Dan Creswell wrote:
Ah, I know Sim is gonna hate this but I feel the need to retain full context
for now....
On 1 February 2011 10:55, Peter Firmstone <j...@zeus.net.au> wrote:
Dan,
I've appended the updated class after our earlier discussion,
MarshalledServiceItem, now only has one method as suggested.
An option is to consider giving Entry's their own jar files, and to
consider doing something about providing a new codebase URL Handler, for
separating identity from location, so we can discover the same jar files in
other locations,
I don't think I understand the separation of identity from location fully
yet so let's try a framing statement:
A Service's ID and it's type plus some minimum set of Entrys to identify
pertinent service details is enough isn't it?. And any client wishing to
identify such a service would require those minimal Entry's (agreed it
mightn't want the others, could filter that at Lookup Service side such that
it never gets to the client thus doesn't need downloading).
I don't see the need to play marshalling games so much as delay opening up
the proxy and limiting the Entry's a client sees to those it declares
interest in.
Clarification: Separate the location and identity of a jar file or
archive containing class files:
We have integrity constraints, which rely on message digests to confirm
files have not been tampered with.
The message digest (although now considered a weak form of encryption),
is the identity of a jar file, well we use this type of information to
identify it's the jar we expect.
For httpmd, we use a URL string annotation (which could be an IP address
and port, or a DNS hostname and port), a path and file name, followed by
the message digest.
The IP address or dns hostname , port and path represent the location of
the jar file, while the file name and message digest represent it's
identity.
If we separate the location + port, it can be discovered using DNS-SRV
records, allowing redundant codebase servers, while the identity is
limited to the file name and message digest.
Then the RMI codebase property only needs to be a domain in which a
suitable codebase can be discovered and queried.
similar to Codebase Services with message digests or Maven Provisioning as
suggested by Dennis. This doesn't mean I don't support the idea that the
best location for a codebase is at the originating service node, just that
it's likely that many service proxy's could share identical jar files, so
why should the client re-download something it already has?
But, if service proxy's are sharing jar files, what does that mean? Someone
somewhere chose to package them all together like that for some reason. What
reason, what problem are they solving?
Deployment reasons.
Again, I'm sitting here thinking as a deployer of services, if I want to
widget around and consolidate .jars I can do that ahead of time and then
tweak service codebases via config just prior to deploy.
But it would be faster for the client to receive unconsolidated jar's,
since other services might use some of these jars also.
What matters is the proxy gets the correct class files, in its own
private namespace that are not shared with other proxy's (except for
service api, which may include Entry's), but we can save duplicate
downloads.
Maven provisioning is interesting.
This is why I'm interested to investigate separating jar file identity
from location, to simplify deployment and redundancy. I'm putting my
thoughts out on the list, to gather responses, to see if there's a
better way.
Jini's lookup service lack of AND / OR querying capability is due to
security, the avoidance of instantiating foreign objects.
Delayed unmarshalling of the service proxy allows service entry's to be
compared as objects, without requiring a codebase download for the proxy if
it's not the service we want, so it's not quite just returning a
MarshalledInstance. This should be done without compromising the good
security features of the existing lookup service.
I don't follow - I could tweak ServiceItem to hold the proxy as a
MarshalledInstance and still expose all other "service identifying
information". That MarshalledInstance mightn't even immediately carry the
proxy code, could still be on the server and pulled down at point the
consumer actually wants the proxy. Feels like some simple
interface/sub-classing.
This is true for cases where the service types don't matter to the
client, I think Gregg wanted to elimate all codebase downloads until he
was sure he had the correct service. Use of a MarshalledInstance could
be an acceptable compromise, if Entry's have their own codebase
annotations. How would this affect the lookup semantics if we're
looking for particular service types though?
Over the internet, we could potentially have very large lookup services, by
allowing clients to remove unwanted services from their results before
unmarshalling, we can reduce the resources required of the client:
* Network Bandwidth, clients don't need to download unwanted codebases.
* Memory (ClassLoader and unwanted classes are not loaded into memory).
If we were to go across internet have we squared away use of e.g. DNS-SD?
i.e. Is it a given we'll expose a classic JINI LUS?
The semantics of DNS-SD make it well suited to discovery of a lookup
service. I think Sim highlighted earlier that its difficult to get
domain administrators to do Dynamic Updated DNS-SD, they're comfortable
with DNS-SRV records, so it would appear easier to rely on DNS-SD as a
lookup locator / domain browser, not as a lookup service replacement.
Cheers,
Peter.
