Re: [jbosscache-dev] Critical CacheMarshaller issue

On 12 Nov 2007, at 17:18, Jason T. Greene wrote: > 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). Of course, yeah, you'd read 3 more bytes. But that would mean (with the adding of more bytes) this would break backward compatibility for > 32767 refs anyway. Existing code wouldn't be able to deserialize such a stream. Then again, for such cases, it is currently *broken* and even existing code wouldn't be able to deserialize such a stream anyway!!

Still I'd prefer to make the change to the stream explicit though, as a separate marshaller for 2.1.0 - I do like the variable int approach since for a small number of refs (< 128, which is probably the majority of use cases) I'd just encode a single byte.