+1 to eliminate the value wrappers.
-1 to adding a dependency from core to server-core, if you feel creating
and maintaining a separate MigrationRemoteCacheStore is too much work I'd
rather we moved CacheValue to core.
If we move CacheValue to core, I think we can do the re-wrapping on the
rawValues branch and avoid adding another setting to the RCS configuration.
On Tue, Dec 11, 2012 at 3:02 PM, Tristan Tarrant <ttarrant(a)redhat.com>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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