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From Mark Thomas <ma...@apache.org>
Subject Re: [Proposal] Preparatory refactoring for Resource handling refactoring
Date Sun, 19 Aug 2012 18:35:08 GMT
On 17/08/2012 16:50, Konstantin Kolinko wrote:
> (regarding DefaultServlet)
>>> If it were using the servlet API only, the code were more reusable,
>>> both inside and outside of Tomcat. There must be specific issue why
>>> Servlet API is not used there. Is the API lacking?
>>
>> In lot of ways. For example, where is the API to create or delete
>> resources? That is deliberately left as a container implementation detail.
>>
> 
> OK.  One way is to use getRealPath() + manipulate files on the file
> system directly. I agree that using the API (be it DirContext or
> something else) is a bit more nice.

getRealPath() won't work for read/write storage that isn't backed by a
file system (e.g. database backed)

It also gets rather expensive when you have to deal with static
resources from multiple sources. You end up needing a several calls to
work out a) if the resource exists, b) if it can be put / deleted
depending on whether the resource is from a resource JAR, collides with
one from a resource JAR etc. Not impossible, but you need quite a few
calls to get it right. This gets more complicated when you add overlays
to the mix.

>> The jndi layer has caused performance problems as well. The special
>> handling for JARs is a direct result of that. The new approach will
>> simplify a lot of that and hopefully improve performance as well. Like
>> we have with the current approach, if there are specific areas causing
>> problems, we can take a look. I hope and expect that far fewer of these
>> 'tweaks' would be required with the new implementation.
>>
>>>>> One good thing with "jndi:" URLs returned via Servlet API is that they
>>>>> are backed by an instance of ProxyDirContext class and it has a cache
>>>>> (*).  If we change implementation and return "true" URLs, they will
>>>>> bypass the cache.  I suspect that using a "jar:" URL directly (in case
>>>>> of an unpacked WAR file) may have poor performance.
>>>>
>>>> The new Resources implementation will include caching where necessary.
>>>> At the moment there is none. I intend to add it as required. I agree
>>>> JARs/WARs are likely to need it to have performance that is comparable
>>>> with expanded WARs.
>>>>
> 
> Regarding this caching:
> 1. Currently calling getInputStream() on a jndi URL may return
> resource directly from memory (cached by ProxyDirContext).
> 
> If a file URL is used that would involve direct reading from hard
> drive.  If a jar URL is used - I wonder whether opening of the JAR
> file by the system had to be performed regardless of independent
> access to the same file.
> 
> One may argue though that there is
> ServletContext.getResourceAsStream() method available.
> 
> 2. If I query some attributes of the resource via
> getResource().openConnection().getHeaderField(),  for jndi URLs they
> are returned from a cache. I wonder what performance will be there
> with file and jar URLs.

I remain to be convinced how much caching is required. My preference is
to add a very simple (possibly even NO-OP) caching layer that we can
extend as necessary to address actual performance issues rather than
guessing where they might occur.

>> I can put a patch of the changes so far on people.a.o if you are
>> interested (note: it won't compile yet).
>>
> 
> I would like to look and review.  I think that at the current point
> this refactoring should not be done directly on trunk without some
> review and consensus first.

I've almost got something working. I'll see if I can finish it off
first. Either way I'll aim to post something this coming weekend.

> Maybe create a branch for this work (create a "tomcat/branches/"
> directory, or use existing "tomcat/sandbox")?

I'm happy with that if required. I suggest we hold of on the how until
folks have had a chance to look at the patch first.

Mark


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