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From Dain Sundstrom <d...@iq80.com>
Subject Artifact patch system [was Re: replacing tomcat classes]
Date Thu, 11 May 2006 18:39:16 GMT
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


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