[Yanel-dev] New XMLDB repository

[Andreas Wuest]

Hi Josias

On 12.2.2007 15:55 Uhr, Josias Thöny wrote:

> Andreas Wuest wrote:
>> Hi
>>
>> I've finished and checked in a basic implementation of the XMLDB 
>> repository, based on the XML:DB API.
> 
> Cool :)
> 
>>
>> Unfortunately, Yarep is documented really bad, so I couldn't find out 
>> what the exact contracts for the various methods are. For example, 
>> should getSize() or delete() throw a repository exception if the 
>> resource does not exist, or return 0 or false, etc.
>>
>> I've extensively documented the XMLDBStorage class, so you can see 
>> what it does on the first glance.
>>
>> The Reader/Writer and InputStream/OutputStream are implemented using 
>> aggregation. Don't know if it would be more desireable to e.g. 
>> subclass StringReader and override the close() method instead.
>>
>> Also, there are some other API related problems: Yanel always seems to 
>> call getInputStream to directly read from the repo. Now, this is all 
>> fine and dandy on a file based repo, but the XML database stores XML 
>> documents as character data, and returns them as strings. With other 
>> words, in order for the OutputStream to work, we have to convert the 
>> string to bytes, which, of course, involves character encoding. I just 
>> use UTF-8 to de- and encode, but of you really want to read an XML 
>> resource, the getReader method should be used.
>>
>> The same goes for writing, but with some additional complication. You 
>> should NEVER use getOutputStream to write an XML document. 
>> getOutputStream creates a binary resource in the database. Use 
>> getWriter instead to write character data, which creates an XML resource.
> 
> Well, I didn't realize that some repository implementations might handle 
> binary data differently than text data. But I guess it makes sense.
> So probably we should change yanel to use the reader/writer methods for 
> text data, and add reader/writer methods to the node-based api, too.
> Would that help?

That would help for sure. Although I don't know how Yanel can find out 
which method to call for reading, because it does not know in the first 
place if a requested resource is character-based or binary.

One possible way would be for the repository implementation to guide 
Yanel, because the repository should generally know what type of 
resource is being requested (at least, XMLDB knows, we may see other 
back-ends in the future which do not even know this one though). If 
Yanel uses getInputStream(), and the repo decides that this is not a 
binary resource, it could throw an exception, and Yanel would then try 
getReader(), or vice versa. We could also introduce a flag on those two 
methods, e.g. forceRead, which would prevent the repo impl from throwing 
if the resource to be read is of the wrong type, but read anyway.

For writing, there should basically be no problem, since Yanel can 
decide based on the MIME-type if it is going to write a character-based 
or a binary resource.

-- 
Kind regards,
Andi