<s>
In	O
databases	O
,	O
change	B-General_Concept
data	I-General_Concept
capture	I-General_Concept
(	O
CDC	O
)	O
is	O
a	O
set	O
of	O
software	O
design	O
patterns	O
used	O
to	O
determine	O
and	O
track	O
the	O
data	O
that	O
has	O
changed	O
(	O
the	O
"	O
deltas	O
"	O
)	O
so	O
that	O
action	O
can	O
be	O
taken	O
using	O
the	O
changed	O
data	O
.	O
</s>
<s>
CDC	O
is	O
an	O
approach	O
to	O
data	B-General_Concept
integration	I-General_Concept
that	O
is	O
based	O
on	O
the	O
identification	O
,	O
capture	O
and	O
delivery	O
of	O
the	O
changes	O
made	O
to	O
enterprise	O
data	O
sources	O
.	O
</s>
<s>
Timestamps	O
on	O
rows	O
are	O
also	O
frequently	O
used	O
for	O
optimistic	B-Operating_System
locking	I-Operating_System
so	O
this	O
column	O
is	O
often	O
available	O
.	O
</s>
<s>
(	O
Do	O
not	O
confuse	O
this	O
technique	O
with	O
row-level	O
versioning	O
used	O
for	O
optimistic	B-Operating_System
locking	I-Operating_System
.	O
</s>
<s>
For	O
optimistic	B-Operating_System
locking	I-Operating_System
each	O
row	O
has	O
an	O
independent	O
version	O
number	O
,	O
typically	O
a	O
sequential	O
counter	O
.	O
</s>
<s>
May	O
include	O
a	O
publish/subscribe	B-Language
pattern	O
to	O
communicate	O
the	O
changed	O
data	O
to	O
multiple	O
targets	O
.	O
</s>
<s>
In	O
this	O
approach	O
,	O
triggers	B-Language
log	O
events	O
that	O
happen	O
to	O
the	O
transactional	O
table	O
into	O
another	O
queue	O
table	O
that	O
can	O
later	O
be	O
"	O
played	O
back	O
"	O
.	O
</s>
<s>
For	O
example	O
,	O
imagine	O
an	O
Accounts	O
table	O
,	O
when	O
transactions	O
are	O
taken	O
against	O
this	O
table	O
,	O
triggers	B-Language
would	O
fire	O
that	O
would	O
then	O
store	O
a	O
history	O
of	O
the	O
event	O
or	O
even	O
the	O
deltas	O
into	O
a	O
separate	O
queue	O
table	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
technique	O
is	O
the	O
pattern	O
known	O
as	O
the	O
log	B-Application
trigger	I-Application
.	O
</s>
<s>
Although	O
this	O
method	O
involves	O
programming	O
vs.	O
more	O
easily	O
implemented	O
"	O
dumb	O
"	O
triggers	B-Language
,	O
it	O
may	O
provide	O
more	O
accurate	O
and	O
desirable	O
CDC	O
,	O
such	O
as	O
only	O
after	O
a	O
COMMIT	O
,	O
or	O
only	O
after	O
certain	O
columns	O
changed	O
to	O
certain	O
values	O
-	O
just	O
what	O
the	O
target	O
system	O
is	O
looking	O
for	O
.	O
</s>
<s>
Most	O
database	O
management	O
systems	O
manage	O
a	O
transaction	B-General_Concept
log	I-General_Concept
that	O
records	O
changes	O
made	O
to	O
the	O
database	O
contents	O
and	O
to	O
metadata	O
.	O
</s>
<s>
By	O
scanning	O
and	O
interpreting	O
the	O
contents	O
of	O
the	O
database	O
transaction	B-General_Concept
log	I-General_Concept
,	O
one	O
can	O
capture	O
the	O
changes	O
made	O
to	O
the	O
database	O
in	O
a	O
non-intrusive	O
manner	O
.	O
</s>
<s>
Using	O
transaction	B-General_Concept
logs	I-General_Concept
for	O
change	B-General_Concept
data	I-General_Concept
capture	I-General_Concept
offers	O
a	O
challenge	O
in	O
that	O
the	O
structure	O
,	O
contents	O
and	O
use	O
of	O
a	O
transaction	B-General_Concept
log	I-General_Concept
is	O
specific	O
to	O
a	O
database	O
management	O
system	O
.	O
</s>
<s>
Unlike	O
data	O
access	O
,	O
no	O
standard	O
exists	O
for	O
transaction	B-General_Concept
logs	I-General_Concept
.	O
</s>
<s>
Most	O
database	O
management	O
systems	O
do	O
not	O
document	O
the	O
internal	O
format	O
of	O
their	O
transaction	B-General_Concept
logs	I-General_Concept
,	O
although	O
some	O
provide	O
programmatic	O
interfaces	O
to	O
their	O
transaction	B-General_Concept
logs	I-General_Concept
(	O
for	O
example	O
:	O
Oracle	O
,	O
DB2	O
,	O
SQL/MP	O
,	O
SQL/MX	O
and	O
SQL	O
Server	O
2008	O
)	O
.	O
</s>
<s>
Other	O
challenges	O
in	O
using	O
transaction	B-General_Concept
logs	I-General_Concept
for	O
change	B-General_Concept
data	I-General_Concept
capture	I-General_Concept
include	O
:	O
</s>
<s>
Coordinating	O
the	O
reading	O
of	O
the	O
transaction	B-General_Concept
logs	I-General_Concept
and	O
the	O
archiving	O
of	O
log	O
files	O
(	O
database	O
management	O
software	O
typically	O
archives	O
log	O
files	O
off-line	O
on	O
a	O
regular	O
basis	O
)	O
.	O
</s>
<s>
Translation	O
between	O
physical	O
storage	O
formats	O
that	O
are	O
recorded	O
in	O
the	O
transaction	B-General_Concept
logs	I-General_Concept
and	O
the	O
logical	O
formats	O
typically	O
expected	O
by	O
database	O
users	O
(	O
e.g.	O
,	O
some	O
transaction	B-General_Concept
logs	I-General_Concept
save	O
only	O
minimal	O
buffer	O
differences	O
that	O
are	O
not	O
directly	O
useful	O
for	O
change	O
consumers	O
)	O
.	O
</s>
<s>
Dealing	O
with	O
changes	O
to	O
the	O
format	O
of	O
the	O
transaction	B-General_Concept
logs	I-General_Concept
between	O
versions	O
of	O
the	O
database	O
management	O
system	O
.	O
</s>
<s>
Eliminating	O
uncommitted	O
changes	O
that	O
the	O
database	O
wrote	O
to	O
the	O
transaction	B-General_Concept
log	I-General_Concept
and	O
later	O
rolled	B-Application
back	I-Application
.	O
</s>
<s>
CDC	O
solutions	O
based	O
on	O
transaction	B-General_Concept
log	I-General_Concept
files	O
have	O
distinct	O
advantages	O
that	O
include	O
:	O
</s>
<s>
minimal	O
impact	O
on	O
the	O
database	O
(	O
even	O
more	O
so	O
if	O
one	O
uses	O
log	B-General_Concept
shipping	I-General_Concept
to	O
process	O
the	O
logs	O
on	O
a	O
dedicated	O
host	O
)	O
.	O
</s>
<s>
Change	B-General_Concept
data	I-General_Concept
capture	I-General_Concept
both	O
increases	O
in	O
complexity	O
and	O
reduces	O
in	O
value	O
if	O
the	O
source	O
system	O
saves	O
metadata	O
changes	O
when	O
the	O
data	O
itself	O
is	O
not	O
modified	O
.	O
</s>
<s>
For	O
example	O
,	O
some	O
Data	B-Application
models	I-Application
track	O
the	O
user	O
who	O
last	O
looked	O
at	O
but	O
did	O
not	O
change	O
the	O
data	O
in	O
the	O
same	O
structure	O
as	O
the	O
data	O
.	O
</s>
<s>
This	O
results	O
in	O
noise	B-Algorithm
in	O
the	O
Change	B-General_Concept
Data	I-General_Concept
Capture	I-General_Concept
.	O
</s>
<s>
If	O
the	O
data	O
is	O
being	O
persisted	O
in	O
a	O
modern	O
database	O
then	O
Change	B-General_Concept
Data	I-General_Concept
Capture	I-General_Concept
is	O
a	O
simple	O
matter	O
of	O
permissions	O
.	O
</s>
<s>
Reading	O
the	O
transaction	B-General_Concept
log	I-General_Concept
as	O
,	O
or	O
shortly	O
after	O
,	O
it	O
is	O
written	O
.	O
</s>
<s>
Sometimes	O
the	O
slowly	B-Application
changing	I-Application
dimension	I-Application
is	O
used	O
as	O
an	O
alternative	O
method	O
.	O
</s>
<s>
CDC	O
and	O
SDC	O
are	O
similar	O
in	O
that	O
both	O
methods	O
can	O
detect	O
changes	O
in	O
a	O
data	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
a	O
delta-driven	O
data	B-General_Concept
set	I-General_Concept
.	O
</s>
