I don't like #1.  Seems more complicated, harder to maintain & debug than the others.

In my opinion the best option would be #4 (eliminate the different formats), but that probably can't be done in a minor release?

Between 2 and 3, I'd prefer #2, handling it in the base class so it's automatically inherited by any custom classes that extend it.
Since the use case isn't limited to rolling upgrades; you could have a HotRod cache with a full-time RemoteCacheStore.

-Dennis

On 12/11/2012 07:02 AM, Tristan Tarrant wrote:

So,
I thought we had everything ready to go for HotRod rolling upgrades:

• have HotRod server full of data (the "source")
• configure a new HotRod server (the "target") with a RemoteCacheStore pointing to the "source" (using "rawValues")
• clients switch over to the "target" server which on cache misses should seamlessly fetch entries from the "source"
• issue a "dump keys" on the source
• fetch the "dumped keys" from the target
• disable the RCS on the target and switch off the "source" for good
  
• PROFIT$$$

Unfortunately there is a teeny tiny flaw in the plan: entries in a HotRod-managed cache are ByteArrayKey/CacheValue pairs and unfortunately, when the "target" reads from the RCS they get unwrapped into their byte[] equivalents.

The solutions we have are:

1. have a special marshaller placed on the RemoteCacheStore's RemoteCacheManager which rewraps the entries. Unfortunately marshallers can't distinguish between keys and values, so this would probably require some horrid ThreadLocal trickery
2. Add a new option to RemoteCacheStore so that it rewraps entries in the ByteArrayKey/CacheValue format. Unfortunately the CacheValue class is part of server-core, but the dependency could be made optional, and in the context of the Rolling Upgrade scenario it is a non-issue, since it will be in the classpath
   
3. Introduce a new MigrationRemoteCacheStore which does the same as the above, but without changing RCS itself.

My personal favourite is number 2, but I trust your better judgement.

I think these are merely workarounds and we should have a better way for "entry wrappers" (such as the cache servers) to "localize" the entries for their own particular needs. Also I believe we need a better way to attach metadata to entries in a portable way so that we don't need these value wrappers.

Tristan

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