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

Manik Surtani manik at jboss.org
Tue May 15 10:26:47 EDT 2012


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 <dan.berindei at gmail.com> wrote:
>> 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...
> 
> 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
>>>> 
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--
Manik Surtani
manik at jboss.org
twitter.com/maniksurtani

Lead, Infinispan
http://www.infinispan.org






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