Re: [infinispan-dev] Stream encoding of Flags and future compatibility
On Fri, May 11, 2012 at 7:23 PM, Sanne Grinovero <sanne(a)infinispan.org> wrote:
On 11 May 2012 16:37, Galder Zamarreño <galder(a)redhat.com>
wrote:
> Quickly tried this and caused no issues:
>
https://github.com/galderz/infinispan/commit/7718926e5a4a6763506250362d7b...
Looks good! I'm sure this doesn't solve all future migration problems,
but if we could keep this kind of tricks around it should improve
odds.
IMHO, this is a kind of sensitivity that we should apply across all
areas (not just flags).
Looks interesting, but then you have the opposite problem: not all new
flags can be ignored, so you need a way to specify that a new flag is
"required". E.g. if we had just added a ZERO_LOCK_ACQUISITION_TIMEOUT
flag then the client would be expecting spurious failures, but not
extra long delays.
On a totally different page, why are we serializing Flags one-by-one
?
We mostly need to serialize EnumSets right?
An EnumSet can be encoded by using the bits of a couple of bytes.
Three bytes looks like enough for all our needs.. we could even be
clever and reserve a special Externalizer-ID for the empty set, to
avoid 3 bytes where none are needed.
While currently we need an integer (4 bytes) to encode the header for
"EnumSet", plus (4 bytes header + 1 byte value) * each flag -> a lot.
RiverMarshaller already has an optimization for the empty set:
https://github.com/dmlloyd/jboss-marshalling/blob/master/river/src/main/j...
I'm not sure why it doesn't encode each element as a bit, it might be
to keep wire compatibility when the order of values in an enum
changes.
However, because there is only one EnumSet for all Enum types, a
hypothetical EnumSetExternalizer also needs to write the name of the
enum class - if we wanted to serialize EnumSet<Flag> in 2 bytes then
we'd need to make the transformation in ReplicableCommandExternalizer.
Cheers
Dan