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

[Dan Berindei]

On Mon, May 14, 2012 at 2:11 PM, Sanne Grinovero <sanne at infinispan.org> wrote:
> On 11 May 2012 22:30, Dan Berindei <dan.berindei at gmail.com> wrote:
>> 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.
>
> 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.
>

Right, the flag serialized format should tell the receiver whether it
can be ignored. But if we encode an EnumSet<Flag> as a bitset, each
flag will have only one bit, so we wouldn't have a place for the
"migration safe" bit. I guess we could send two bit sets, one with the
flags and one with the "migration safe" bits...

>
>>> 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
>
> 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 think we need to encode the Flag class name as long as we consider
the flag set as just another parameter in the Object[] parameter array
- so the externalizer doesn't know how to distinguish between an
EnumSet<Flag> and an EnumSet<UserEnumType>.

>> 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.
>

I think the UNKNOWN flag is useful as long as we parse each flag
independently, but if we start encoding the flags as a bitset (or
two), then we don't need UNKNOWN any more. We would only need to
ensure the position of a flag in the bitset remains constant between
versions, which means either relying on the declaration order and
keeping that constant (no removing or reordering of flags) or adding a
position field to the enum.

Assuming the order in the bitset remains constant from one version to
the next, there are only two scenarios:
a) The added flags are not important, and the receiver can just ignore
them - in which case the parsed flag set will only include the flags
that the receiver knows about.
b) The added flags are important, and the receiver should reject the
command - in which case the externalizer should just throw an
exception instead of reading the flag set and setting the UNKNOWN
flag.


>> 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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