On Sat, May 8, 2010 at 11:09 AM, Emmanuel Lecharny <elecharny@gmail.com> wrote:
On 5/8/10 9:43 AM, Alex Karasulu wrote:
Hi all,

Any thoughts about using the globally visible UUID in the XDBM partition
design for the primary key for Entries instead of using a partition specific
 Long ID?

I'm thinking we need one day to implement certain features. Let me list then
and also point out why using the globally unique UUID might be advantageous:

(1) System wide DN and Entry Cache

      Rather than having each partition manage it's own cache a central DN
and Entry cache makes sense. In this case a global identifier for an entry
might come in handy for hashing cached values.

(2) Nested Partitions, Default Root Partition, Hash Partitioning and Range
Partitioning

      At some point we will want to have nestable partitions. This means we
can have one ADS Partition mounted under another ADS Partition with
operation routing taking place properly to the nested partition where
appropriate.

      Nested partitions will also allow us to also have a default root
partition from which we can mount other partitions.  The default root
partition is nice to have since it allows us to add administrative areas and
their administrative points with subentries onto the root empty string DN.
 It also makes it so the RootDSE is now stored in this partition properly
with persistence.  Right now the RootDSE is generated and not mutable.

      Hash partitioning and range partitioning entails distributing entries
across partitions under some container entry based on some value. Hash
partitioning uses the value's hash to distribute entries where as range
partitioning uses ranges of values to distribute the entries.  So it's not
really the DN that determines which partition the entry is pushed into but
this hash or range value. This makes it so we can scale to very large
numbers of entries in the DIT while also distributing the disk access load
across several disk spindles as does Oracle's RDBMS in these kinds of
configurations.

(3) Global Indices

      If we use a globally unique UUID instead of a partition specific Long
ID then we can expose index segments managed by partitions to higher layers
to construct global indices.  These global indices can then be used to
conduct searches outside of the partition one step higher.  This makes it
possible for us to implement certain virtual directory strategies
irregardless of the partition implementations used in a server's
configuration.  The XDBM search algorithm can leverage these global indices
or delegate sub partition search to a partition if a partition uses it's own
search mechanism.  There's a lot to be said here but this is neither the
time or the place to expand on this topic. But global indices is a key
factor for several things including virtualization.

Thoughts?
 
One other advantage will be that we won't need anymore to store an increment on the disk. Atm, each time we add an element in the backend, we have to ask for a Long, which has to be unique. This is potentially a bottleneck, and it's costly, as this unique Long has to be stored on disk.

I don't yet see any other negative impact we can get by using UUID instead of Long, except that it will requires more disk space (slightly).



Aye very good point I forgot that the Long ID incrementation is a bottleneck for add operations on disk. This goes away and should improve adds significantly.
 
--
Alex Karasulu
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