<s>
Dimensional	B-Application
modeling	I-Application
(	O
DM	O
)	O
is	O
part	O
of	O
the	O
Business	O
Dimensional	O
Lifecycle	O
methodology	O
developed	O
by	O
Ralph	O
Kimball	O
which	O
includes	O
a	O
set	O
of	O
methods	O
,	O
techniques	O
and	O
concepts	O
for	O
use	O
in	O
data	O
warehouse	O
design	O
.	O
</s>
<s>
An	O
alternative	O
approach	O
from	O
Inmon	O
advocates	O
a	O
top	O
down	O
design	O
of	O
the	O
model	O
of	O
all	O
the	O
enterprise	O
data	O
using	O
tools	O
such	O
as	O
entity-relationship	B-Architecture
modeling	O
(	O
ER	O
)	O
.	O
</s>
<s>
Dimensional	B-Application
modeling	I-Application
always	O
uses	O
the	O
concepts	O
of	O
facts	O
(	O
measures	O
)	O
,	O
and	O
dimensions	O
(	O
context	O
)	O
.	O
</s>
<s>
Dimensional	B-Application
modeling	I-Application
does	O
not	O
necessarily	O
involve	O
a	O
relational	O
database	O
.	O
</s>
<s>
The	O
dimensional	O
model	O
is	O
built	O
on	O
a	O
star-like	B-Application
schema	I-Application
or	O
snowflake	B-Language
schema	I-Language
,	O
with	O
dimensions	O
surrounding	O
the	O
fact	O
table	O
.	O
</s>
<s>
The	O
process	O
of	O
dimensional	B-Application
modeling	I-Application
builds	O
on	O
a	O
4-step	O
design	O
method	O
that	O
helps	O
to	O
ensure	O
the	O
usability	O
of	O
the	O
dimensional	O
model	O
and	O
the	O
use	O
of	O
the	O
data	O
warehouse	O
.	O
</s>
<s>
To	O
describe	O
the	O
business	O
process	O
,	O
one	O
can	O
choose	O
to	O
do	O
this	O
in	O
plain	O
text	O
or	O
use	O
basic	O
Business	B-Architecture
Process	I-Architecture
Modeling	I-Architecture
Notation	I-Architecture
(	O
BPMN	B-Architecture
)	O
or	O
other	O
design	O
guides	O
like	O
the	O
Unified	B-Language
Modeling	I-Language
Language	I-Language
(	O
UML	B-Language
)	O
.	O
</s>
<s>
Dimensional	O
normalization	O
or	O
snowflaking	B-Language
removes	O
redundant	O
attributes	O
,	O
which	O
are	O
known	O
in	O
the	O
normal	O
flatten	O
de-normalized	O
dimensions	O
.	O
</s>
<s>
Snowflaking	B-Language
has	O
an	O
influence	O
on	O
the	O
data	O
structure	O
that	O
differs	O
from	O
many	O
philosophies	O
of	O
data	O
warehouses	O
.	O
</s>
<s>
Query	B-Language
optimization	I-Language
for	O
star-joined	O
databases	O
is	O
simple	O
,	O
predictable	O
,	O
and	O
controllable	O
.	O
</s>
<s>
We	O
still	O
get	O
the	O
benefits	O
of	O
dimensional	O
models	O
on	O
Hadoop	B-Application
and	O
similar	O
big	B-Application
data	I-Application
frameworks	O
.	O
</s>
<s>
However	O
,	O
some	O
features	O
of	O
Hadoop	B-Application
require	O
us	O
to	O
slightly	O
adapt	O
the	O
standard	O
approach	O
to	O
dimensional	O
modelling	O
.	O
</s>
<s>
The	O
Hadoop	B-Application
File	O
System	O
is	O
immutable	B-Application
.	O
</s>
<s>
Slowly	B-Application
Changing	I-Application
Dimensions	I-Application
on	O
Hadoop	B-Application
become	O
the	O
default	O
behavior	O
.	O
</s>
<s>
First	O
,	O
we	O
can	O
create	O
a	O
View	B-Language
that	O
retrieves	O
the	O
latest	O
record	O
using	O
windowing	O
functions	O
.	O
</s>
<s>
Third	O
,	O
we	O
can	O
store	O
our	O
dimension	O
tables	O
in	O
mutable	B-Application
storage	O
,	O
e.g.	O
</s>
<s>
In	O
a	O
distributed	O
relational	O
database	O
(	O
MPP	B-Operating_System
)	O
we	O
can	O
co-locate	O
records	O
with	O
the	O
same	O
primary	O
and	O
foreign	O
keys	O
on	O
the	O
same	O
node	O
in	O
a	O
cluster	O
.	O
</s>
<s>
This	O
is	O
very	O
different	O
on	O
Hadoop	B-Application
and	O
HDFS	O
.	O
</s>
<s>
As	O
a	O
result	O
joins	O
on	O
Hadoop	B-Application
for	O
two	O
very	O
large	O
tables	O
are	O
quite	O
expensive	O
as	O
data	O
has	O
to	O
travel	O
across	O
the	O
network	O
.	O
</s>
