote service has three bundles loaded;
>>>> server-impl, smart-proxy & service-api.
>>>>
>>>> The client only has the service api bundle installed at deployment and the
>>>> smart proxy is resolved and provisioned before the service is made
>>>> available via the local OSGi service registry, where the client will learn
>>>> of it's existence using ServiceTracker.
>>>>
>>>> At first glance only the smart proxy bundle needs to be provisioned at the
>>>> client, however for cases where a dynamic proxy is required to implement
>>>> interfaces from different packages, where class visibility issues may
>>>> exist, it may be beneficial in these cases to utilise and provision a
>>>> proxy bundle that imports all these interfaces, one might do that by
>>>> taking advantage of java's interface multiple inheritance; create a bundle
>>>> that contains one interface (annotated with @ProviderType) which extends
>>>> all interfaces, which the bundle doesn't export, so we ensure that the
>>>> dynamic proxy has a proper bundle manifest with all package imports and
>>>> version ranges correctly defined.
>>>>
>>>> The inbuilt remote invocation protocol has server and client endpoints,
>>>> the protocol is extensible and has a number of implementations (for
>>>> example https, http, tls, kerberos, tcp). Each endpoint is assigned a
>>>> ClassLoader when it's created.
>>>>
>>>> For classes installed at the client, these are typically installed in a
>>>> URLClassLoader, typically with the Application loader as parent loader.
>>>> In an OSGi environment however, the smart proxy bundle will be installed
>>>> at the client, it's ClassLoader utilised by the client endpoint, the smart
>>>> proxy bundle will also be installed at the server and it's ClassLoader
>>>> utilised by the server endpoint. In this case the visibility of the
>>>> bundles at each endpoint will be utilised to resolve serializable classes.
>>>> Private smart proxy serializable classes will be resolvable at each
>>>> end, but only public classes from imported packages will be
>>>> deserializable, since the client interacts using the Service API, all
>>>> serializable classes in the Service API packages will need to be exported
>>>> and public and imported by the client and smart proxy.
>>>>
>>>> Once a bundle has been provisioned its ClassLoader will be given to the
>>>> client endpoint and the marshalled state of the proxy unmarshalled into
>>>> it. At this point the service that the proxy provides would be registered
>>>> with the OSGi service registry for the client to discover and consume.
>>>> The smart proxy communicates with it's server via an internal dynamic
>>>> proxy (java.lang.reflect.Proxy), it's used to invoke methods on the server.
>>>>
>>>> While the existing protocol uses Java serialization, it doesn't use Java
>>>> serialization's method of resolving classes. Java Serialization walks the
>>>> stack and finds the first non system classloader (looking for the
>>>> application ClassLoader). The existing class resolution method isn't
>>>> suitable for OSGi, however the mechanism is extensible, so can be replaced
>>>> with something suitable.
>>>>
>>>>
>>>>
>>>> Does anyone have any advise or experience utilising the OSGi Enterprise
>>>> Resolver Service Specification (chapter 136) and the OSGi Enterprise
>>>> Repository Service Specification (chapter 132) to resolve and provision a
>>>> bundle for the smart proxy at the client?
>>>>
>>>>
>>>>
>>>> The intent here is the bundle manifests at each endpoint will be used to
>>>> determine class visiblity, so the resolution and provisioning process will
>>>> be of critical importance.
>>>>
>>>> For anyone curios, the application is a fork of Apache River / Jini and
>>>> I'm experimenting with support for OSGi. I'm also a committer and PMC
>>>> member of Apache River. This isn't the old Jini we all know and love
>>>> however, there are some additional features that allow provisioning to
>>>> occur using a feature called delayed unmarshalling, so we can avoid the
>>>> need for codebase annotations and URLClassLoaders.
>>>>
>>>> The work in progress can be found here, for anyone who's curious:
>>>>
>>>> https://github.com/pfirmstone/JGDMS/tree/Maven_build/modularize/JGDMS
>>>> <https://github.com/pfirmstone/JGDMS/tree/Maven_build/modularize/JGDMS>
>>>>
>>>> Regards,
>>>>
>>>> Peter.
>>>>
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>>>
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> Matt Sicker <boa...@gmail.com <mailto:boa...@gmail.com>>
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