On 11 May 2012 22:30, Dan Berindei <dan.berindei(a)gmail.com> wrote:
On Fri, May 11, 2012 at 7:23 PM, Sanne Grinovero
<sanne(a)infinispan.org> wrote:
> On 11 May 2012 16:37, Galder ZamarreƱo <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.
You're right - but did you read the conversation on github? We already
pointed this out, still I believe we should have an option to ignore
unknown flags if/when/exclusively we think the migration is safe: we
should be able to tell after the fact, possibly even write migration
tests, but can't predict the future.
We could also use a single bit in the externalized representation of a
flag to mean "safe to be ignored" for any flag, but I'm not sure that
all cases would be black/white .. more likely it will depend on use
case or actual configuration.
> 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...
That code makes perfectly sense in a general purpose use case, but it
still needs to serialize the Class definition: we can avoid that, so
we should!
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.
That's a safe behaviour, expected for a default use case. But if we
decide to add the UNKNOWN flag, we could use bitsets.
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
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