<s>
Sedna	B-Application
is	O
an	O
open-source	B-Application
database	B-Application
management	I-Application
system	I-Application
that	O
provides	O
native	B-Application
storage	B-General_Concept
for	O
XML	B-Protocol
data	B-General_Concept
.	O
</s>
<s>
The	O
distinctive	O
design	O
decisions	O
employed	O
in	O
Sedna	B-Application
are	O
(	O
i	O
)	O
schema-based	O
clustering	B-Algorithm
storage	B-General_Concept
strategy	O
for	O
XML	B-Protocol
data	B-General_Concept
and	O
(	O
ii	O
)	O
memory	B-General_Concept
management	I-General_Concept
based	O
on	O
layered	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Data	B-General_Concept
organization	O
in	O
Sedna	B-Application
is	O
designed	O
with	O
the	O
goal	O
of	O
providing	O
a	O
balance	O
in	O
performance	O
between	O
XML	B-Protocol
queries	B-Language
and	O
updates	O
execution	O
.	O
</s>
<s>
The	O
two	O
primary	O
design	O
decisions	O
in	O
data	B-General_Concept
organization	O
in	O
Sedna	B-Application
are	O
:	O
</s>
<s>
Direct	O
pointers	O
are	O
used	O
to	O
represent	O
XML	B-Protocol
node	O
relationships	O
such	O
as	O
parent	O
,	O
child	O
,	O
and	O
sibling	O
ones	O
.	O
</s>
<s>
Unlike	O
relational-based	O
approaches	O
that	O
require	O
performing	O
joins	B-Language
for	O
traversing	O
an	O
XML	B-Protocol
document	I-Protocol
,	O
traversing	O
in	O
Sedna	B-Application
is	O
performed	O
by	O
simply	O
following	O
a	O
direct	O
pointer	O
.	O
</s>
<s>
A	O
descriptive	O
schema-driven	O
storage	B-General_Concept
strategy	O
is	O
developed	O
which	O
consists	O
of	O
clustering	B-Algorithm
nodes	O
of	O
an	O
XML	B-Protocol
document	I-Protocol
according	O
to	O
their	O
positions	O
in	O
the	O
descriptive	O
schema	B-Language
of	O
the	O
document	O
.	O
</s>
<s>
In	O
contrast	O
to	O
a	O
prescriptive	O
schema	B-Language
that	O
is	O
known	O
in	O
advance	O
and	O
is	O
usually	O
specified	O
in	O
DTD	B-Language
or	O
XML	B-Language
Schema	I-Language
,	O
the	O
descriptive	O
schema	B-Language
is	O
generated	O
from	O
data	B-General_Concept
dynamically	O
(	O
and	O
is	O
maintained	O
incrementally	O
)	O
and	O
represents	O
a	O
concise	O
and	O
an	O
accurate	O
structure	B-General_Concept
summary	O
for	O
data	B-General_Concept
.	O
</s>
<s>
Using	O
the	O
descriptive	O
schema	B-Language
instead	O
of	O
the	O
prescriptive	O
one	O
makes	O
the	O
storage	B-General_Concept
strategy	O
applicable	O
to	O
any	O
XML	B-Protocol
document	I-Protocol
,	O
even	O
a	O
one	O
that	O
comes	O
with	O
no	O
prescriptive	O
schema	B-Language
.	O
</s>
<s>
The	O
following	O
figure	O
illustrates	O
the	O
overall	O
principles	O
of	O
data	B-General_Concept
organization	O
in	O
Sedna	B-Application
.	O
</s>
<s>
The	O
descriptive	O
schema	B-Language
represented	O
as	O
a	O
tree	B-Application
of	O
schema	B-Language
nodes	O
is	O
the	O
central	O
component	O
in	O
the	O
data	B-General_Concept
organization	O
.	O
</s>
<s>
Each	O
schema	B-Language
node	O
is	O
labeled	O
with	O
an	O
XML	B-Protocol
node	O
kind	O
(	O
e.g.	O
</s>
<s>
and	O
has	O
a	O
pointer	O
to	O
data	B-General_Concept
blocks	B-General_Concept
that	O
store	O
XML	B-Protocol
nodes	O
corresponding	O
to	O
the	O
given	O
schema	B-Language
node	O
.	O
</s>
<s>
Depending	O
on	O
their	O
node	O
kind	O
,	O
some	O
schema	B-Language
nodes	O
are	O
also	O
labeled	O
with	O
names	O
(	O
e.g.	O
,	O
element	O
nodes	O
,	O
attribute	O
nodes	O
)	O
.	O
</s>
<s>
Data	B-General_Concept
blocks	B-General_Concept
related	O
to	O
a	O
common	O
schema	B-Language
node	O
are	O
linked	O
via	O
pointers	O
into	O
a	O
bidirectional	O
list	O
.	O
</s>
<s>
Node	O
descriptors	O
in	O
a	O
list	O
of	O
blocks	B-General_Concept
are	O
partly	O
ordered	O
according	O
to	O
document	O
order	O
.	O
</s>
