Interesting discussion for sure. So what have we done in this regard? I see (a) an optimisation for marshalling collections of Flags, and the ability to mark unknown flags accordingly (and log this rather than crash).
On 15 May 2012, at 11:11, Sanne Grinovero wrote: > On 15 May 2012 09:36, Dan Berindei <[email protected]> wrote: >> On Mon, May 14, 2012 at 2:11 PM, Sanne Grinovero <[email protected]> >> wrote: >>> On 11 May 2012 22:30, Dan Berindei <[email protected]> wrote: >>>> On Fri, May 11, 2012 at 7:23 PM, Sanne Grinovero <[email protected]> >>>> wrote: >>>>> On 11 May 2012 16:37, Galder Zamarreño <[email protected]> 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... > > I know I proposed this myself, but I would rather avoid this path.. > even if some flags are unsafe, they might be safe in the configuration > / use case, and we might want to force it to proceed nevertheless. I > think we should always have the option to "force it to proceed". > In worst case one might want to block all transactions and kill the > clients, but you have to propose a way to migrate the data. > > #On two bitsets being sent.. > > First problem is that we're sending redundant information; second is > that it should not be responsibility of the older node (in terms of > version of the code being run) to properly understand the newer nodes, > it should be the other way around. > > So rather than sending two bitsets, in this case the responsibility > belongs to the *sender*: since it's the one containing the newer code, > it might know how to deal with older nodes, or might be patched/fixed > to know how to deal with older nodes. It's obviously harder to patch > existing nodes! Especially if they are running already (and you don't > have Byteman). > If we could keep track of the version of other nodes in the View, we > could have a translation table plugged in on how to talk to a > different version node.. maintaining such a table would be much > simpler as it would live independently from the main code, and be > fixed and hacked on.. possibly even loaded dynamically. So that's what > I would do in the long term.. in the short term, let's just save some > bytes using optimal encoding for the EnumSet. > > Especially of note, if the sender (newer node) knows by looking into > the translaction table that an operation/command can't be possibly > sent to the older node, it should be easier to find an alternative or > workaround. > when migrating an existing cluster by a rolling upgrade, for sure the > existing application is NOT using features which exist only in the > never version.. that's granted and we can take advantage of that. > >> >>> >>>>> 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>. > > Can't we write it using the special-purpose Externalizer explicitly? > To read it, it will have it's own Externalizer ID. > > >>>> 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. > > Right, I only asked for the UNKNOWN for the time being, as we don't > have anything better yet. > And I guess it's safe to have such a flag for future.. you never know. > >> 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 >>>> >>>> _______________________________________________ >>>> infinispan-dev mailing list >>>> [email protected] >>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev >>> >>> _______________________________________________ >>> infinispan-dev mailing list >>> [email protected] >>> https://lists.jboss.org/mailman/listinfo/infinispan-dev >> >> _______________________________________________ >> infinispan-dev mailing list >> [email protected] >> https://lists.jboss.org/mailman/listinfo/infinispan-dev > > _______________________________________________ > infinispan-dev mailing list > [email protected] > https://lists.jboss.org/mailman/listinfo/infinispan-dev -- Manik Surtani [email protected] twitter.com/maniksurtani Lead, Infinispan http://www.infinispan.org _______________________________________________ infinispan-dev mailing list [email protected] https://lists.jboss.org/mailman/listinfo/infinispan-dev
