[jbosscache-dev] VAM integration in JDBCCL - updating an empty node from database results on a put() on Collections.emptyMap()

[Galder Zamarreno]

Hi,

I'm in the process of integrating VersionAwareMarshaller with JBDCCacheLoader and I've found an issue in TreeCacheMarshaller200. unmarshallMap().

Let's say I make a put (single key/value pair) in the cache which results in an insert of a marshalled map in the db. I then call remove(key) which results on the map still being in the db but containing no data.

If I evict the data from memory and call put() again, unmarshallMap() would return Collections.emptyMap() which returns an immutable empty map. When I try putting my new data into the map, it throws java.lang.UnsupportedOperationException which is expected as the map is immutable.

I had a quick chat with Brian and the first solution was to check for isEmpty() but as Brian pointed out, 

"If VAM is meant to be a general purpose tool, it needs to work in a general purpose way, if code that uses it needs to understand it's internal details and code around them, that's improper"

He also said: "well VAM is just assuming that the only purpose of the map is to be an argument in a put call.  if VAM is to be general purpose it can't make such assumptions, or needs to expose an API to turn them on/off per request"

I think Brian is right. Currently, JDBCCL checks values from JDBCCL.loadNode() based on the contract of this method, and therefore, should not really now what could come from VAM.

Thoughts?

Galder Zamarreño
Sr. Software Maintenance Engineer
JBoss, a division of Red Hat

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