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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