[jbosscache-dev] Critical CacheMarshaller issue

[Jason T. Greene]

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.

-- 
Jason T. Greene
JBoss, a division of Red Hat