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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: In AbstractRocksDBState, why write a byte 42 between key and namespace?
Date Mon, 18 Jul 2016 09:51:21 GMT
In which cases is it not solved? Because then we should make sure to solve
it.

On Mon, 18 Jul 2016 at 10:33 Stephan Ewen <sewen@apache.org> wrote:

> Got it. But the ambiguity is not really solved by that, just lessened.
>
> On Sun, Jul 17, 2016 at 2:10 PM, Aljoscha Krettek <aljoscha@apache.org>
> wrote:
>
> > @Stephan It's not about the serializers not being able to read the key.
> The
> > key/namespace are never read again. It's just about the serialized form
> > possibly being ambiguous since we don't control the TypeSerializers and
> > there might be wanky var-length encoding schemes and what not.
> >
> > On Fri, 15 Jul 2016 at 19:20 Timothy Farkas <
> timothytiborfarkas@gmail.com>
> > wrote:
> >
> > > I've faced a similar issue when serializing data two a key value store.
> > Not
> > > sure how helpful it is for this case but two possible solutions I've
> used
> > > for persisting keys and values under different namespaces to the same
> key
> > > value store are:
> > >
> > > - have all namespaces be the same number of bytes and prefix each key
> > with
> > > its namespace.
> > > - Include the number of bytes in the name space and key. So the bytes
> > would
> > > look like this:
> > >
> > > [name space num bytes] [ name space] [key num bytes] [key]
> > >
> > > Thanks,
> > > Tim
> > >
> > > On Fri, Jul 15, 2016 at 9:45 AM, Stephan Ewen <sewen@apache.org>
> wrote:
> > >
> > > > Every serializer should know how many bytes to consume. The key
> > > serializer
> > > > should not need to look for 42 to know where to terminate.
> > > >
> > > > Otherwise this would be a problem case:
> > > > key[42, 42] - 42 - namespace [42, 42, 42]
> > > > key[42, 42, 42] - 42 - namespace [42, 42]
> > > >
> > > >
> > > >
> > > > On Fri, Jul 15, 2016 at 5:38 PM, Aljoscha Krettek <
> aljoscha@apache.org
> > >
> > > > wrote:
> > > >
> > > > > I left that in on purpose to protect against cases where the
> > > combination
> > > > > of key and namespace can be ambiguous. For example, these two
> > > > combinations
> > > > > of key and namespace have the same written representation:
> > > > > key [0 1 2] namespace [3 4 5] (values in brackets are byte arrays)
> > > > > key [0 1] namespace [2 3 4 5]
> > > > >
> > > > > having the "magic number" in there protects against such cases.
> > > > >
> > > > > On Fri, 15 Jul 2016 at 16:31 Stephan Ewen <sewen@apache.org>
> wrote:
> > > > >
> > > > >> My assumption is that this was a sanity check that actually just
> > stuck
> > > > in
> > > > >> the code.
> > > > >>
> > > > >> It can probably be removed.
> > > > >>
> > > > >> PS: Moving this to the dev@flink.apache.org list...
> > > > >>
> > > > >>
> > > > >>
> > > > >> On Fri, Jul 15, 2016 at 11:05 AM, 刘彪 <mmyy1110@gmail.com>
wrote:
> > > > >>
> > > > >> > In AbstractRocksDBState.writeKeyAndNamespace():
> > > > >> >
> > > > >> > protected void writeKeyAndNamespace(DataOutputView out)
throws
> > > > >> IOException
> > > > >> > {
> > > > >> > backend.keySerializer().serialize(backend.currentKey(),
out);
> > > > >> > out.writeByte(42);
> > > > >> > namespaceSerializer.serialize(currentNamespace, out);
> > > > >> > }
> > > > >> >
> > > > >> > Why write a byte 42 between key and namespace? The keySerializer
> > and
> > > > >> > namespaceSerializer know their lengths. It seems we don't
need
> > this
> > > > >> byte.
> > > > >> >
> > > > >> > Could anybody tell me what it is for?  Is there any situation
> that
> > > we
> > > > >> must
> > > > >> > have this separator?
> > > > >> >
> > > > >>
> > > > >
> > > >
> > >
> >
>

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