<s>
Appia	B-Operating_System
is	O
a	O
free	B-License
and	I-License
open-source	I-License
layered	O
communication	O
toolkit	O
implemented	O
in	O
Java	B-Language
,	O
and	O
licensed	O
under	O
the	O
Apache	B-Application
License	I-Application
,	I-Application
version	I-Application
2.0	I-Application
.	O
</s>
<s>
Appia	B-Operating_System
is	O
composed	O
by	O
a	O
core	O
that	O
is	O
used	O
to	O
compose	O
protocols	O
,	O
and	O
a	O
set	O
of	O
protocols	O
that	O
provide	O
group	O
communication	O
,	O
ordering	O
guaranties	O
,	O
atomic	O
broadcast	O
,	O
among	O
other	O
properties	O
.	O
</s>
<s>
The	O
Appia	B-Operating_System
core	O
offers	O
a	O
clean	O
way	O
for	O
the	O
application	O
to	O
express	O
inter-channel	O
constraints	O
.	O
</s>
<s>
Thus	O
,	O
Appia	B-Operating_System
retains	O
a	O
flexible	O
and	O
modular	O
design	O
that	O
allows	O
communication	O
stacks	O
to	O
be	O
composed	O
and	O
reconfigured	O
in	O
run-time	O
.	O
</s>
<s>
The	O
existing	O
protocols	O
include	O
interface	O
with	O
TCP	B-Protocol
and	O
UDP	B-Protocol
sockets	I-Protocol
,	O
virtual	O
synchrony	O
,	O
several	O
implementations	O
of	O
total	O
order	O
,	O
causal	O
order	O
,	O
among	O
others	O
.	O
</s>
