[infinispan-dev] Stream encoding of Flags and future compatibility

[Dan Berindei]

On Fri, May 11, 2012 at 7:23 PM, Sanne Grinovero <sanne at infinispan.org> wrote:
> On 11 May 2012 16:37, Galder Zamarreño <galder at redhat.com> wrote:
>> Quickly tried this and caused no issues:
>> https://github.com/galderz/infinispan/commit/7718926e5a4a6763506250362d7bd5cbdccd2931
>
> 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/java/org/jboss/marshalling/river/RiverMarshaller.java#L613

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