<s>
Push	B-General_Concept
technology	I-General_Concept
or	O
server	B-General_Concept
push	I-General_Concept
is	O
a	O
style	O
of	O
Internet-based	O
communication	O
where	O
the	O
request	O
for	O
a	O
given	O
transaction	O
is	O
initiated	O
by	O
the	O
publisher	O
or	O
central	O
server	B-Application
.	O
</s>
<s>
It	O
is	O
contrasted	O
with	O
pull	O
/	O
get	O
,	O
where	O
the	O
request	O
for	O
the	O
transmission	O
of	O
information	O
is	O
initiated	O
by	O
the	O
receiver	O
or	O
client	B-Protocol
.	O
</s>
<s>
Push	O
services	O
are	O
often	O
based	O
on	O
information	O
and	O
data	O
preferences	O
expressed	O
in	O
advance	O
,	O
called	O
a	O
publish-subscribe	B-Operating_System
model	O
.	O
</s>
<s>
A	O
client	B-Protocol
"	O
subscribes	O
"	O
to	O
various	O
information	O
"	O
channels	O
"	O
provided	O
by	O
a	O
server	B-Application
;	O
whenever	O
new	O
content	O
is	O
available	O
on	O
one	O
of	O
those	O
channels	O
,	O
the	O
server	B-General_Concept
pushes	I-General_Concept
that	O
information	O
out	O
to	O
the	O
client	B-Protocol
.	O
</s>
<s>
Push	O
is	O
sometimes	O
emulated	O
with	O
a	O
polling	B-General_Concept
technique	O
,	O
particularly	O
under	O
circumstances	O
where	O
a	O
real	O
push	O
is	O
not	O
possible	O
,	O
such	O
as	O
sites	O
with	O
security	O
policies	O
that	O
reject	O
incoming	O
HTTP/S	O
requests	O
.	O
</s>
<s>
Synchronous	B-Protocol
conferencing	I-Protocol
and	O
instant	B-Application
messaging	I-Application
are	O
examples	O
of	O
push	O
services	O
.	O
</s>
<s>
Chat	B-Protocol
messages	O
and	O
sometimes	O
files	B-Operating_System
are	O
pushed	O
to	O
the	O
user	O
as	O
soon	O
as	O
they	O
are	O
received	O
by	O
the	O
messaging	O
service	O
.	O
</s>
<s>
Both	O
decentralized	O
peer-to-peer	O
programs	O
(	O
such	O
as	O
WASTE	O
)	O
and	O
centralized	O
programs	O
(	O
such	O
as	O
IRC	B-Protocol
or	O
XMPP	B-Protocol
)	O
allow	O
pushing	O
files	B-Operating_System
,	O
which	O
means	O
the	O
sender	O
initiates	O
the	O
data	O
transfer	O
rather	O
than	O
the	O
recipient	O
.	O
</s>
<s>
Email	O
may	O
also	O
be	O
a	O
push	O
system	O
:	O
SMTP	B-Protocol
is	O
a	O
push	B-General_Concept
protocol	I-General_Concept
(	O
see	O
Push	B-General_Concept
e-mail	I-General_Concept
)	O
.	O
</s>
<s>
However	O
,	O
the	O
last	O
step	O
—	O
from	O
mail	O
server	B-Application
to	O
desktop	O
computer	O
—	O
typically	O
uses	O
a	O
pull	O
protocol	O
like	O
POP3	B-Protocol
or	O
IMAP	B-Protocol
.	O
</s>
<s>
Modern	O
e-mail	O
clients	O
make	O
this	O
step	O
seem	O
instantaneous	O
by	O
repeatedly	O
polling	B-General_Concept
the	O
mail	O
server	B-Application
,	O
frequently	O
checking	O
it	O
for	O
new	O
mail	O
.	O
</s>
<s>
The	O
IMAP	B-Protocol
protocol	O
includes	O
the	O
IDLE	B-Protocol
command	O
,	O
which	O
allows	O
the	O
server	B-Application
to	O
tell	O
the	O
client	B-Protocol
when	O
new	O
messages	O
arrive	O
.	O
</s>
<s>
The	O
original	O
BlackBerry	B-Operating_System
was	O
the	O
first	O
popular	O
example	O
of	O
push-email	O
in	O
a	O
wireless	O
context	O
.	O
</s>
<s>
Another	O
example	O
is	O
the	O
PointCast	B-Application
Network	I-Application
,	O
which	O
was	O
widely	O
covered	O
in	O
the	O
1990s	O
.	O
</s>
<s>
Both	O
Netscape	B-Protocol
and	O
Microsoft	O
integrated	O
push	B-General_Concept
technology	I-General_Concept
through	O
the	O
Channel	B-Application
Definition	I-Application
Format	I-Application
(	O
CDF	O
)	O
into	O
their	O
software	O
at	O
the	O
height	O
of	O
the	O
browser	B-Application
wars	I-Application
,	O
but	O
it	O
was	O
never	O
very	O
popular	O
.	O
</s>
<s>
CDF	O
faded	O
away	O
and	O
was	O
removed	O
from	O
the	O
browsers	B-Application
of	O
the	O
time	O
,	O
replaced	O
in	O
the	O
2000s	O
with	O
RSS	O
(	O
a	O
pull	O
system	O
.	O
)	O
</s>
<s>
Other	O
uses	O
of	O
push-enabled	O
web	B-Application
applications	I-Application
include	O
software	O
updates	O
distribution	O
(	O
"	O
push	O
updates	O
"	O
)	O
,	O
market	O
data	O
distribution	O
(	O
stock	O
tickers	O
)	O
,	O
online	O
chat/messaging	O
systems	O
(	O
webchat	B-Protocol
)	O
,	O
auctions	O
,	O
online	O
betting	O
and	O
gaming	O
,	O
sport	O
results	O
,	O
monitoring	O
consoles	O
,	O
and	O
sensor	B-Architecture
network	I-Architecture
monitoring	O
.	O
</s>
<s>
The	O
Web	O
push	O
proposal	O
of	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
is	O
a	O
simple	O
protocol	O
using	O
HTTP	B-Protocol
version	I-Protocol
2	I-Protocol
to	O
deliver	O
real	O
time	O
events	O
,	O
such	O
as	O
incoming	O
calls	O
or	O
messages	O
,	O
which	O
can	O
be	O
delivered	O
(	O
or	O
“	O
pushed	O
”	O
)	O
in	O
a	O
timely	O
fashion	O
.	O
</s>
<s>
The	O
protocol	O
consolidates	O
all	O
real-time	B-General_Concept
events	O
into	O
a	O
single	O
session	O
which	O
ensures	O
more	O
efficient	O
use	O
of	O
network	O
and	O
radio	O
resources	O
.	O
</s>
<s>
Web	O
Notifications	O
are	O
part	O
of	O
W3C	O
standard	O
and	O
defines	O
an	O
API	B-General_Concept
for	O
end-user	O
notifications	O
.	O
</s>
<s>
As	O
part	O
of	O
this	O
standard	O
Push	O
API	B-General_Concept
defined	O
by	O
W3C	O
,	O
is	O
now	O
being	O
implemented	O
by	O
Chrome	B-Application
,	O
Firefox	B-Application
,	O
Edge	B-Application
,	O
and	O
Safari	B-Application
.	O
</s>
<s>
HTTP	B-Application
server	I-Application
push	O
(	O
also	O
known	O
as	O
HTTP	B-General_Concept
streaming	I-General_Concept
)	O
is	O
a	O
mechanism	O
for	O
sending	O
unsolicited	O
(	O
asynchronous	O
)	O
data	O
from	O
a	O
web	B-Application
server	I-Application
to	O
a	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
HTTP	B-Application
server	I-Application
push	O
can	O
be	O
achieved	O
through	O
any	O
of	O
several	O
mechanisms	O
.	O
</s>
<s>
As	O
a	O
part	O
of	O
HTML5	B-Application
the	O
Web	B-Protocol
Socket	I-Protocol
API	B-General_Concept
allows	O
a	O
web	B-Application
server	I-Application
and	O
client	B-Protocol
to	O
communicate	O
over	O
a	O
full-duplex	O
TCP	O
connection	O
.	O
</s>
<s>
Generally	O
the	O
web	B-Application
server	I-Application
does	O
not	O
terminate	O
a	O
connection	O
after	O
response	O
data	O
has	O
been	O
served	O
to	O
a	O
client	B-Protocol
.	O
</s>
<s>
The	O
web	B-Application
server	I-Application
leaves	O
the	O
connection	O
open	O
so	O
that	O
if	O
an	O
event	O
occurs	O
(	O
for	O
example	O
,	O
a	O
change	O
in	O
internal	O
data	O
which	O
needs	O
to	O
be	O
reported	O
to	O
one	O
or	O
multiple	O
clients	O
)	O
,	O
it	O
can	O
be	O
sent	O
out	O
immediately	O
;	O
otherwise	O
,	O
the	O
event	O
would	O
have	O
to	O
be	O
queued	O
until	O
the	O
client	B-Protocol
's	O
next	O
request	O
is	O
received	O
.	O
</s>
<s>
Most	O
web	B-Application
servers	I-Application
offer	O
this	O
functionality	O
via	O
CGI	B-Language
(	O
e.g.	O
,	O
Non-Parsed	O
Headers	O
scripts	O
on	O
Apache	B-Application
HTTP	I-Application
Server	I-Application
)	O
.	O
</s>
<s>
The	O
underlying	O
mechanism	O
for	O
this	O
approach	O
is	O
chunked	B-Protocol
transfer	I-Protocol
encoding	I-Protocol
.	O
</s>
<s>
Another	O
mechanism	O
is	O
related	O
to	O
a	O
special	O
MIME	B-Protocol
type	O
called	O
multipart/x	O
-mixed-replace	O
,	O
which	O
was	O
introduced	O
by	O
Netscape	B-Protocol
in	O
1995	O
.	O
</s>
<s>
Web	B-Application
browsers	I-Application
interpret	O
this	O
as	O
a	O
document	O
that	O
changes	O
whenever	O
the	O
server	B-General_Concept
pushes	I-General_Concept
a	O
new	O
version	O
to	O
the	O
client	B-Protocol
.	O
</s>
<s>
It	O
is	O
still	O
supported	O
by	O
Firefox	B-Application
,	O
Opera	B-Application
,	O
and	O
Safari	B-Application
today	O
,	O
but	O
it	O
is	O
ignored	O
by	O
Internet	B-Application
Explorer	I-Application
and	O
is	O
only	O
partially	O
supported	O
by	O
Google	B-Application
Chrome	I-Application
.	O
</s>
<s>
It	O
can	O
be	O
applied	O
to	O
HTML	B-Language
documents	O
,	O
and	O
also	O
for	O
streaming	O
images	O
in	O
webcam	B-Protocol
applications	O
.	O
</s>
<s>
The	O
WHATWG	B-Language
Web	B-Application
Applications	I-Application
1.0	I-Application
proposal	O
includes	O
a	O
mechanism	O
to	O
push	O
content	O
to	O
the	O
client	B-Protocol
.	O
</s>
<s>
On	O
September	O
1	O
,	O
2006	O
,	O
the	O
Opera	B-Application
web	I-Application
browser	I-Application
implemented	O
this	O
new	O
experimental	O
system	O
in	O
a	O
feature	O
called	O
"	O
Server-Sent	B-Language
Events	I-Language
"	O
.	O
</s>
<s>
It	O
is	O
now	O
part	O
of	O
the	O
HTML5	B-Application
standard	O
.	O
</s>
<s>
In	O
this	O
technique	O
,	O
the	O
server	B-Application
takes	O
advantage	O
of	O
persistent	B-Protocol
HTTP	I-Protocol
connections	I-Protocol
,	O
leaving	O
the	O
response	O
perpetually	O
"	O
open	O
"	O
(	O
i.e.	O
,	O
the	O
server	B-Application
never	O
terminates	O
the	O
response	O
)	O
,	O
effectively	O
fooling	O
the	O
browser	B-Application
to	O
remain	O
in	O
"	O
loading	O
"	O
mode	O
after	O
the	O
initial	O
page	O
load	O
could	O
be	O
considered	O
complete	O
.	O
</s>
<s>
The	O
server	B-Application
then	O
periodically	O
sends	O
snippets	O
of	O
JavaScript	B-Language
to	O
update	O
the	O
content	O
of	O
the	O
page	O
,	O
thereby	O
achieving	O
push	O
capability	O
.	O
</s>
<s>
By	O
using	O
this	O
technique	O
,	O
the	O
client	B-Protocol
does	O
n't	O
need	O
Java	B-Application
applets	I-Application
or	O
other	O
plug-ins	O
in	O
order	O
to	O
keep	O
an	O
open	O
connection	O
to	O
the	O
server	B-Application
;	O
the	O
client	B-Protocol
is	O
automatically	O
notified	O
about	O
new	O
events	O
,	O
pushed	O
by	O
the	O
server	B-Application
.	O
</s>
<s>
One	O
serious	O
drawback	O
to	O
this	O
method	O
,	O
however	O
,	O
is	O
the	O
lack	O
of	O
control	O
the	O
server	B-Application
has	O
over	O
the	O
browser	B-Application
timing	O
out	O
;	O
a	O
page	O
refresh	O
is	O
always	O
necessary	O
if	O
a	O
timeout	O
occurs	O
on	O
the	O
browser	B-Application
end	O
.	O
</s>
<s>
Long	O
polling	B-General_Concept
is	O
itself	O
not	O
a	O
true	O
push	O
;	O
long	O
polling	B-General_Concept
is	O
a	O
variation	O
of	O
the	O
traditional	O
polling	B-General_Concept
technique	O
,	O
but	O
it	O
allows	O
emulating	O
a	O
push	O
mechanism	O
under	O
circumstances	O
where	O
a	O
real	O
push	O
is	O
not	O
possible	O
,	O
such	O
as	O
sites	O
with	O
security	O
policies	O
that	O
require	O
rejection	O
of	O
incoming	O
HTTP/S	O
Requests	O
.	O
</s>
<s>
With	O
long	O
polling	B-General_Concept
,	O
the	O
client	B-Protocol
requests	O
information	O
from	O
the	O
server	B-Application
exactly	O
as	O
in	O
normal	O
polling	B-General_Concept
,	O
but	O
with	O
the	O
expectation	O
the	O
server	B-Application
may	O
not	O
respond	O
immediately	O
.	O
</s>
<s>
If	O
the	O
server	B-Application
has	O
no	O
new	O
information	O
for	O
the	O
client	B-Protocol
when	O
the	O
poll	O
is	O
received	O
,	O
instead	O
of	O
sending	O
an	O
empty	O
response	O
,	O
the	O
server	B-Application
holds	O
the	O
request	O
open	O
and	O
waits	O
for	O
response	O
information	O
to	O
become	O
available	O
.	O
</s>
<s>
Once	O
it	O
does	O
have	O
new	O
information	O
,	O
the	O
server	B-Application
immediately	O
sends	O
an	O
HTTP/S	O
response	O
to	O
the	O
client	B-Protocol
,	O
completing	O
the	O
open	O
HTTP/S	O
Request	O
.	O
</s>
<s>
Upon	O
receipt	O
of	O
the	O
server	B-Application
response	O
,	O
the	O
client	B-Protocol
often	O
immediately	O
issues	O
another	O
server	B-Application
request	O
.	O
</s>
<s>
In	O
this	O
way	O
the	O
usual	O
response	O
latency	O
(	O
the	O
time	O
between	O
when	O
the	O
information	O
first	O
becomes	O
available	O
at	O
the	O
next	O
client	B-Protocol
request	O
)	O
otherwise	O
associated	O
with	O
polling	B-General_Concept
clients	O
is	O
eliminated	O
.	O
</s>
<s>
For	O
example	O
,	O
BOSH	B-Protocol
is	O
a	O
popular	O
,	O
long-lived	O
HTTP	B-Protocol
technique	O
used	O
as	O
a	O
long-polling	O
alternative	O
to	O
a	O
continuous	O
TCP	O
connection	O
when	O
such	O
a	O
connection	O
is	O
difficult	O
or	O
impossible	O
to	O
employ	O
directly	O
(	O
e.g.	O
,	O
in	O
a	O
web	B-Application
browser	I-Application
)	O
;	O
it	O
is	O
also	O
an	O
underlying	O
technology	O
in	O
the	O
XMPP	B-Protocol
,	O
which	O
Apple	O
uses	O
for	O
its	O
iCloud	O
push	O
support	O
.	O
</s>
<s>
This	O
technique	O
,	O
used	O
by	O
chat	B-Application
applications	I-Application
,	O
makes	O
use	O
of	O
the	O
XML	O
Socket	B-Protocol
object	O
in	O
a	O
single-pixel	O
Adobe	B-Operating_System
Flash	I-Operating_System
movie	O
.	O
</s>
<s>
Under	O
the	O
control	O
of	O
JavaScript	B-Language
,	O
the	O
client	B-Protocol
establishes	O
a	O
TCP	O
connection	O
to	O
a	O
unidirectional	B-Device
relay	O
on	O
the	O
server	B-Application
.	O
</s>
<s>
The	O
relay	O
server	B-Application
does	O
not	O
read	O
anything	O
from	O
this	O
socket	B-Protocol
;	O
instead	O
,	O
it	O
immediately	O
sends	O
the	O
client	B-Protocol
a	O
unique	O
identifier	O
.	O
</s>
<s>
Next	O
,	O
the	O
client	B-Protocol
makes	O
an	O
HTTP	B-Protocol
request	O
to	O
the	O
web	B-Application
server	I-Application
,	O
including	O
with	O
it	O
this	O
identifier	O
.	O
</s>
<s>
The	O
web	B-Application
application	I-Application
can	O
then	O
push	B-General_Concept
messages	I-General_Concept
addressed	O
to	O
the	O
client	B-Protocol
to	O
a	O
local	O
interface	O
of	O
the	O
relay	O
server	B-Application
,	O
which	O
relays	O
them	O
over	O
the	O
Flash	O
socket	B-Protocol
.	O
</s>
<s>
The	O
advantage	O
of	O
this	O
approach	O
is	O
that	O
it	O
appreciates	O
the	O
natural	O
read-write	O
asymmetry	O
that	O
is	O
typical	O
of	O
many	O
web	B-Application
applications	I-Application
,	O
including	O
chat	B-Protocol
,	O
and	O
as	O
a	O
consequence	O
it	O
offers	O
high	O
efficiency	O
.	O
</s>
<s>
Since	O
it	O
does	O
not	O
accept	O
data	O
on	O
outgoing	O
sockets	O
,	O
the	O
relay	O
server	B-Application
does	O
not	O
need	O
to	O
poll	O
outgoing	O
TCP	O
connections	O
at	O
all	O
,	O
making	O
it	O
possible	O
to	O
hold	O
open	O
tens	O
of	O
thousands	O
of	O
concurrent	O
connections	O
.	O
</s>
<s>
In	O
this	O
model	O
,	O
the	O
limit	O
to	O
scale	O
is	O
the	O
TCP	O
stack	O
of	O
the	O
underlying	O
server	B-Application
operating	O
system	O
.	O
</s>
<s>
A	O
push	B-General_Concept
notification	I-General_Concept
is	O
a	O
message	O
that	O
is	O
"	O
pushed	O
"	O
from	O
back-end	O
server	B-Application
or	O
application	O
to	O
user	O
interface	O
,	O
e.g.	O
</s>
<s>
Push	B-General_Concept
notifications	I-General_Concept
were	O
first	O
introduced	O
by	O
Apple	O
in	O
2009	O
.	O
</s>
<s>
In	O
2010	O
Google	B-Application
released	O
its	O
own	O
service	O
,	O
Google	B-Application
Cloud	O
to	O
Device	O
Messaging	O
(	O
since	O
superseded	O
first	O
by	O
Google	B-Application
Cloud	O
Messaging	O
and	O
then	O
Firebase	O
Cloud	O
Messaging	O
)	O
.	O
</s>
<s>
November	O
2015	O
,	O
Microsoft	O
announced	O
that	O
the	O
Windows	O
Notification	O
Service	O
would	O
be	O
expanded	O
to	O
make	O
use	O
of	O
the	O
Universal	O
Windows	O
Platform	O
architecture	O
,	O
allowing	O
for	O
push	O
data	O
to	O
be	O
sent	O
to	O
Windows	B-Operating_System
10	I-Operating_System
,	O
Windows	B-Operating_System
10	I-Operating_System
Mobile	I-Operating_System
,	O
Xbox	B-Protocol
,	O
as	O
well	O
as	O
other	O
supported	O
platforms	O
using	O
universal	O
API	B-General_Concept
calls	O
and	O
POST	O
requests	O
.	O
</s>
<s>
Push	B-General_Concept
notifications	I-General_Concept
are	O
mainly	O
divided	O
into	O
two	O
approaches	O
,	O
local	O
notifications	O
and	O
remote	O
notifications	O
.	O
</s>
<s>
Remote	O
notifications	O
are	O
handled	O
by	O
a	O
remote	O
server	B-Application
.	O
</s>
<s>
Under	O
this	O
scenario	O
,	O
the	O
client	B-Protocol
application	I-Protocol
needs	O
to	O
be	O
registered	O
on	O
the	O
server	B-Application
with	O
a	O
unique	O
key	O
(	O
e.g.	O
,	O
a	O
UUID	O
)	O
.	O
</s>
<s>
The	O
server	B-Application
then	O
fires	O
the	O
message	O
against	O
the	O
unique	O
key	O
to	O
deliver	O
the	O
message	O
to	O
the	O
client	B-Protocol
application	I-Protocol
via	O
an	O
agreed	O
client/server	B-Operating_System
protocol	I-Operating_System
such	O
as	O
HTTP	B-Protocol
or	O
XMPP	B-Protocol
and	O
the	O
client	B-Protocol
displays	O
the	O
message	O
received	O
.	O
</s>
<s>
When	O
the	O
push	B-General_Concept
notification	I-General_Concept
arrives	O
,	O
it	O
can	O
transmit	O
short	O
notifications	O
and	O
messages	O
,	O
set	O
badges	O
on	O
application	O
icons	O
,	O
blink	O
or	O
continuously	O
light	O
up	O
the	O
notification	B-Application
LED	I-Application
,	O
or	O
play	O
alert	O
sounds	O
to	O
attract	O
user	O
's	O
attention	O
.	O
</s>
<s>
Push	B-General_Concept
notifications	I-General_Concept
are	O
usually	O
used	O
by	O
applications	O
to	O
bring	O
information	O
to	O
users	O
 '	O
attention	O
.	O
</s>
<s>
Chat	B-Protocol
messages	O
,	O
e.g.	O
</s>
<s>
:	O
messages	O
from	O
Facebook	B-Application
messenger	O
sent	O
by	O
other	O
users	O
.	O
</s>
<s>
Real-time	B-General_Concept
push	B-General_Concept
notifications	I-General_Concept
may	O
raise	O
privacy	O
issues	O
since	O
they	O
can	O
be	O
used	O
to	O
bind	O
virtual	O
identities	O
of	O
social	O
network	O
pseudonyms	O
to	O
the	O
real	O
identities	O
of	O
the	O
smartphone	O
owners	O
.	O
</s>
<s>
The	O
use	O
of	O
unnecessary	O
push	B-General_Concept
notifications	I-General_Concept
for	O
promotional	O
purposes	O
has	O
been	O
criticized	O
as	O
an	O
example	O
of	O
attention	O
theft	O
.	O
</s>
