I think that technically this is a very easy problem to solve. The
difficult problem will be our recommendation to users on how to use
the feature.
Technical
---------
I think the best way to implement this is to add another method
getPatches(Artifact artifact) the Repository interface (this is
analogous to the getDependecies method which appends to the class
path). Then we just modify the class loader building code in
Configuration to put the patch jars before the artifact in the
classpath.
One thing to note, this proposed patch system will only address class
loading of artifacts. If a user wants to patch a library inside of a
web application, they will need to modify the web application
directly. This is particularly tricky since the load order of jars
in WEB-INF/lib is not specified and updating it requires a full
redeploy (not a restart as most would expect). Additionally, this
system would not address patching resources inside of a war. For
that, the user would have to overwrite the files in the unpacked
deployment.
The only tricky part of implementing this system will be deciding how
we want to associate patches with artifacts. A single flat directory
is easiest for users, but it difficult to avoid name collisions. It
would be very easy for us to have some sort of foo-1.1-23456.patch
files in the normal repository structure, but that requires an
administrator to know where to put files which is error prone. I'm
personally leaning toward the single patch directory simply because
it will make it easier for admins to see which patches the server has
installed.
Finally, this system will impact any tool that is using the
repository. I'm specifically thinking of the plugin packaging and
download code which will have to be modified to grab the patches. I
also suspect it will effect the eclipse tooling also.
Recomendations
--------------
I agree with David that it is a bad idea to replace only a few
classes in a jar. The process is inherently error prone, and only
provides a very risky stop gap measure. I also agree with Matt that
it is important be able to patch just a few classes in an emergency,
and as soon as the emergency is over, work should start to roll the
changes into a full jar update.
I think we should recommend that our users don't use the patch
feature unless there is an emergency. Further, I don't think this
project should ever ship class level patches, since it is so easy for
us to ship a whole jar.
BTW, does anyone know if maven has a patch system in the pipeline?
-dain
On May 11, 2006, at 9:44 AM, David Jencks wrote:
On May 11, 2006, at 9:16 AM, Joe Bohn wrote:
Bumping up the version should work for the jar approach. However,
I was still trying to figure out a way to honor the tomcat
recommendation of replacing just the modified classes. Is there
some way to make the version independent classloaders pick up
individual classes rather than entire jars?
No, and I think that's a good thing. I think the tomcat team is
giving bad advice.
thanks
david jencks
Joe
David Jencks wrote:
On May 11, 2006, at 8:29 AM, Joe Bohn wrote:
Thanks for the quick response Jeff.
I like the idea of a "system patch" location in the classpath
where we can pick up patches for anything we might include in a
geronimo assembly.
I think this "system patch" idea will only work in environments
with only one classloader, i.e. not geronimo. The problem is
that the patched classes need to get into the correct
classloader, "before" the normal versions. We'd need a patch
directory for each module. I also think any solution that
relies on the order of stuff in a classpath is inherently
unstable and unreliable.
Basically I think this is a terrible idea and we should avoid it
at all costs. I think instead we should use our new version
independence and replace jars with patched jars with slightly
higher version numbers. IIUC this is what you propose doing
below. This should not require removing the standard tomcat
jars: the hight version number should be enough to get the
correct version picked up.
thanks
david jencks
I too was confused by the tomcat recommendation but it does
seem that they have a strategy for addressing necessary changes
with minimal interference in tomcat. I have also noticed some
things that make me wonder if my local tomcat build of 5.5.15
really does match the official 5.5.15 build. For example, the
only source for 5.5.15 that I could find was a zip file rather
than a svn branch or tag. I am not able to build from the
unpacked zip without making a change to move the contents of
jasper/jasper2 into the jasper directory itself. And the
version that is displayed when I hit tomcat with my rebuilt
image is 5.5 rather than 5.5.15 as with the official image.
Until we figure out the correct approach for Geronimo I'm
thinking of using a compromise solution. The changes I need
in tomcat result in 4 of the 13 tomcat jars getting rebuilt.
Rather than replacing all of the tomcat jars with my local
build I have verified that replacing just the 4 changed jars
appears to work fine. I'm hoping this hybrid solution keeps
most of the official tomcat image and our local changes. I
haven't noticed any problems. Assuming the source is mostly
identical (apart from our changes) does anybody know of a
reason that I should definitely not take this approach?
Joe
Jeff Genender wrote:
Ultimately, we probably would need to somehow build a "patch"
directory
or lib directory where we can ensure the URLClassLoader picks
that up
before all other classes. I think this is probably a good idea
to have
as well, so that we could release "service paks" or patches.
I would be
interested in others' thoughts on this, but I think this would
be a nice
feature to have.
Right now I think your only choices are to either hard set a
classpath
to be sure the patches get picked up first or build a hacked
Tomcat
version, or rebuild Tomcat. Dain or David Jencks may be able
to verify
if the classpath solution would work or not as I have not dug
into the
new G classloaders to know if this would even be possible.
The best solution right now may be to just build TC. I am a little
confused as to why the TC guys say not to build the Tomcat
from source
(after its hacked). It seems like just an ant build script, so
I don't
understand why this is being discouraged. This way you can
replace the
Tomcat jars in the repo and you are good to go.
Jeff
Joe Bohn wrote:
Jeff,
I am working with a user that is moving some applications
from tomcat to
geronimo. Due to some problems they have had to modify
tomcat source.
I was chatting with jasonb on the tomcat irc channel and he
recommended
that we only build the classes rather than rebuilding all of
tomcat. He
discouraged rebuilding all of tomcat because there are many
permutations
that can result in different build images and we should run
with as much
of the official tomcat build as possible to avoid problems.
He also
indicated that Tomcat's directory structure provides a place
to put
these "patch classes" in CATALINA_HOME/server/classes .
Is there a similar place that we can put classes when tomcat
is running
under geronimo to have them picked up? Adding the tomcat
classes to our
new sharedlib doesn't seem to be the right place because it would
require a dependency from the tomcat config on sharelib. The
net result
would be that all tomcat apps would potentially pick up user
classes
added in sharedlib even if the user only intended these
classes for some
subset of the apps.
Joe
--
Joe Bohn
joe.bohn at earthlink.net
"He is no fool who gives what he cannot keep, to gain what he
cannot lose." -- Jim Elliot
--
Joe Bohn
joe.bohn at earthlink.net
"He is no fool who gives what he cannot keep, to gain what he
cannot lose." -- Jim Elliot
