Hi Dennis,
It sounds like you remain unconvinced or don't have a need to use common
interfaces for your service proxy's?
So I guess it's a tread lightly approach, make the feature available to
those that want common Service API (The same interface instance for all
proxy's implementing that interface, so they can be used in collections
or batch operations), for maximum sharing of proxy's with differing
implementations, but let those that don't want to publish their API
continue doing what they usually do. A configuration parameter should
be able to set the desired behaviour.
The approach I've taken is a simple approach to a complex problem,
alternative approaches leave the complexity in the hands of the
implementer, my approach will enable them to practically ignore it.
Before you write it off though, to answer your earlier question, I'll
further explain the ClassLoader structure between multiple nodes.
CLIENT NODE
________________________________________________
| |
| System |
| ClassLoader |
| | |
| Extension |
| ClassLoader |
| | |
| Jini Platform & |
| Service API |
| ClassLoader |
| | |
| _________|___ |
| | | |
| Application Smart Proxy |
| ClassLoader ClassLoader's |
|________________________________________________|
SERVICE NODE
________________________________________________
| |
| System |
| ClassLoader |
| | |
| Extension |
| ClassLoader |
| | |
| Jini Platform & |
| Service API |
| ClassLoader |
| | |
| _________|_____________________ |
| | | | |
| Service Imp Smart Proxy Parameter Impl |
| ClassLoader ClassLoader's ClassLoader's |
|________________________________________________|
All Proxy and Service implementations are free to vary at will, proxy
instances can be shared in collections or iterative operations based on
common Service API supertype's All Proxy's and Services are isolated in
their own Domain, the only way to communicate externally is by using
interfaces and classes in upper level ClassLoaders, they can utilise as
many third party libraries jar archives as they need, these will all be
loaded into a single ClassLoader unique to that Service or Proxy's
Codebase and Principles. The Service or Proxy's namespace will be
totally separate from Application or other Proxy implementation's,
except in the case where sharing is permitted for identical
implementations by the implementation developer.
Service API should be carefully considered and designed, it forms the
basis of network Dependency Injection, you can discover ANY Service
implementation variant using the same Service API.
The Service API may include other Service API or Java platform classes.
public interface SimpleBookService {
public Book get( Library lib, String name);
}
In the SimpleBookService above, given a Library and String name of a
Book, a Book instance is returned by the proxy.
The contents of simpleBookService-api.jar:
SimpleBookService.class
Book.class
Library.class
Let's say a client has extended the Library, with a class called
PrivateLibrary, this class is an implementation class by the client,
that the Service knows nothing about, the Service only uses the Library
API, but it needs the PrivateLibrary.class
The client makes an archive containing the PrivateLibrary.class publicly
available in an archive called privateLibrary-param.jar. This archive
depends on simpleBookService-api.jar The jar URL is marshalled with the
class when the parameters are sent back to the service. When it gets to
the service implementation, privateLibrary-param.jar is given it's own
ClassLoader and ProtectionDomain and isn't given any Permissions. It is
used by a SimpleBookService implementation to decide which Book to
return to the client, after which, it's garbage collected, eventually if
PrivateLibrary isn't used again its ClassLoader is garbage collected too.
Each SimpleBookService proxy implementation will have their own
ClassLoader namespace, but Client Application classes can still use any
Service implementation or as many Service implementations as it can
handle at the same time, all the while treating them as the same Type.
One SimpleBookService implementation proxy, contains its own
implementation of Book, and while the client is reading the book, it
remains available from the proxy ClassLoader via the Service API Book
interface, when finished reading the book, the proxy, book and
ClassLoader can be garbage collected. Tomorrow, the client might read a
book from another SimpleBookService
Most Service API will be relatively small bytecodes as most will be
abstract or have simple implementations, the fact they're not garbage
collected, doesn't matter much if the function of the node doesn't
change, the API will remain limited to the subset in use, by the node.
Note that Jini Platform service implementations like Reggie, Outrigger
etc, will exist in Service Impl ClassLoaders and Smart Proxy
ClassLoaders, only the api will be in the Jini Platform ClassLoader.
Think of it as an expandable platform, everything is shared using
implementations of Service API classes.
It's actually a good policy to be liberal with interfaces when building
the Service API classes, even with parameters and return types. Since
extending interfaces is relatively straight forward, you new interfaces
will be discovered by older client software as the old interface while
new implementation code discovers the new interface of your service.
Older nodes will load your new interface classes into the Service API
space when your new proxy versions are unmarshalled. That's why it's
important to maintain backward compatibility in the Service API space.
So best practise would be to create an experimental djinn group until
your interfaces are stabilised and be prepared to restart your
experimental group on a regular basis.
I have thought about using OSGi for Service API classes to be served up
so they can be garbage collected, this might work for serialization too
using the context ClassLoader. I have also thought about using
ClassLoader Tree's using bytecode dependency analysis as per Tim
Blackman's research. These things start to get very complicated, just
to be able to flush the Service API classes. Wouldn't it just be better
to use mutiple services that are load balanced, enabling the jvm to be
restarted if we want to?
There are other ways to make the Service API classes garbage
collectable, such as having a tier filled with Service API ClassLoaders
where each Service and Proxy lives in a child ClassLoader in the tree,
however this presents the problem of what if an application wants to use
many Service API's or a combination, the different Service API classes
couldn't see each other from separate ClassLoaders.
Something to consider, best regards,
Peter.
Dennis Reedy wrote:
On May 25, 2010, at 710PM, Peter Firmstone wrote:
This is a good question, which gets to the heart of the Jini's pattern.
I think the proposed ClassLoader structure will benefit Rio, by enabling
increased API commonality and class sharing among Services and their clients.
You can get around having to shutdown your jvm if you manage evolution of your
API interfaces correctly, set up a separate testing Registrar, to keep new API
interfaces out of your deployment, until they have stabilised.
Right now the JVM doesnt need to be shut down at all, services can be loaded
with different versions, unloaded, etc ... I think you're making assumptions
here.
Classes, once loaded into a ClassLoader, cannot be garbage collected, but if
your API classes don't change there is no problem, when was the last time
ServiceRegistrar changed it's public API? Unlike Jini's platform classes
which are set in stone, new API classes can be introduced into older
environments.
Right, which is why service implementations get loaded into their own class loader. You define the 'platform' as whatever that needs to be for your case. For Rio it includes requisite bootstrapping and infrastructure technology. For River it most likely just includes the River 'platform', or nothing at all.
Consider ServiceStarter and the class loader created from that bootstrapping
process. Please explain what is missing from that approach? Each service has
it's own security policy. Why does this need to change? What and how does your
approach improve on? To my eyes it seems overly complicated.
Lets take Jini Platform services as an example, in Rio's ClassLoader tree
below, the Interfaces for the Platform services exist in the CommonClassLoader,
all classes in the CommonClassLoader are visible to any class in any child
ClassLoader below in the tree.
Platform services can be shared freely among all child ClassLoaders.
Now take Service-1CL and Service-2CL, lets imagine for a moment that these two
services both provide the same service, from different or the same node, it
doesn't matter, let's imagine now another node with the same ClassLoader tree
structure, which consumes these services.
These services have their service interfaces bundled with their CodeSources,
both on the client and at the Service, lets say that Service-2CL provides the
same service, but has a different implementation. Now there's a client service
that consumes these services, performs an operation then discards the service.
Now which common API do the two service proxy's share?
Common API? The service proxies dont share anything. They are each loaded from
an implementation of RMIClassLoaderApi
This forces you to load both proxy's into the same ClassLoader, making their
implementations visible to each other and the client.
Not so sure about that Peter.
By separating the API into, in your case the CommonClassLoader,
APIs are not added to the CommonClassLoader, and I would argue that it should
not happen. You generally do not want to add classes into a class loader that
does not get GC'd.
each with their own ProtectionDomains, all Services and clients in that node,
share the same API classes and can be isolated in their own ClassLoader's and
can have different implementations but share the same common API types.
The client service-param.jar is for clients who create new implementations /
extend parameters in API methods, the Service server node will require these
classes to unmarshall the parameters. Client parameter classes will never be
granted permissions.
I'll make up some separate ClassLoader tree diagrams showing the client node,
the service node and the relationships between remote ClassLoaders.
Peter.
Dennis Reedy wrote:
If I understand correctly I think this is the crux of the issue. I dont
understand why you need to load all API classes with the same class loader.
FWIW, in Rio we handle the loading (and unloading) of services with the
following structure
(http://www.rio-project.org/apidocs/org/rioproject/boot/package-summary.html#package_description):
AppCL
|
CommonClassLoader (http:// URLs of common JARs)
+
|
+
+-------+-------+----...---+
| | |
Service-1CL Service-2CL Service-nCL
AppCL - Contains the main() class of the container. Main-Class in manifest points to com.sun.jini.start.ServiceStarter
Classpath: boot.jar, start.jar, jsk-platform.jar
Codebase: none
CommonClassLoader - Contains the common Rio and Jini technology classes (and
other declared common platform JARs) to be made available to its children.
Classpath: Common JARs such as rio.jar
Codebase: Context dependent. The codebase returned is the codebase of the
specific child CL that is the current context of the request.
Service-nCL - Contains the service specific implementation classes.
Classpath: serviceImpl.jar
Codebase: "serviceX-dl.jar rio-dl.jar jsk-lib-dl.jar"
Certainly not as sophisticated as OSGi (or what you are targeting), but it meets the requirements of allowing multiple service versions, applying security context per class loader using the same approach as ActivateWrapper, and allows the JVM to stay running.
