[infinispan-dev] [ISPN-125] Reading part of dynamic externalizer mappings

[Galder Zamarreno]

Hi,

Re: https://jira.jboss.org/jira/browse/ISPN-125

I've been thinking more about this and I don't have the reading part 
very clear. At runtime, it's very easy to check the externalizer map to 
see if there's externalizer for class A and if there isn't any, inspect 
the class for @Marshallable annotation, instantiate the A's Externalizer 
class and add it to the map.

However, the reading part is a fairly difficult problem to crack cos all 
you'd get would be an Id which you'd need to map back to an Externalizer 
that will be used for reading. However, how does the reader know what's 
the mapping Id to Externalizer mapping?

One option, rather brutal, would be to look up the entire classpath for 
a class containing a @Marshallable with the index you're after.

Another option would be that the 1st time someone writes an instance of 
that class A, it adds a String form of the fully qualified class name to 
the wire. So, the reader can check the externalizer map and if the id 
not present, read the class name and resolve it and add the mapping. 
However, this breaks the moment you have two different nodes sending a 
class for the 1st time. They would both add the class name but only the 
1st one to arrive would resolve the class name correctly, the 2nd time 
it arrives on the node, cos the mapping would already be there, it would 
read the object directly, failing cos there's also the class name on the 
wire.

Anyone has any other ideas?

It's worth bearing in mind that adding this dynamic behaviour adds 
complexity the code and it forces previously non-concurrent collections 
to be concurrent, hence performance will degrade compared to the current 
solution. It's worth keeping in mind that users will always be able to 
implement their j.i.Externalizable, which will be less performant than 
our internal solution but might be reasonable enough.

One thing is for sure here, the indexes need to be tied down and I'm 
already working on it.
-- 
Galder Zamarreño
Sr. Software Engineer
Infinispan, JBoss Cache