<s>
Distributed	B-Operating_System
data	I-Operating_System
flow	I-Operating_System
(	O
also	O
abbreviated	O
as	O
distributed	O
flow	O
)	O
refers	O
to	O
a	O
set	O
of	O
events	O
in	O
a	O
distributed	B-Architecture
application	I-Architecture
or	O
protocol	O
.	O
</s>
<s>
Distributed	B-Operating_System
data	I-Operating_System
flows	I-Operating_System
serve	O
a	O
purpose	O
analogous	O
to	O
variables	O
or	O
method	O
parameters	O
in	O
programming	O
languages	O
such	O
as	O
Java	B-Language
,	O
in	O
that	O
they	O
can	O
represent	O
state	O
that	O
is	O
stored	O
or	O
communicated	O
by	O
a	O
layer	O
of	O
software	O
.	O
</s>
<s>
As	O
such	O
,	O
distributed	O
flows	O
are	O
a	O
more	O
natural	O
way	O
of	O
modeling	O
the	O
semantics	O
and	O
inner	O
workings	O
of	O
certain	O
classes	O
of	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
In	O
particular	O
,	O
the	O
distributed	B-Operating_System
data	I-Operating_System
flow	I-Operating_System
abstraction	O
has	O
been	O
used	O
as	O
a	O
convenient	O
way	O
of	O
expressing	O
the	O
high-level	O
logical	O
relationships	O
between	O
parts	O
of	O
distributed	O
protocols	O
.	O
</s>
<s>
A	O
distributed	B-Operating_System
data	I-Operating_System
flow	I-Operating_System
satisfies	O
the	O
following	O
informal	O
properties	O
.	O
</s>
<s>
Asynchronous	O
,	O
non-blocking	B-Operating_System
,	O
and	O
one-way	O
.	O
</s>
<s>
Each	O
event	O
represents	O
a	O
single	O
instance	O
of	O
a	O
non-blocking	B-Operating_System
,	O
one-way	O
,	O
asynchronous	B-Operating_System
method	I-Operating_System
invocation	I-Operating_System
or	O
other	O
form	O
of	O
explicit	O
or	O
implicit	O
message	B-Architecture
passing	I-Architecture
between	O
two	O
layers	B-Architecture
or	O
software	B-Architecture
components	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
each	O
event	O
might	O
represent	O
a	O
single	O
request	O
to	O
multicast	O
a	O
packet	B-Protocol
,	O
issued	O
by	O
an	O
application	B-Application
layer	I-Application
to	O
an	O
underlying	O
multicast	B-Operating_System
protocol	I-Operating_System
.	O
</s>
<s>
All	O
events	O
in	O
the	O
distributed	O
flow	O
serve	O
the	O
same	O
functional	O
and	O
logical	O
purpose	O
,	O
and	O
are	O
related	O
to	O
one	O
another	O
;	O
generally	O
,	O
we	O
require	O
that	O
they	O
represent	O
method	O
calls	O
or	O
message	O
exchanges	O
between	O
instances	O
of	O
the	O
same	O
functional	O
layers	B-Architecture
,	O
or	O
instances	O
of	O
the	O
same	O
components	B-Architecture
,	O
but	O
perhaps	O
on	O
different	O
nodes	B-Protocol
within	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
a	O
set	O
of	O
events	O
that	O
includes	O
all	O
multicast	O
requests	O
issued	O
by	O
the	O
same	O
application	B-Application
layer	I-Application
to	O
the	O
same	O
multicast	B-Operating_System
protocol	I-Operating_System
is	O
a	O
distributed	O
flow	O
.	O
</s>
<s>
The	O
flow	O
usually	O
includes	O
all	O
events	O
that	O
flow	O
between	O
the	O
two	O
layers	B-Architecture
of	O
software	O
,	O
simultaneously	O
at	O
different	O
locations	O
,	O
and	O
over	O
a	O
finite	O
or	O
infinite	O
period	O
of	O
time	O
.	O
</s>
<s>
Thus	O
,	O
in	O
general	O
,	O
events	O
in	O
a	O
distributed	O
flow	O
are	O
distributed	O
both	O
in	O
space	O
(	O
they	O
occur	O
at	O
different	O
nodes	B-Protocol
)	O
and	O
in	O
time	O
(	O
they	O
occur	O
at	O
different	O
times	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
flow	O
of	O
multicast	O
requests	O
would	O
include	O
all	O
such	O
requests	O
made	O
by	O
instances	O
of	O
the	O
given	O
application	O
on	O
different	O
nodes	B-Protocol
;	O
normally	O
,	O
such	O
flow	O
would	O
include	O
events	O
that	O
occur	O
on	O
all	O
nodes	B-Protocol
participating	O
in	O
the	O
given	O
multicast	B-Operating_System
protocol	I-Operating_System
.	O
</s>
