<s>
In	O
fault-tolerant	B-General_Concept
distributed	B-Architecture
computing	I-Architecture
,	O
an	O
atomic	B-Operating_System
broadcast	I-Operating_System
or	O
total	O
order	O
broadcast	B-Operating_System
is	O
a	O
broadcast	B-Operating_System
where	O
all	O
correct	O
processes	O
in	O
a	O
system	O
of	O
multiple	O
processes	O
receive	O
the	O
same	O
set	O
of	O
messages	O
in	O
the	O
same	O
order	O
;	O
that	O
is	O
,	O
the	O
same	O
sequence	O
of	O
messages	O
.	O
</s>
<s>
The	O
broadcast	B-Operating_System
is	O
termed	O
"	O
atomic	B-General_Concept
"	O
because	O
it	O
either	O
eventually	O
completes	O
correctly	O
at	O
all	O
participants	O
,	O
or	O
all	O
participants	O
abort	O
without	O
side	O
effects	O
.	O
</s>
<s>
Atomic	B-Operating_System
broadcasts	I-Operating_System
are	O
an	O
important	O
distributed	B-Architecture
computing	I-Architecture
primitive	O
.	O
</s>
<s>
The	O
following	O
properties	O
are	O
usually	O
required	O
from	O
an	O
atomic	B-Operating_System
broadcast	I-Operating_System
protocol	O
:	O
</s>
<s>
Validity	O
:	O
if	O
a	O
correct	O
participant	O
broadcasts	B-Operating_System
a	O
message	O
,	O
then	O
all	O
correct	O
participants	O
will	O
eventually	O
receive	O
it	O
.	O
</s>
<s>
Uniform	O
Integrity	O
:	O
a	O
message	O
is	O
received	O
by	O
each	O
participant	O
at	O
most	O
once	O
,	O
and	O
only	O
if	O
it	O
was	O
previously	O
broadcast	B-Operating_System
.	O
</s>
<s>
define	O
the	O
integrity	O
and	O
validity	O
properties	O
of	O
atomic	B-Operating_System
broadcast	I-Operating_System
slightly	O
differently	O
.	O
</s>
<s>
Note	O
that	O
total	O
order	O
is	O
not	O
equivalent	O
to	O
FIFO	B-Operating_System
order	O
,	O
which	O
requires	O
that	O
if	O
a	O
process	O
sent	O
message	O
1	O
before	O
it	O
sent	O
message	O
2	O
,	O
then	O
all	O
participants	O
must	O
receive	O
message	O
1	O
before	O
receiving	O
message	O
2	O
.	O
</s>
<s>
Designing	O
an	O
algorithm	O
for	O
atomic	B-Operating_System
broadcasts	I-Operating_System
is	O
relatively	O
easy	O
if	O
it	O
can	O
be	O
assumed	O
that	O
computers	O
will	O
not	O
fail	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
there	O
are	O
no	O
failures	O
,	O
atomic	B-Operating_System
broadcast	I-Operating_System
can	O
be	O
achieved	O
simply	O
by	O
having	O
all	O
participants	O
communicate	O
with	O
one	O
"	O
leader	O
"	O
which	O
determines	O
the	O
order	O
of	O
the	O
messages	O
,	O
with	O
the	O
other	O
participants	O
following	O
the	O
leader	O
.	O
</s>
<s>
In	O
such	O
an	O
environment	O
achieving	O
atomic	B-Operating_System
broadcasts	I-Operating_System
is	O
difficult	O
.	O
</s>
<s>
A	O
number	O
of	O
protocols	O
have	O
been	O
proposed	O
for	O
performing	O
atomic	B-Operating_System
broadcast	I-Operating_System
,	O
under	O
various	O
assumptions	O
about	O
the	O
network	O
,	O
failure	O
models	O
,	O
availability	O
of	O
hardware	O
support	O
for	O
multicast	O
,	O
and	O
so	O
forth	O
.	O
</s>
<s>
In	O
order	O
for	O
the	O
conditions	O
for	O
atomic	B-Operating_System
broadcast	I-Operating_System
to	O
be	O
satisfied	O
,	O
the	O
participants	O
must	O
effectively	O
"	O
agree	O
"	O
on	O
the	O
order	O
of	O
receipt	O
of	O
the	O
messages	O
.	O
</s>
<s>
Such	O
considerations	O
indicate	O
that	O
in	O
systems	O
with	O
crash	O
failures	O
,	O
atomic	B-Operating_System
broadcast	I-Operating_System
and	O
consensus	B-Operating_System
are	O
equivalent	O
problems	O
.	O
</s>
<s>
A	O
value	O
can	O
be	O
proposed	O
by	O
a	O
process	O
for	O
consensus	B-Operating_System
by	O
atomically	O
broadcasting	B-Operating_System
it	O
,	O
and	O
a	O
process	O
can	O
decide	O
a	O
value	O
by	O
selecting	O
the	O
value	O
of	O
the	O
first	O
message	O
which	O
it	O
atomically	O
receives	O
.	O
</s>
<s>
Thus	O
,	O
consensus	B-Operating_System
can	O
be	O
reduced	O
to	O
atomic	B-Operating_System
broadcast	I-Operating_System
.	O
</s>
<s>
Conversely	O
,	O
a	O
group	O
of	O
participants	O
can	O
atomically	O
broadcast	B-Operating_System
messages	O
by	O
achieving	O
consensus	B-Operating_System
regarding	O
the	O
first	O
message	O
to	O
be	O
received	O
,	O
followed	O
by	O
achieving	O
consensus	B-Operating_System
on	O
the	O
next	O
message	O
,	O
and	O
so	O
forth	O
until	O
all	O
the	O
messages	O
have	O
been	O
received	O
.	O
</s>
<s>
Thus	O
,	O
atomic	B-Operating_System
broadcast	I-Operating_System
reduces	O
to	O
consensus	B-Operating_System
.	O
</s>
<s>
A	O
fundamental	O
result	O
in	O
distributed	B-Architecture
computing	I-Architecture
is	O
that	O
achieving	O
consensus	B-Operating_System
in	O
asynchronous	O
systems	O
in	O
which	O
even	O
one	O
crash	O
failure	O
can	O
occur	O
is	O
impossible	O
in	O
the	O
most	O
general	O
case	O
.	O
</s>
<s>
Since	O
consensus	B-Operating_System
and	O
atomic	B-Operating_System
broadcast	I-Operating_System
are	O
equivalent	O
,	O
FLP	O
applies	O
also	O
to	O
atomic	B-Operating_System
broadcast	I-Operating_System
.	O
</s>
<s>
The	O
FLP	O
result	O
does	O
not	O
prohibit	O
the	O
implementation	O
of	O
atomic	B-Operating_System
broadcast	I-Operating_System
in	O
practice	O
,	O
but	O
it	O
does	O
require	O
making	O
less	O
stringent	O
assumptions	O
than	O
FLP	O
in	O
some	O
respect	O
,	O
such	O
as	O
about	O
processor	O
and	O
communication	O
timings	O
.	O
</s>
<s>
The	O
Chandra-Toueg	B-Operating_System
algorithm	I-Operating_System
is	O
a	O
consensus-based	O
solution	O
to	O
atomic	B-Operating_System
broadcast	I-Operating_System
.	O
</s>
<s>
The	O
Zookeeper	B-Language
Atomic	B-Operating_System
Broadcast	I-Operating_System
(	O
ZAB	O
)	O
protocol	O
is	O
the	O
basic	O
building	O
block	O
for	O
Apache	B-Language
ZooKeeper	I-Language
,	O
a	O
fault-tolerant	B-General_Concept
distributed	O
coordination	O
service	O
which	O
underpins	O
Hadoop	B-Application
and	O
many	O
other	O
important	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
Ken	O
Birman	O
has	O
proposed	O
the	O
virtual	O
synchrony	O
execution	O
model	O
for	O
distributed	B-Architecture
systems	I-Architecture
,	O
the	O
idea	O
of	O
which	O
is	O
that	O
all	O
processes	O
observe	O
the	O
same	O
events	O
in	O
the	O
same	O
order	O
.	O
</s>
<s>
A	O
total	O
ordering	O
of	O
the	O
messages	O
being	O
received	O
,	O
as	O
in	O
atomic	B-Operating_System
broadcast	I-Operating_System
,	O
is	O
one	O
(	O
though	O
not	O
the	O
only	O
)	O
method	O
for	O
attaining	O
virtually	O
synchronous	O
message	O
receipt	O
.	O
</s>
