<s>
A	O
web	B-Application
application	I-Application
(	O
or	O
web	B-Application
app	I-Application
)	O
is	O
application	B-Application
software	I-Application
that	O
is	O
accessed	O
using	O
a	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Web	B-Application
applications	I-Application
are	O
delivered	O
on	O
the	O
World	O
Wide	O
Web	O
to	O
users	O
with	O
an	O
active	O
network	O
connection	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
an	O
application	O
had	O
its	O
own	O
pre-compiled	O
client	O
program	O
which	O
served	O
as	O
its	O
user	B-Application
interface	I-Application
and	O
had	O
to	O
be	O
separately	O
installed	O
on	O
each	O
user	O
's	O
personal	B-Device
computer	I-Device
.	O
</s>
<s>
An	O
upgrade	O
to	O
the	O
server-side	O
code	O
of	O
the	O
application	O
would	O
typically	O
also	O
require	O
an	O
upgrade	O
to	O
the	O
client-side	O
code	O
installed	O
on	O
each	O
user	O
workstation	O
,	O
adding	O
to	O
the	O
support	B-General_Concept
cost	O
and	O
decreasing	O
productivity	O
.	O
</s>
<s>
In	O
addition	O
,	O
both	O
the	O
client	O
and	O
server	O
components	O
of	O
the	O
application	O
were	O
usually	O
tightly	O
bound	O
to	O
a	O
particular	O
computer	B-General_Concept
architecture	I-General_Concept
and	O
operating	B-General_Concept
system	I-General_Concept
and	O
porting	O
them	O
to	O
others	O
was	O
often	O
prohibitively	O
expensive	O
for	O
all	O
but	O
the	O
largest	O
applications	O
(	O
Nowadays	O
,	O
native	O
apps	O
for	O
mobile	O
devices	O
are	O
also	O
hobbled	O
by	O
some	O
or	O
all	O
of	O
the	O
foregoing	O
issues	O
)	O
.	O
</s>
<s>
In	O
1995	O
,	O
Netscape	B-Protocol
introduced	O
a	O
client-side	O
scripting	O
language	O
called	O
JavaScript	B-Language
allowing	O
programmers	O
to	O
add	O
some	O
dynamic	B-General_Concept
elements	I-General_Concept
to	O
the	O
user	B-Application
interface	I-Application
that	O
ran	O
on	O
the	O
client	O
side	O
.	O
</s>
<s>
In	O
1999	O
,	O
the	O
"	O
web	B-Application
application	I-Application
"	O
concept	O
was	O
introduced	O
in	O
the	O
Java	O
language	O
in	O
the	O
Servlet	B-Application
Specification	O
version	O
2.2	O
.	O
</s>
<s>
At	O
that	O
time	O
both	O
JavaScript	B-Language
and	O
XML	B-Protocol
had	O
already	O
been	O
developed	O
,	O
but	O
Ajax	B-General_Concept
had	O
still	O
not	O
yet	O
been	O
coined	O
and	O
the	O
XMLHttpRequest	B-Protocol
object	O
had	O
only	O
been	O
recently	O
introduced	O
on	O
Internet	O
Explorer	O
5	O
as	O
an	O
ActiveX	B-Application
object	I-Application
.	O
</s>
<s>
Applications	O
like	O
Gmail	B-Application
started	O
to	O
make	O
their	O
client	O
sides	O
more	O
and	O
more	O
interactive	O
since	O
early	O
2000s	O
.	O
</s>
<s>
The	O
practice	O
became	O
known	O
as	O
Ajax	B-General_Concept
in	O
2005	O
.	O
</s>
<s>
"	O
Progressive	B-Application
web	I-Application
apps	I-Application
"	O
,	O
the	O
term	O
coined	O
by	O
designer	O
Frances	O
Berriman	O
and	O
Google	B-Application
Chrome	I-Application
engineer	O
Alex	O
Russell	O
in	O
2015	O
,	O
refers	O
to	O
apps	O
taking	O
advantage	O
of	O
new	O
features	O
supported	O
by	O
modern	O
browsers	B-Application
,	O
which	O
initially	O
run	O
inside	O
a	O
web	B-Application
browser	I-Application
tab	O
but	O
later	O
can	O
run	O
completely	O
offline	O
and	O
can	O
be	O
launched	O
without	O
entering	O
app	O
URL	O
in	O
the	O
browser	B-Application
.	O
</s>
<s>
Traditional	O
PC	O
applications	O
consist	O
only	O
of	O
1	O
tier	O
,	O
which	O
resides	O
on	O
the	O
client	O
machine	O
,	O
but	O
web	B-Application
applications	I-Application
lend	O
themselves	O
to	O
a	O
multi-tiered	O
approach	O
by	O
nature	O
.	O
</s>
<s>
A	O
web	B-Application
browser	I-Application
is	O
the	O
first	O
tier	O
(	O
presentation	O
)	O
,	O
an	O
engine	O
using	O
some	O
dynamic	O
Web	O
content	O
technology	O
(	O
such	O
as	O
ASP	B-General_Concept
,	O
CGI	B-Language
,	O
ColdFusion	B-Language
,	O
Dart	B-Language
,	O
JSP/Java	B-Application
,	O
Node.js	B-Language
,	O
PHP	B-Application
,	O
Python	B-Language
or	O
Ruby	B-Application
on	I-Application
Rails	I-Application
)	O
is	O
the	O
middle	O
tier	O
(	O
application	O
logic	O
)	O
,	O
and	O
a	O
database	O
is	O
the	O
third	O
tier	O
(	O
storage	O
)	O
.	O
</s>
<s>
The	O
web	B-Application
browser	I-Application
sends	O
requests	O
to	O
the	O
middle	O
tier	O
,	O
which	O
services	O
them	O
by	O
making	O
queries	O
and	O
updates	O
against	O
the	O
database	O
and	O
generates	O
a	O
user	B-Application
interface	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
client	O
data	O
would	O
be	O
accessed	O
by	O
calling	O
a	O
"	O
list_clients( )	O
"	O
function	O
instead	O
of	O
making	O
an	O
SQL	B-Language
query	O
directly	O
against	O
the	O
client	O
table	O
on	O
the	O
database	O
.	O
</s>
<s>
There	O
are	O
some	O
who	O
view	O
a	O
web	B-Application
application	I-Application
as	O
a	O
two-tier	O
architecture	O
.	O
</s>
<s>
Writing	O
web	B-Application
applications	I-Application
is	O
simplified	O
with	O
the	O
use	O
of	O
web	B-Application
application	I-Application
frameworks	I-Application
.	O
</s>
<s>
Many	O
of	O
the	O
frameworks	O
in	O
use	O
are	O
open-source	B-Application
software	I-Application
.	O
</s>
<s>
In	O
addition	O
,	O
there	O
is	O
potential	O
for	O
the	O
development	O
of	O
applications	O
on	O
Internet	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
although	O
currently	O
there	O
are	O
not	O
many	O
viable	O
platforms	O
that	O
fit	O
this	O
model	O
.	O
</s>
