Yes that's the beauty of Services, they provide opportunity for
pluggable replacement implementations. That's the "Service
Pattern" As we have seen it is possible to use the Service
Pattern to solve a number of different problems. Eg Netbean
Plugins, SPI, OSGi, Jini.
I'm looking at OSGi to wire up services inside the JVM as you
say. When I say package, I mean a java package residing in the
local JVM it may or may not be part of a Jini service, it may be a
purely local JVM package, eg a library dependency or local domain
package. For example, I have package X, version 1 loaded in my
local JVM, I need to have package X version 2 loaded as version 1
isn't compatible with the new Objects (domain data) I'm recieving
in serialized form. I need to share this information locally with
Package Y that currently has references to objects in Package X
version 1. The Objects in Package X version 1 that Package Y
references need to have their class files upgraded. Without OSGi
I can do this by persisting state, stopping the JVM, restarting
and loading package X version 2.
I'm not looking at distributed OSGi, but I can see a use case for
utilising a Jini Service, when a local OSGi bundle that performs
some task that could be done optimally if the processing can be
moved to where the data resides, this is just an example there are
probably 10 other ways of doing this:
A local application bundle that provides an OSGi service locally
queries a remote database using JDBC and performs a considerable
amount of manipulation to that data prior to returning a subset.
The query and its result are sent over the network using a
database JDBC connection.
The processing for that data, if shifted to the machine that has
the database data, would consume significantly less network
resources. EG the data transferred over the network is reduced by
a factor of 100 by processing the data on the database machine
after querying. A bundle that provides a "local JVM application"
an "OSGi service" could utilise a "Jini Service" to request the
data be processed at the Database machine in a particular manner
before receiving the result. This function could be locally
available as an OSGi service to some other local application, that
application doesn't need to know about Jini, it is an
implementation detail that is abstracted.
My objectives are all based around codebase services (objects
aren't locked to their http codebase origin), in combination with
OSGi or something like it, to ensure compatible classes and
packages are loaded among separate JVM instances. Yes Newton does
something similar, however it is AGPLv3 licensed.
I envision a distributed environment where nodes can have the
majority of their packages downloaded and upgraded via codebase
servcies. Providing an evolving cluster, that upgrades it's
bundles incrementally, while maintaining the maximum level of
class and package compatibility. Think Agile Cluster Running
System component upgrades.
People, who are jumping in now because I've mentioned OSGi, are
making assumptions and haven't been following the discussions I've
posted previously about Versioned Classes, Classloader trees,
Static Analysis and Codebase Services, this is frustrating as I
was hoping for some participation. It seems I can only get
attention when I mention a controversial subject. What I want is
attention to solving the problems that will make River better.
In my note below when I'm referring to the "Service Pattern", I
mean the service pattern that OSGi implements, enables bundles to
be upgraded by loading the replacement bundle in a new
classloader, The service is a common interface, the new upgraded
service is discovered after it is started. The alternative is to
use delegates to update references between objects when the
Classloader changes as per some of the other patches I've uploaded.
Jini also utilises a "Service Pattern", but to solve a different
problem.
I knew this was going to be a difficult topic to present.
What we need are separate lists, where people who want to
participate in constructive development to solve problems can do
so and another list where people can pontificate about software
ideals and have disrespectful arguments with each other without
holding up development. While we're developing we can keep an eye
on the argument list without getting embroiled.
Anyway I've said enough, I'm going back to doing the things I need
to do, if someone who has been following my posts to date has
implementation ideas, but are afraid to mention it, please feel
free to contact me directly to discuss, I do need some input to
gain confidence that I'm approaching these problems in the right
manner.
Peter.
Dennis Reedy wrote:
On Nov 8, 2009, at 1251AM, Peter Firmstone wrote:
I had avoided OSGi purely due to the controversy it generates on
this list, however without the Service Pattern one cannot
upgrade a package without first persisting everything and
shutting down the entire JVM, then restarting. At least OSGi
allows you to stop a bundle and any dependents, persist what you
need to then start with a later bundle version if desired,
without having to persist or shut down the entire JVM.
If thats all you want you dont need OSGi. Service lifecycles are
supported with a variety of container approaches, from JEE,
Spring to Rio. You also do not need to shutdown the JVM to load
new service classes.
Adopting OSGi as a micro-kernel architecture for wiring up
services inside the JVM is a different thing. Looking at
distributed OSGi is a totally different thing on top of that.
IMO, if you want to consider OSGi for River, you focus on the
former, not the latter.
