<s>
The	O
Application	B-Application
Interface	I-Application
Specification	I-Application
(	O
AIS	O
)	O
is	O
a	O
collection	O
of	O
open	O
specifications	O
that	O
define	O
the	O
application	B-Application
programming	I-Application
interfaces	I-Application
(	O
APIs	B-Application
)	O
for	O
high-availability	O
application	O
computer	O
software	O
.	O
</s>
<s>
The	O
original	O
specifications	O
,	O
released	O
on	O
April	O
14	O
,	O
2003	O
,	O
were	O
the	O
Availability	O
Management	O
Framework	O
(	O
AMF	O
)	O
,	O
the	O
Cluster	O
Membership	O
Service	O
(	O
CLM	O
)	O
and	O
four	O
other	O
utility	O
services	O
(	O
Checkpoint	B-General_Concept
,	O
Event	O
,	O
Message	O
,	O
Lock	O
)	O
.	O
</s>
<s>
Release	O
3	O
(	O
January	O
18	O
,	O
2006	O
)	O
added	O
the	O
first	O
set	O
of	O
management	O
services	O
:	O
Log	O
,	O
Notification	O
and	O
Information	B-Application
Model	I-Application
Management	O
(	O
IMM	O
)	O
.	O
</s>
<s>
Release	O
6	O
(	O
October	O
21	O
,	O
2008	O
)	O
added	O
the	O
Platform	O
Management	O
Service	O
to	O
close	O
the	O
gap	O
between	O
AIS	O
and	O
HPI	O
(	O
Hardware	B-Application
Platform	I-Application
Interface	I-Application
)	O
.	O
</s>
<s>
Initially	O
,	O
the	O
APIs	B-Application
were	O
defined	O
in	O
the	O
C	B-Language
programming	I-Language
language	I-Language
only	O
,	O
but	O
as	O
of	O
July	O
2008	O
,	O
the	O
Java	B-Language
mapping	O
of	O
the	O
different	O
service	O
APIs	B-Application
is	O
being	O
released	O
incrementally	O
.	O
</s>
<s>
It	O
is	O
expected	O
that	O
all	O
AIS	O
Services	O
should	O
use	O
the	O
AIS	O
Management	O
Services	O
to	O
expose	O
their	O
administrative	O
interfaces	O
,	O
configuration	O
,	O
and	O
runtime	B-Library
management	O
information	O
(	O
fig2	O
)	O
.	O
</s>
<s>
Hardware	O
Element	O
(	O
HE	O
)	O
-	O
A	O
hardware	O
element	O
is	O
a	O
logical	O
entity	O
that	O
represents	O
any	O
kind	O
of	O
hardware	O
entity	O
,	O
which	O
can	O
be	O
,	O
for	O
instance	O
,	O
a	O
chassis	O
,	O
a	O
CPU	B-Architecture
blade	I-Architecture
,	O
or	O
an	O
I/O	O
device	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
CPU	B-Architecture
blade	I-Architecture
or	O
an	O
SMP	B-Operating_System
machine	O
runs	O
a	O
single	O
operating	O
system	O
instance	O
modeled	O
as	O
an	O
execution	O
environment	O
.	O
</s>
<s>
PLM	O
maintains	O
the	O
state	O
of	O
these	O
entities	O
in	O
the	O
information	B-Application
model	I-Application
and	O
provides	O
means	O
to	O
control	O
them	O
and	O
track	O
any	O
state	O
changes	O
.	O
</s>
<s>
The	O
CLM	O
provides	O
APIs	B-Application
to	O
retrieve	O
the	O
current	O
cluster	O
membership	O
information	O
and	O
to	O
track	O
membership	O
changes	O
(	O
e.g.	O
</s>
<s>
All	O
cluster-wide	O
AIS	O
services	O
must	O
use	O
the	O
CLM	O
track	O
API	B-Application
to	O
determine	O
the	O
membership	O
.	O
</s>
<s>
are	O
represented	O
as	O
managed	O
objects	O
in	O
the	O
SA	O
Forum	O
Information	B-Application
Model	I-Application
(	O
IM	O
)	O
,	O
which	O
can	O
be	O
seen	O
as	O
a	O
configuration	B-Application
management	I-Application
database	I-Application
.	O
</s>
<s>
The	O
logical	O
entities	O
represented	O
by	O
the	O
objects	O
in	O
the	O
IM	O
are	O
not	O
generally	O
implemented	O
by	O
the	O
IMM	O
Service	O
itself	O
;	O
instead	O
,	O
user	O
applications	O
and	O
the	O
AIS	O
Services	O
,	O
such	O
as	O
the	O
Checkpoint	B-General_Concept
Service	O
or	O
the	O
Availability	O
Management	O
Framework	O
provide	O
their	O
implementation	O
.	O
</s>
<s>
There	O
are	O
two	O
categories	O
of	O
objects	O
and	O
attributes	O
in	O
the	O
IM	O
:	O
runtime	B-Library
and	O
configuration	O
.	O
</s>
<s>
Runtime	B-Library
objects	O
and	O
attributes	O
reflect	O
the	O
current	O
state	O
of	O
the	O
entities	O
they	O
represent	O
–	O
they	O
are	O
of	O
descriptive	O
nature	O
.	O
</s>
<s>
Configuration	O
objects	O
may	O
include	O
both	O
configuration	O
and	O
runtime	B-Library
attributes	O
while	O
runtime	B-Library
objects	O
may	O
include	O
only	O
runtime	B-Library
attributes	O
.	O
</s>
<s>
Accordingly	O
,	O
the	O
IMM	O
Service	O
exposes	O
a	O
“	O
southbound	B-Architecture
”	O
interface	O
–	O
the	O
IMM-OI	O
API	B-Application
–	O
to	O
object	O
implementers	O
and	O
a	O
“	O
northbound	B-Architecture
”	O
interface	O
–	O
the	O
IMM-OM	O
API	B-Application
–	O
to	O
management	O
applications	O
(	O
fig	O
.	O
</s>
<s>
The	O
NTF	O
service	O
is	O
based	O
on	O
the	O
publish-subscribe	B-Operating_System
paradigm	O
.	O
</s>
<s>
Notifications	O
are	O
generated/published	O
by	O
producers	O
using	O
the	O
notification	O
producer	O
API	B-Application
.	O
</s>
<s>
The	O
notification	O
consumers	O
can	O
be	O
either	O
subscribers	O
,	O
who	O
subscribe	O
for	O
notifications	O
and	O
receive	O
them	O
as	O
they	O
occur	O
;	O
or	O
readers	O
,	O
who	O
retrieve	O
notifications	O
from	O
persisted	O
logs	O
using	O
the	O
notification	O
consumer	O
API	B-Application
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
the	O
application	O
needs	O
to	O
be	O
described	O
according	O
to	O
the	O
information	B-Application
model	I-Application
specified	O
for	O
AMF	O
.	O
</s>
<s>
The	O
basic	O
logical	O
entity	O
of	O
this	O
information	B-Application
model	I-Application
is	O
the	O
component	O
,	O
which	O
represents	O
a	O
set	O
of	O
resources	O
to	O
the	O
Availability	O
Management	O
Framework	O
that	O
encapsulate	O
some	O
specific	O
application	O
functionality	O
.	O
</s>
<s>
The	O
fundamental	O
principle	O
of	O
fault	B-General_Concept
tolerant	I-General_Concept
design	I-General_Concept
is	O
to	O
provide	O
the	O
services	O
by	O
a	O
set	O
of	O
redundant	O
entities	O
and	O
therefore	O
components	O
need	O
to	O
be	O
able	O
to	O
act	O
as	O
a	O
standby	O
on	O
behalf	O
of	O
the	O
CSI	O
.	O
</s>
<s>
Accordingly	O
,	O
the	O
APIs	B-Application
provided	O
by	O
the	O
Availability	O
Management	O
Framework	O
enable	O
component	O
registration	O
,	O
life	O
cycle	O
management	O
and	O
workload	O
assignments	O
.	O
</s>
<s>
These	O
range	O
from	O
the	O
simple	O
2N	O
model	O
(	O
also	O
known	O
as	O
1+1	O
,	O
or	O
active-standby	O
)	O
to	O
more	O
sophisticated	O
ones	O
such	O
as	O
the	O
N-way	B-Operating_System
redundancy	O
model	O
,	O
which	O
allows	O
for	O
more	O
than	O
one	O
standby	O
assignment	O
on	O
behalf	O
of	O
the	O
same	O
component	O
service	O
instance	O
or	O
the	O
N-way-active	O
that	O
allows	O
multiple	O
active	O
assignments	O
.	O
</s>
<s>
The	O
deployment	O
configuration	O
constitutes	O
an	O
essential	O
part	O
of	O
the	O
information	B-Application
model	I-Application
managed	O
by	O
the	O
IMM	O
Service	O
.	O
</s>
<s>
The	O
Software	O
Management	O
Framework	O
(	O
SMF	O
)	O
maintains	O
the	O
part	O
of	O
the	O
information	B-Application
model	I-Application
that	O
describes	O
the	O
software	O
available	O
for	O
,	O
and	O
deployed	O
in	O
,	O
the	O
cluster	O
.	O
</s>
<s>
In	O
SMF	O
terms	O
,	O
this	O
migration	O
process	B-Operating_System
is	O
called	O
an	O
upgrade	O
campaign	O
.	O
</s>
<s>
An	O
SMF	O
implementation	O
migrates	O
the	O
system	O
from	O
one	O
deployment	O
configuration	O
to	O
a	O
new	O
desired	O
one	O
based	O
on	O
such	O
an	O
XML	O
file	O
,	O
which	O
is	O
essentially	O
a	O
script	B-Language
of	O
ordered	O
actions	O
and	O
configuration	O
changes	O
that	O
lead	O
to	O
the	O
new	O
configuration	O
.	O
</s>
<s>
To	O
accomplish	O
all	O
these	O
tasks	O
,	O
the	O
SMF	O
implementation	O
interacts	O
at	O
least	O
(	O
1	O
)	O
with	O
AMF	O
in	O
order	O
to	O
maintain	O
availability	O
,	O
(	O
2	O
)	O
with	O
IMM	O
to	O
carry	O
out	O
changes	O
to	O
the	O
information	B-Application
model	I-Application
,	O
and	O
(	O
3	O
)	O
with	O
NTF	O
to	O
receive	O
notifications	O
that	O
may	O
indicate	O
error	O
situations	O
caused	O
by	O
the	O
ongoing	O
campaign	O
.	O
</s>
<s>
The	O
Software	O
Management	O
Framework	O
also	O
provides	O
an	O
API	B-Application
for	O
client	O
processes	O
to	O
register	O
their	O
interest	O
in	O
receiving	O
callbacks	O
when	O
a	O
relevant	O
upgrade	O
campaign	O
is	O
initiated	O
in	O
the	O
cluster	O
and	O
as	O
it	O
progresses	O
through	O
significant	O
milestones	O
.	O
</s>
<s>
The	O
Checkpoint	B-General_Concept
Service	O
provides	O
a	O
facility	O
for	O
processes	O
to	O
record	O
checkpoint	B-General_Concept
data	O
incrementally	O
,	O
which	O
can	O
be	O
used	O
to	O
protect	O
an	O
application	O
against	O
failures	O
.	O
</s>
<s>
When	O
a	O
process	B-Operating_System
recovers	O
from	O
a	O
failure	O
(	O
with	O
a	O
restart	O
or	O
a	O
failover	B-General_Concept
procedure	O
)	O
,	O
the	O
Checkpoint	B-General_Concept
Service	O
can	O
be	O
used	O
to	O
retrieve	O
the	O
previously	O
checkpointed	O
data	O
and	O
resume	O
execution	O
from	O
the	O
recorded	O
state	O
,	O
thus	O
minimizing	O
the	O
impact	O
of	O
the	O
failure	O
.	O
</s>
<s>
A	O
copy	O
of	O
the	O
data	O
stored	O
in	O
a	O
checkpoint	B-General_Concept
is	O
called	O
a	O
checkpoint	B-General_Concept
replica	O
,	O
which	O
is	O
typically	O
stored	O
in	O
main	O
memory	O
rather	O
than	O
on	O
disk	O
for	O
performance	O
reasons	O
.	O
</s>
<s>
A	O
checkpoint	B-General_Concept
may	O
have	O
several	O
checkpoint	B-General_Concept
replicas	O
stored	O
on	O
different	O
nodes	O
in	O
the	O
cluster	O
to	O
protect	O
it	O
against	O
node	O
failures	O
.	O
</s>
<s>
The	O
process	B-Operating_System
creating	O
the	O
checkpoint	B-General_Concept
may	O
choose	O
between	O
synchronous	O
and	O
asynchronous	O
replica-update	O
policies	O
.	O
</s>
<s>
The	O
Event	O
Service	O
is	O
a	O
publish/subscribe	B-Operating_System
multipoint-to-multipoint	O
communication	O
mechanism	O
that	O
is	O
based	O
on	O
the	O
concept	O
of	O
event	O
channels	O
:	O
one	O
or	O
more	O
publishers	O
communicate	O
asynchronously	O
with	O
one	O
or	O
more	O
anonymous	O
subscribers	O
by	O
using	O
events	O
over	O
an	O
event	O
channel	O
.	O
</s>
<s>
The	O
Event	O
Service	O
API	B-Application
does	O
not	O
impose	O
a	O
specific	O
layout	O
for	O
the	O
published	O
event	O
data	O
.	O
</s>
<s>
When	O
a	O
process	B-Operating_System
subscribes	O
on	O
an	O
event	O
channel	O
to	O
receive	O
published	O
events	O
,	O
it	O
specifies	O
the	O
filters	O
to	O
apply	O
on	O
the	O
published	O
events	O
.	O
</s>
<s>
Events	O
are	O
only	O
delivered	O
to	O
the	O
process	B-Operating_System
if	O
they	O
satisfy	O
the	O
provided	O
filters	O
.	O
</s>
<s>
The	O
Lock	O
Service	O
is	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
service	I-Operating_System
,	O
which	O
is	O
intended	O
for	O
use	O
in	O
a	O
cluster	O
where	O
processes	O
in	O
different	O
nodes	O
might	O
compete	O
with	O
each	O
other	O
for	O
access	O
to	O
a	O
shared	O
resource	O
.	O
</s>
<s>
The	O
Message	O
Service	O
specifies	O
APIs	B-Application
for	O
a	O
cluster-wide	O
inter-process	O
communicationsystem	O
.	O
</s>
<s>
Any	O
number	O
of	O
processes	O
can	O
send	O
messages	O
to	O
a	O
message	O
queue	O
,	O
but	O
one	O
process	B-Operating_System
at	O
a	O
time	O
at	O
most	O
can	O
open	O
it	O
for	O
receiving	O
.	O
</s>
<s>
The	O
single	O
message	O
queue	O
thus	O
supports	O
point-to-point	B-Architecture
or	O
multi-point-to-point	O
communication	O
patterns	O
.	O
</s>
<s>
Processes	O
sending	O
messages	O
to	O
a	O
message	O
queue	O
are	O
unaware	O
of	O
the	O
identity	O
of	O
the	O
receiving	O
process	B-Operating_System
;	O
therefore	O
,	O
the	O
process	B-Operating_System
that	O
was	O
originally	O
receiving	O
these	O
messages	O
may	O
have	O
been	O
replaced	O
by	O
another	O
process	B-Operating_System
during	O
a	O
failover	B-General_Concept
or	O
switch-over	O
.	O
</s>
<s>
They	O
are	O
identified	O
by	O
logical	O
names	O
so	O
that	O
a	O
sender	O
process	B-Operating_System
is	O
unaware	O
of	O
the	O
number	O
of	O
message	O
queues	O
and	O
of	O
the	O
location	O
of	O
the	O
message	O
queues	O
within	O
the	O
cluster	O
with	O
which	O
it	O
is	O
communicating	O
.	O
</s>
<s>
MSG	O
defines	O
three	O
unicast	B-Protocol
distribution	O
policies	O
–	O
equal	O
load	O
distribution	O
,	O
local	O
equal	O
load	O
distribution	O
and	O
local	O
best	O
queue	O
–	O
and	O
the	O
broadcast	O
(	O
multicast	O
)	O
policy	O
.	O
</s>
<s>
on	O
message	O
queues	O
and	O
on	O
unicast	B-Protocol
message	O
queue	O
groups	O
.	O
</s>
<s>
Periodic	O
timers	O
will	O
expire	O
each	O
time	O
a	O
specified	O
duration	O
is	O
reached	O
,	O
and	O
the	O
process	B-Operating_System
is	O
notified	O
about	O
the	O
expirations	O
.	O
</s>
<s>
The	O
same	O
naming	O
conventions	O
,	O
standard	O
predefined	O
types	O
and	O
constants	O
,	O
API	B-Application
semantics	O
,	O
library	O
life	O
cycle	O
control	O
,	O
etc	O
.	O
</s>
<s>
The	O
SA	O
Forum	O
Application	O
Interface	O
occurs	O
between	O
a	O
process	B-Operating_System
and	O
a	O
library	O
that	O
implements	O
the	O
interface	O
.	O
</s>
<s>
The	O
term	O
'	O
process	B-Operating_System
 '	O
can	O
be	O
regarded	O
as	O
being	O
equivalent	O
to	O
a	O
process	B-Operating_System
defined	O
by	O
the	O
POSIX	O
standard	O
;	O
however	O
,	O
AIS	O
does	O
not	O
mandate	O
a	O
POSIX	O
process	B-Operating_System
,	O
but	O
rather	O
,	O
any	O
equivalent	O
entity	O
that	O
a	O
system	O
provides	O
to	O
manage	O
executing	O
software	O
.	O
</s>
<s>
The	O
area	O
server	O
is	O
an	O
abstraction	O
that	O
represents	O
the	O
server	O
that	O
provides	O
services	O
for	O
a	O
specification	O
area	O
(	O
Availability	O
Management	O
Framework	O
,	O
Cluster	O
Membership	O
Service	O
,	O
Checkpoint	B-General_Concept
Service	O
,	O
and	O
so	O
on	O
)	O
.	O
</s>
<s>
The	O
area	O
implementation	O
libraries	O
may	O
be	O
implemented	O
in	O
one	O
or	O
several	O
physical	O
libraries	O
;	O
however	O
,	O
a	O
process	B-Operating_System
is	O
required	O
to	O
initialize	O
,	O
register	O
,	O
and	O
obtain	O
an	O
operating	O
system	O
selection	O
object	O
separately	O
for	O
each	O
area	O
's	O
implementation	O
library	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
Service	O
Availability	O
library	O
starts	O
with	O
a	O
call	O
to	O
initialize	O
the	O
library	O
,	O
which	O
potentially	O
loads	O
any	O
dynamic	O
code	O
and	O
binds	O
the	O
asynchronous	O
calls	O
implemented	O
by	O
the	O
process	B-Operating_System
.	O
</s>
<s>
When	O
the	O
process	B-Operating_System
no	O
longer	O
requires	O
the	O
use	O
of	O
the	O
area	O
functions	O
,	O
it	O
calls	O
the	O
area	O
finalization	O
function	O
,	O
which	O
disassociates	O
the	O
process	B-Operating_System
from	O
the	O
interface	O
area	O
implementation	O
instance	O
and	O
recovers	O
any	O
associated	O
resources	O
.	O
</s>
<s>
Synchronous	O
APIs	B-Application
are	O
generally	O
used	O
for	O
library	O
and	O
association	O
housekeeping	O
interfaces	O
.	O
</s>
<s>
The	O
API	B-Application
track	O
typically	O
consists	O
of	O
three	O
functions	O
:	O
the	O
client-invoked	O
initiate	O
and	O
stop	O
tracking	O
of	O
an	O
entity	O
;	O
and	O
the	O
service-invoked	O
callback	O
to	O
notify	O
the	O
client	O
about	O
(	O
pending	O
)	O
changes	O
of	O
a	O
tracked	O
entity	O
.	O
</s>
<s>
Considering	O
an	O
AIS	O
implementation	O
that	O
supports	O
both	O
versions	O
Vx	O
and	O
Vy	O
,	O
a	O
process	B-Operating_System
can	O
initialize	O
the	O
library	O
specifying	O
either	O
Vx	O
or	O
Vy	O
:	O
</s>
<s>
if	O
the	O
process	B-Operating_System
initializes	O
a	O
library	O
handle	O
with	O
Vx	O
,	O
this	O
handle	O
does	O
not	O
provide	O
access	O
to	O
functions	O
that	O
have	O
been	O
introduced	O
in	O
versions	O
newer	O
than	O
Vx	O
.	O
</s>
<s>
if	O
the	O
process	B-Operating_System
initializes	O
a	O
library	O
handle	O
with	O
Vy	O
,	O
this	O
handle	O
does	O
not	O
provide	O
access	O
to	O
a	O
function	O
introduced	O
in	O
versions	O
older	O
than	O
Vy	O
and	O
then	O
replaced	O
by	O
a	O
newer	O
variant	O
of	O
the	O
same	O
function	O
.	O
</s>
<s>
In	O
particular	O
,	O
this	O
handle	O
will	O
not	O
enable	O
the	O
process	B-Operating_System
to	O
successfully	O
invoke	O
f( )	O
.	O
</s>
<s>
Note	O
,	O
however	O
,	O
that	O
a	O
process	B-Operating_System
may	O
initialize	O
the	O
library	O
multiple	O
times	O
each	O
time	O
with	O
the	O
version	O
appropriate	O
to	O
the	O
functionality	O
it	O
intends	O
to	O
obtain	O
.	O
</s>
<s>
Releases	O
with	O
major	O
number	O
change	O
may	O
introduce	O
new	O
features	O
and	O
change	O
the	O
API	B-Application
in	O
a	O
backward-compatible	O
way	O
as	O
described	O
above	O
.	O
</s>
<s>
Releases	O
with	O
a	O
minor	O
number	O
change	O
do	O
not	O
change	O
the	O
API	B-Application
.	O
</s>
<s>
The	O
SA	O
Forum	O
Implementation	O
Registry	O
is	O
a	O
process	B-Operating_System
that	O
enables	O
implementations	O
of	O
the	O
SA	O
Forum	O
specifications	O
to	O
be	O
registered	O
and	O
made	O
publicly	O
available	O
.	O
</s>
