[jbosscache-dev] Critical CacheMarshaller issue

[Manik Surtani]

On 8 Nov 2007, at 03:00, Jason T. Greene wrote:

> Manik Surtani wrote:
>> A nasty bug, spotted by someone in the user forum (initially as a  
>> CCE)
>>    http://jira.jboss.org/jira/browse/JBCACHE-1211
>> Copying from the JIRA:
>> "This is a nasty. What started life as an optimisation for certain  
>> types of objects in a marshalled stream (Fqn, GlobalTransactio,  
>> String and Serializable) has become a major limitation in that a  
>> single stream can only hold up to 32767 different (not equal())  
>> instances of such objects.
>> Basically the optimisation was, for example, instead of writing  
>> "hello" to a stream twice, just write it once and use a reference  
>> for all subsequent times. Unfortunately this reference was encoded  
>> as a short, hence the limitation of 32767.
>> Fixing this will definitely break wire compatibility with JBoss  
>> Cache 2.0.0, although JBC does allow backward compatibility by  
>> specifying replication version in your configuration, thanks to the  
>> VersionAwareMarshaller. "
>> So I guess this mandates the need for a CacheMarshaller210.  The  
>> question is how do we fix this.  The obvious thing is to replace  
>> the short references with integers.  The 2 ^ 31 - 1 number of  
>> references this would allow should be plenty!  The drawback though,  
>> is larger streams.  4-byte refs instead of 2-byte refs can be an  
>> unnecessary overhead especially if objects aren't repeated much.
>
> I wouldn't worry too much about the extra bytes. However, you could  
> maintain backwards compatibility, and save the 2 bytes, by stealing  
> the sign bit on the short. If byte1 & 0x80 then read 3 more bytes,  
> else read only 1 more.
>

Still wouldn't help if you needed a million Strings in a  
collection.  :-)  Since we have the VersionAwareMarshaller that is  
able to switch between marshallers, I'm not so worried about backward  
compatibility.

Cheers
--
Manik Surtani
Lead, JBoss Cache
manik at jboss.org