<s>
In	O
database	O
technologies	O
,	O
a	O
rollback	B-Application
is	O
an	O
operation	O
which	O
returns	O
the	O
database	O
to	O
some	O
previous	O
state	O
.	O
</s>
<s>
Rollbacks	B-Application
are	O
important	O
for	O
database	O
integrity	O
,	O
because	O
they	O
mean	O
that	O
the	O
database	O
can	O
be	O
restored	O
to	O
a	O
clean	O
copy	O
even	O
after	O
erroneous	O
operations	O
are	O
performed	O
.	O
</s>
<s>
They	O
are	O
crucial	O
for	O
recovering	O
from	O
database	O
server	O
crashes	O
;	O
by	O
rolling	O
back	O
any	O
transaction	B-General_Concept
which	O
was	O
active	O
at	O
the	O
time	O
of	O
the	O
crash	O
,	O
the	O
database	O
is	O
restored	O
to	O
a	O
consistent	O
state	O
.	O
</s>
<s>
The	O
rollback	B-Application
feature	O
is	O
usually	O
implemented	O
with	O
a	O
transaction	B-General_Concept
log	I-General_Concept
,	O
but	O
can	O
also	O
be	O
implemented	O
via	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
.	O
</s>
<s>
A	O
cascading	O
rollback	B-Application
occurs	O
in	O
database	O
systems	O
when	O
a	O
transaction	B-General_Concept
(	O
T1	O
)	O
causes	O
a	O
failure	O
and	O
a	O
rollback	B-Application
must	O
be	O
performed	O
.	O
</s>
<s>
That	O
is	O
,	O
one	O
transaction	B-General_Concept
's	O
failure	O
causes	O
many	O
to	O
fail	O
.	O
</s>
<s>
Practical	O
database	O
recovery	O
techniques	O
guarantee	O
cascadeless	O
rollback	B-Application
,	O
therefore	O
a	O
cascading	O
rollback	B-Application
is	O
not	O
a	O
desirable	O
result	O
.	O
</s>
<s>
Cascading	O
rollback	B-Application
is	O
scheduled	O
by	O
dba	O
.	O
</s>
<s>
SQL	B-Language
refers	O
to	O
Structured	B-Language
Query	I-Language
Language	I-Language
,	O
a	O
kind	O
of	O
language	O
used	O
to	O
access	O
,	O
update	O
and	O
manipulate	O
database	O
.	O
</s>
<s>
In	O
SQL	B-Language
,	O
ROLLBACK	B-Application
is	O
a	O
command	O
that	O
causes	O
all	O
data	O
changes	O
since	O
the	O
last	O
START	B-General_Concept
TRANSACTION	I-General_Concept
or	O
BEGIN	O
to	O
be	O
discarded	O
by	O
the	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
(	O
RDBMS	O
)	O
,	O
so	O
that	O
the	O
state	O
of	O
the	O
data	O
is	O
"	O
rolled	O
back	O
"	O
to	O
the	O
way	O
it	O
was	O
before	O
those	O
changes	O
were	O
made	O
.	O
</s>
<s>
A	O
ROLLBACK	B-Application
statement	O
will	O
also	O
release	O
any	O
existing	O
savepoints	B-General_Concept
that	O
may	O
be	O
in	O
use	O
.	O
</s>
<s>
In	O
most	O
SQL	B-Language
dialects	O
,	O
ROLLBACKs	B-Application
are	O
connection	O
specific	O
.	O
</s>
<s>
This	O
means	O
that	O
if	O
two	O
connections	O
are	O
made	O
to	O
the	O
same	O
database	O
,	O
a	O
ROLLBACK	B-Application
made	O
in	O
one	O
connection	O
will	O
not	O
affect	O
any	O
other	O
connections	O
.	O
</s>
<s>
This	O
is	O
vital	O
for	O
proper	O
concurrency	B-Architecture
.	O
</s>
