<s>
A	O
data	B-General_Concept
stream	I-General_Concept
management	I-General_Concept
system	I-General_Concept
(	O
DSMS	B-General_Concept
)	O
is	O
a	O
computer	O
software	O
system	O
to	O
manage	O
continuous	O
data	B-General_Concept
streams	I-General_Concept
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
a	O
database	B-Application
management	I-Application
system	I-Application
(	O
DBMS	O
)	O
,	O
which	O
is	O
,	O
however	O
,	O
designed	O
for	O
static	O
data	O
in	O
conventional	O
databases	B-Application
.	O
</s>
<s>
A	O
DBMS	O
also	O
offers	O
a	O
flexible	O
query	B-Application
processing	I-Application
so	O
that	O
the	O
information	O
needed	O
can	O
be	O
expressed	O
using	O
queries	B-Library
.	O
</s>
<s>
However	O
,	O
in	O
contrast	O
to	O
a	O
DBMS	O
,	O
a	O
DSMS	B-General_Concept
executes	O
a	O
continuous	B-General_Concept
query	I-General_Concept
that	O
is	O
not	O
only	O
performed	O
once	O
,	O
but	O
is	O
permanently	O
installed	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
query	B-Library
is	O
continuously	O
executed	O
until	O
it	O
is	O
explicitly	O
uninstalled	O
.	O
</s>
<s>
Since	O
most	O
DSMS	B-General_Concept
are	O
data-driven	O
,	O
a	O
continuous	B-General_Concept
query	I-General_Concept
produces	O
new	O
results	O
as	O
long	O
as	O
new	O
data	O
arrive	O
at	O
the	O
system	O
.	O
</s>
<s>
This	O
basic	O
concept	O
is	O
similar	O
to	O
Complex	B-Operating_System
event	I-Operating_System
processing	I-Operating_System
so	O
that	O
both	O
technologies	O
are	O
partially	O
coalescing	O
.	O
</s>
<s>
One	O
important	O
feature	O
of	O
a	O
DSMS	B-General_Concept
is	O
the	O
possibility	O
to	O
handle	O
potentially	O
infinite	O
and	O
rapidly	O
changing	O
data	B-General_Concept
streams	I-General_Concept
by	O
offering	O
flexible	O
processing	O
at	O
the	O
same	O
time	O
,	O
although	O
there	O
are	O
only	O
limited	O
resources	O
such	O
as	O
main	O
memory	O
.	O
</s>
<s>
The	O
following	O
table	O
provides	O
various	O
principles	O
of	O
DSMS	B-General_Concept
and	O
compares	O
them	O
to	O
traditional	O
DBMS	O
.	O
</s>
<s>
One	O
of	O
the	O
biggest	O
challenges	O
for	O
a	O
DSMS	B-General_Concept
is	O
to	O
handle	O
potentially	O
infinite	O
data	B-General_Concept
streams	I-General_Concept
using	O
a	O
fixed	O
amount	O
of	O
memory	O
and	O
no	O
random	O
access	O
to	O
the	O
data	O
.	O
</s>
<s>
The	O
idea	O
behind	O
compression	O
techniques	O
is	O
to	O
maintain	O
only	O
a	O
synopsis	O
of	O
the	O
data	O
,	O
but	O
not	O
all	O
(	O
raw	O
)	O
data	O
points	O
of	O
the	O
data	B-General_Concept
stream	I-General_Concept
.	O
</s>
<s>
Instead	O
of	O
using	O
synopses	O
to	O
compress	O
the	O
characteristics	O
of	O
the	O
whole	O
data	B-General_Concept
streams	I-General_Concept
,	O
window	O
techniques	O
only	O
look	O
on	O
a	O
portion	O
of	O
the	O
data	O
.	O
</s>
<s>
Therefore	O
,	O
a	O
window	O
continuously	O
cuts	O
out	O
a	O
part	O
of	O
the	O
data	B-General_Concept
stream	I-General_Concept
,	O
e.g.	O
</s>
<s>
the	O
last	O
ten	O
data	B-General_Concept
stream	I-General_Concept
elements	O
,	O
and	O
only	O
considers	O
these	O
elements	O
during	O
the	O
processing	O
.	O
</s>
<s>
There	O
are	O
different	O
kinds	O
of	O
such	O
windows	O
like	O
sliding	O
windows	O
that	O
are	O
similar	O
to	O
FIFO	B-Operating_System
lists	O
or	O
tumbling	O
windows	O
that	O
cut	O
out	O
disjoint	O
parts	O
.	O
</s>
<s>
Sliding-window	O
query	B-Application
processing	I-Application
is	O
also	O
suitable	O
to	O
being	O
implemented	O
in	O
parallel	O
processors	O
by	O
exploiting	O
parallelism	O
between	O
different	O
windows	O
and/or	O
within	O
each	O
window	O
extent	O
.	O
</s>
<s>
However	O
,	O
most	O
DSMS	B-General_Concept
are	O
based	O
on	O
the	O
query	B-Application
processing	I-Application
in	O
DBMS	O
by	O
using	O
declarative	O
languages	O
to	O
express	O
queries	B-Library
,	O
which	O
are	O
translated	O
into	O
a	O
plan	O
of	O
operators	O
.	O
</s>
<s>
A	O
query	B-Application
processing	I-Application
often	O
consists	O
of	O
the	O
following	O
steps	O
.	O
</s>
<s>
The	O
formulation	O
of	O
queries	B-Library
is	O
mostly	O
done	O
using	O
declarative	O
languages	O
like	O
SQL	B-Language
in	O
DBMS	O
.	O
</s>
<s>
Since	O
there	O
are	O
no	O
standardized	O
query	B-Library
languages	O
to	O
express	O
continuous	B-General_Concept
queries	I-General_Concept
,	O
there	O
are	O
a	O
lot	O
of	O
languages	O
and	O
variations	O
.	O
</s>
<s>
However	O
,	O
most	O
of	O
them	O
are	O
based	O
on	O
SQL	B-Language
,	O
such	O
as	O
the	O
Continuous	B-General_Concept
Query	I-General_Concept
Language	O
(	O
CQL	O
)	O
,	O
StreamSQL	B-Language
and	O
ESP	B-Application
.	O
</s>
<s>
In	O
StreamSQL	B-Language
,	O
a	O
query	B-Library
with	O
a	O
sliding	O
window	O
for	O
the	O
last	O
10	O
elements	O
looks	O
like	O
follows	O
:	O
</s>
<s>
In	O
the	O
next	O
step	O
,	O
the	O
declarative	O
query	B-Library
is	O
translated	O
into	O
a	O
logical	O
query	B-Library
plan	O
.	O
</s>
<s>
A	O
query	B-Library
plan	O
is	O
a	O
directed	O
graph	O
where	O
the	O
nodes	O
are	O
operators	O
and	O
the	O
edges	O
describe	O
the	O
processing	O
flow	O
.	O
</s>
<s>
Each	O
operator	O
in	O
the	O
query	B-Library
plan	O
encapsulates	O
the	O
semantic	O
of	O
a	O
specific	O
operation	O
,	O
such	O
as	O
filtering	O
or	O
aggregation	O
.	O
</s>
<s>
In	O
DSMSs	O
that	O
process	O
relational	O
data	B-General_Concept
streams	I-General_Concept
,	O
the	O
operators	O
are	O
equal	O
or	O
similar	O
to	O
the	O
operators	O
of	O
the	O
Relational	B-Algorithm
algebra	I-Algorithm
,	O
so	O
that	O
there	O
are	O
operators	O
for	O
selection	O
,	O
projection	O
,	O
join	O
,	O
and	O
set	O
operations	O
.	O
</s>
<s>
This	O
operator	O
concept	O
allows	O
the	O
very	O
flexible	O
and	O
versatile	O
processing	O
of	O
a	O
DSMS	B-General_Concept
.	O
</s>
<s>
The	O
logical	O
query	B-Library
plan	O
can	O
be	O
optimized	O
,	O
which	O
strongly	O
depends	O
on	O
the	O
streaming	O
model	O
.	O
</s>
<s>
The	O
basic	O
concepts	O
for	O
optimizing	O
continuous	B-General_Concept
queries	I-General_Concept
are	O
equal	O
to	O
those	O
from	O
database	B-Language
systems	I-Language
.	O
</s>
<s>
If	O
there	O
are	O
relational	O
data	B-General_Concept
streams	I-General_Concept
and	O
the	O
logical	O
query	B-Library
plan	O
is	O
based	O
on	O
relational	O
operators	O
from	O
the	O
Relational	B-Algorithm
algebra	I-Algorithm
,	O
a	O
query	B-Language
optimizer	I-Language
can	O
use	O
the	O
algebraic	O
equivalences	O
to	O
optimize	O
the	O
plan	O
.	O
</s>
<s>
Furthermore	O
,	O
there	O
are	O
also	O
cost-based	O
optimization	O
techniques	O
like	O
in	O
DBMS	O
,	O
where	O
a	O
query	B-Library
plan	O
with	O
the	O
lowest	O
costs	O
is	O
chosen	O
from	O
different	O
equivalent	O
query	B-Library
plans	O
.	O
</s>
<s>
In	O
DBMS	O
this	O
decision	O
is	O
mostly	O
done	O
by	O
certain	O
statistics	O
of	O
the	O
involved	O
databases	B-Application
.	O
</s>
<s>
But	O
,	O
since	O
the	O
data	O
of	O
a	O
data	B-General_Concept
streams	I-General_Concept
is	O
unknown	O
in	O
advance	O
,	O
there	O
are	O
no	O
such	O
statistics	O
in	O
a	O
DSMS	B-General_Concept
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
observe	O
a	O
data	B-General_Concept
stream	I-General_Concept
for	O
a	O
certain	O
time	O
to	O
obtain	O
some	O
statistics	O
.	O
</s>
<s>
Using	O
these	O
statistics	O
,	O
the	O
query	B-Library
can	O
also	O
be	O
optimized	O
later	O
.	O
</s>
<s>
So	O
,	O
in	O
contrast	O
to	O
a	O
DBMS	O
,	O
some	O
DSMS	B-General_Concept
allows	O
to	O
optimize	O
the	O
query	B-Library
even	O
during	O
runtime	O
.	O
</s>
<s>
Therefore	O
,	O
a	O
DSMS	B-General_Concept
needs	O
some	O
plan	O
migration	O
strategies	O
to	O
replace	O
a	O
running	O
query	B-Library
plan	O
with	O
a	O
new	O
one	O
.	O
</s>
<s>
Since	O
a	O
logical	O
operator	O
is	O
only	O
responsible	O
for	O
the	O
semantics	O
of	O
an	O
operation	O
but	O
does	O
not	O
consist	O
of	O
any	O
algorithms	O
,	O
the	O
logical	O
query	B-Library
plan	O
must	O
be	O
transformed	O
into	O
an	O
executable	O
counterpart	O
.	O
</s>
<s>
This	O
is	O
called	O
a	O
physical	O
query	B-Library
plan	O
.	O
</s>
<s>
The	O
join	O
,	O
for	O
example	O
,	O
is	O
logically	O
the	O
same	O
,	O
although	O
it	O
can	O
be	O
implemented	O
by	O
different	O
algorithms	O
like	O
a	O
Nested	B-Algorithm
loop	I-Algorithm
join	I-Algorithm
or	O
a	O
Sort-merge	B-Algorithm
join	I-Algorithm
.	O
</s>
<s>
Finally	O
,	O
the	O
query	B-Library
is	O
available	O
as	O
a	O
physical	O
query	B-Library
plan	O
.	O
</s>
<s>
Since	O
the	O
physical	O
query	B-Library
plan	O
consists	O
of	O
executable	O
algorithms	O
,	O
it	O
can	O
be	O
directly	O
executed	O
.	O
</s>
<s>
For	O
this	O
,	O
the	O
physical	O
query	B-Library
plan	O
is	O
installed	O
into	O
the	O
system	O
.	O
</s>
<s>
The	O
bottom	O
of	O
the	O
graph	O
(	O
of	O
the	O
query	B-Library
plan	O
)	O
is	O
connected	O
to	O
the	O
incoming	O
sources	O
,	O
which	O
can	O
be	O
everything	O
like	O
connectors	O
to	O
sensors	O
.	O
</s>
<s>
Since	O
most	O
DSMSs	O
are	O
data-driven	O
,	O
a	O
query	B-Library
is	O
executed	O
by	O
pushing	O
the	O
incoming	O
data	O
elements	O
from	O
the	O
source	O
through	O
the	O
query	B-Library
plan	O
to	O
the	O
sink	O
.	O
</s>
