<s>
A	O
crawl	B-Application
frontier	I-Application
is	O
a	O
data	O
structure	O
used	O
for	O
storage	O
of	O
URLs	O
eligible	O
for	O
crawling	O
and	O
supporting	O
such	O
operations	O
as	O
adding	O
URLs	O
and	O
selecting	O
for	O
crawl	O
.	O
</s>
<s>
Sometimes	O
it	O
can	O
be	O
seen	O
as	O
a	O
priority	B-Application
queue	I-Application
.	O
</s>
<s>
A	O
crawl	B-Application
frontier	I-Application
is	O
one	O
of	O
the	O
components	O
that	O
make	O
up	O
the	O
architecture	O
of	O
a	O
web	B-Application
crawler	I-Application
.	O
</s>
<s>
The	O
crawl	B-Application
frontier	I-Application
contains	O
the	O
logic	O
and	O
policies	O
that	O
a	O
crawler	B-Application
follows	O
when	O
visiting	O
websites	O
.	O
</s>
<s>
The	O
efficiency	O
of	O
the	O
crawl	B-Application
frontier	I-Application
is	O
especially	O
important	O
since	O
one	O
of	O
the	O
characteristics	O
of	O
the	O
Web	O
that	O
make	O
web	B-Application
crawling	I-Application
a	O
challenge	O
;	O
is	O
that	O
it	O
contains	O
such	O
a	O
large	O
volume	O
of	O
data	O
and	O
it	O
is	O
constantly	O
changing	O
.	O
</s>
<s>
The	O
initial	O
list	O
of	O
URLs	O
contained	O
in	O
the	O
crawler	B-Application
frontier	O
are	O
known	O
as	O
seeds	O
.	O
</s>
<s>
The	O
web	B-Application
crawler	I-Application
will	O
constantly	O
ask	O
the	O
frontier	O
what	O
pages	O
to	O
visit	O
.	O
</s>
<s>
As	O
the	O
crawler	B-Application
visits	O
each	O
of	O
those	O
pages	O
,	O
it	O
will	O
inform	O
the	O
frontier	O
with	O
the	O
response	O
of	O
each	O
page	O
.	O
</s>
<s>
The	O
crawler	B-Application
will	O
also	O
update	O
the	O
crawler	B-Application
frontier	O
with	O
any	O
new	O
hyperlinks	O
contained	O
in	O
those	O
pages	O
it	O
has	O
visited	O
.	O
</s>
<s>
These	O
hyperlinks	O
are	O
added	O
to	O
the	O
frontier	O
and	O
the	O
crawler	B-Application
will	O
visit	O
new	O
web	O
pages	O
based	O
on	O
the	O
policies	O
of	O
the	O
frontier	O
.	O
</s>
<s>
This	O
process	O
continues	O
recursively	O
until	O
all	O
URLs	O
in	O
the	O
crawl	B-Application
frontier	I-Application
are	O
visited	O
.	O
</s>
<s>
The	O
Frontier	O
Manager	O
is	O
the	O
component	O
that	O
the	O
web	B-Application
crawler	I-Application
will	O
use	O
to	O
communicate	O
with	O
the	O
crawl	B-Application
frontier	I-Application
.	O
</s>
<s>
The	O
frontier	O
API	O
can	O
also	O
be	O
used	O
to	O
communicate	O
with	O
the	O
crawl	B-Application
frontier	I-Application
.	O
</s>
<s>
The	O
frontier	O
middlewares	B-General_Concept
sit	O
between	O
the	O
manager	O
and	O
the	O
backend	O
.	O
</s>
<s>
The	O
purpose	O
of	O
middlewares	B-General_Concept
is	O
to	O
manage	O
the	O
communication	O
between	O
frontier	O
and	O
the	O
backend	O
.	O
</s>
<s>
Middlewares	B-General_Concept
are	O
an	O
ideal	O
way	O
to	O
add	O
or	O
extend	O
additional	O
functionality	O
simply	O
by	O
plugging	O
additional	O
code	O
.	O
</s>
