[jbosscache-dev] Critical CacheMarshaller issue

[Jason T. Greene]

Manik Surtani wrote:
> 
> 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.  
> :-)  

Sure it would, since you get the full positive rang of a signed int 
(2^31 - 1). The only difference is that if its <= 32767 you write only 
two bytes, and when it's greater you write an encoded int that can be 
detected (only 4 bytes).

// Writing code
if (n > 32767) {
    n = n | 0x80000000;
    writeInt(n);
} else {
    writeShort(n);
}

// Reading code
int n = readShort();
if ((n & 0x80000000) != 0) {
   // sign bit rolls off
   n = n << 16  & readShort();
}

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