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

[Galder Zamarreño]

On May 15, 2012, at 10:52 AM, Dan Berindei wrote:

> On Tue, May 15, 2012 at 10:44 AM, Galder Zamarreño <galder at redhat.com> wrote:
>> 
>> On May 11, 2012, at 11:30 PM, Dan Berindei 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.
>> 
>> Hmmm, I disagree. If you're adding a new flag, say in 5.2, and you expect a node that runs 5.0 to deal with it properly, really, what you need to be doing is implementing that flag in 5.0.
>> 
> 
> Well, 5.0 is already out there, so modifying it is not an option.

You can always release a 5.0.x :), and I'm pretty sure we might have to do some micro releases to make rolling upgrades work.

> What we can do is ensure that the clients see the incompatibility in
> their testing environment and don't use two versions in production
> without being aware of the problem.
> 
>> We want:
>> - if an old client encounters a new/unknown marshalled value, to not blow up and log a WARN.
>> - if an old client is expected to react to a to a new/unknown marshalled value in the way new versions deal with it, it'll need to implement it.
>> 
>> We don't want:
>> - old client to 'blow up in flames' when they encounter new/unknown options, since this causes problems with potential rolling upgrades.
>> 
> 
> I think there are situations where two versions really are
> incompatible and we really should "blow up in flames".

Any blowing up in flames would stop rolling upgrades from working, so find me a real example of this first and try to understand how rolling upgrade would work in that scenario...

> I'm not saying that's justified in all cases or even in the majority
> of cases, but I'm pretty sure it's not going to be 0% either.
> 
>>> 
>>>> 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.
>> 
>> David?
>> 
>>> 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.
>> 
>> Not necessarily. I think we should do what Sanne suggests but manually in the Flag.Externalizer class, since that's tied to the Flag type.
>> 
>> Within it, we can replicate what an enum set does for marshalling. We already have such code in the Hot Rod server/client (that's how we handle flags there - completely forgot about it when I wrote Flag.Externalizer), so shouldn't be a biggie.
>> 
> 
> The call stack looks like this: ReplicableCommandExternalizer ->
> EnumSet externalizer (in RiverMarshaller) -> Flag.Externalizer.
> You can't change how the EnumSet is serialized in Flag.Externalizer,
> you have to modify either the EnumSet externalizer (e.g. by writing a
> new FlagSet class) or ReplicableCommandExternalizer.

Writing a new FlagSet class is my fav. Easy to do and easy to resgister in the ext table.

> 
> The HotRod flags are serialized directly in Codec10.writeHeader, which
> is the equivalent of ReplicableCommandExternalizer.writeCommandHeader.
> 
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--
Galder Zamarreño
Sr. Software Engineer
Infinispan, JBoss Cache