<s>
Dryad	B-Operating_System
was	O
a	O
research	O
project	O
at	O
Microsoft	O
Research	O
for	O
a	O
general	O
purpose	O
runtime	O
for	O
execution	O
of	O
data	B-Operating_System
parallel	I-Operating_System
applications	O
.	O
</s>
<s>
The	O
research	O
prototypes	O
of	O
the	O
Dryad	B-Operating_System
and	O
DryadLINQ	O
data-parallel	O
processing	O
frameworks	O
are	O
available	O
in	O
source	O
form	O
at	O
GitHub	B-Application
.	O
</s>
<s>
An	O
application	O
written	O
for	O
Dryad	B-Operating_System
is	O
modeled	O
as	O
a	O
directed	O
acyclic	O
graph	O
(	O
DAG	O
)	O
.	O
</s>
<s>
The	O
DAG	O
defines	O
the	O
dataflow	B-Application
of	O
the	O
application	O
,	O
and	O
the	O
vertices	O
of	O
the	O
graph	O
defines	O
the	O
operations	O
that	O
are	O
to	O
be	O
performed	O
on	O
the	O
data	O
.	O
</s>
<s>
The	O
"	O
computational	O
vertices	O
"	O
are	O
written	O
using	O
sequential	O
constructs	O
,	O
devoid	O
of	O
any	O
concurrency	B-Architecture
or	O
mutual	B-Operating_System
exclusion	I-Operating_System
semantics	O
.	O
</s>
<s>
The	O
Dryad	B-Operating_System
runtime	O
parallelizes	O
the	O
dataflow	B-Application
graph	O
by	O
distributing	O
the	O
computational	O
vertices	O
across	O
various	O
execution	O
engines	O
(	O
which	O
can	O
be	O
multiple	O
processor	O
cores	O
on	O
the	O
same	O
computer	O
or	O
different	O
physical	O
computers	O
connected	O
by	O
a	O
network	O
,	O
as	O
in	O
a	O
cluster	B-Architecture
)	O
.	O
</s>
<s>
Scheduling	O
of	O
the	O
computational	O
vertices	O
on	O
the	O
available	O
hardware	O
is	O
handled	O
by	O
the	O
Dryad	B-Operating_System
runtime	O
,	O
without	O
any	O
explicit	O
intervention	O
by	O
the	O
developer	O
of	O
the	O
application	O
or	O
administrator	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
flow	B-Application
of	I-Application
data	I-Application
between	O
one	O
computational	O
vertex	O
to	O
another	O
is	O
implemented	O
by	O
using	O
communication	O
"	O
channels	O
"	O
between	O
the	O
vertices	O
,	O
which	O
in	O
physical	O
implementation	O
is	O
realized	O
by	O
TCP/IP	B-Protocol
streams	O
,	O
shared	B-Operating_System
memory	I-Operating_System
or	O
temporary	O
files	B-Operating_System
.	O
</s>
<s>
A	O
stream	O
is	O
used	O
at	O
runtime	O
to	O
transport	O
a	O
finite	O
number	O
of	O
structured	B-General_Concept
Items	O
.	O
</s>
<s>
Dryad	B-Operating_System
defines	O
a	O
domain-specific	B-Language
language	I-Language
,	O
which	O
is	O
implemented	O
via	O
a	O
C++	B-Language
library	O
,	O
that	O
is	O
used	O
to	O
create	O
and	O
model	O
a	O
Dryad	B-Operating_System
execution	O
graph	O
.	O
</s>
<s>
Computational	O
vertices	O
are	O
written	O
using	O
standard	O
C++	B-Language
constructs	O
.	O
</s>
<s>
To	O
make	O
them	O
accessible	O
to	O
the	O
Dryad	B-Operating_System
runtime	O
,	O
they	O
must	O
be	O
encapsulated	O
in	O
a	O
class	O
that	O
inherits	B-Language
from	O
the	O
GraphNode	O
base	O
class	O
.	O
</s>
<s>
The	O
graph	O
is	O
defined	O
by	O
adding	O
edges	O
;	O
edges	O
are	O
added	O
by	O
using	O
a	O
composition	O
operator	O
(	O
defined	O
by	O
Dryad	B-Operating_System
)	O
that	O
connects	O
two	O
graphs	O
(	O
or	O
two	O
nodes	O
of	O
a	O
graph	O
)	O
with	O
an	O
edge	O
.	O
</s>
<s>
Managed	O
code	O
wrappers	O
for	O
the	O
Dryad	B-Operating_System
API	B-General_Concept
can	O
also	O
be	O
written	O
.	O
</s>
<s>
There	O
exist	O
several	O
high-level	O
language	O
compilers	O
which	O
use	O
Dryad	B-Operating_System
as	O
a	O
runtime	O
;	O
examples	O
include	O
Scope	O
(	O
Structured	B-General_Concept
Computations	O
Optimized	O
for	O
Parallel	O
Execution	O
)	O
and	O
DryadLINQ	O
.	O
</s>
<s>
In	O
October	O
2011	O
,	O
Microsoft	O
discontinued	O
active	O
development	O
on	O
Dryad	B-Operating_System
,	O
shifting	O
focus	O
to	O
the	O
Apache	B-Application
Hadoop	I-Application
framework	O
.	O
</s>
