On Sat, May 8, 2010 at 12:36 PM, Kiran Ayyagari <kayyagari@apache.org> wrote:
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.
besides this I see some more advantages

*if* we keep the entryUUID of entry also as the ID of the entry then,
building the DN using the RDN index will be
a lot easier (cause finding the parent of an entry requires now a full
DN construction which can be avoided
by doing a reverse lookup in RDN idex if we know the entry's ID)

> 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).
yeap, and RDN index also takes more disk space now

Yeah but this disk space is very negligible. Basically the UUID is 16 bytes and the Long is 8 on intel arch. We're talking about 8 extra bytes here. So no need to even worry about it. The benefits will outweigh the disadvantages if this is all we can see for disadvantages. 

Alex Karasulu
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