<s>
A	O
focused	B-Protocol
crawler	I-Protocol
is	O
a	O
web	B-Application
crawler	I-Application
that	O
collects	O
Web	O
pages	O
that	O
satisfy	O
some	O
specific	O
property	O
,	O
by	O
carefully	O
prioritizing	O
the	O
crawl	B-Application
frontier	I-Application
and	O
managing	O
the	O
hyperlink	O
exploration	O
process	O
.	O
</s>
<s>
Other	O
predicates	O
may	O
be	O
softer	O
or	O
comparative	O
,	O
e.g.	O
,	O
"	O
crawl	O
pages	O
about	O
baseball	O
"	O
,	O
or	O
"	O
crawl	O
pages	O
with	O
large	O
PageRank	B-Algorithm
"	O
.	O
</s>
<s>
An	O
important	O
page	O
property	O
pertains	O
to	O
topics	O
,	O
leading	O
to	O
'	O
topical	B-Protocol
crawlers	I-Protocol
 '	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
topical	B-Protocol
crawler	I-Protocol
may	O
be	O
deployed	O
to	O
collect	O
pages	O
about	O
solar	O
power	O
,	O
swine	O
flu	O
,	O
or	O
even	O
more	O
abstract	O
concepts	O
like	O
controversy	O
while	O
minimizing	O
resources	O
spent	O
fetching	O
pages	O
on	O
other	O
topics	O
.	O
</s>
<s>
Crawl	B-Application
frontier	I-Application
management	O
may	O
not	O
be	O
the	O
only	O
device	O
used	O
by	O
focused	B-Protocol
crawlers	I-Protocol
;	O
they	O
may	O
use	O
a	O
Web	B-Application
directory	I-Application
,	O
a	O
Web	B-Algorithm
text	I-Algorithm
index	I-Algorithm
,	O
backlinks	O
,	O
or	O
any	O
other	O
Web	O
artifact	O
.	O
</s>
<s>
A	O
focused	B-Protocol
crawler	I-Protocol
must	O
predict	O
the	O
probability	O
that	O
an	O
unvisited	O
page	O
will	O
be	O
relevant	O
before	O
actually	O
downloading	O
the	O
page	O
.	O
</s>
<s>
coined	O
the	O
term	O
'	O
focused	B-Protocol
crawler	I-Protocol
 '	O
and	O
used	O
a	O
text	O
classifier	O
to	O
prioritize	O
the	O
crawl	B-Application
frontier	I-Application
.	O
</s>
<s>
A	O
form	O
of	O
online	O
reinforcement	O
learning	O
has	O
been	O
used	O
,	O
along	O
with	O
features	O
extracted	O
from	O
the	B-General_Concept
DOM	I-General_Concept
tree	O
and	O
text	O
of	O
linking	O
pages	O
,	O
to	O
continually	O
train	O
classifiers	O
that	O
guide	O
the	O
crawl	O
.	O
</s>
<s>
Another	O
type	O
of	O
focused	B-Protocol
crawlers	I-Protocol
is	O
semantic	O
focused	B-Protocol
crawler	I-Protocol
,	O
which	O
makes	O
use	O
of	O
domain	O
ontologies	O
to	O
represent	O
topical	O
maps	O
and	O
link	O
Web	O
pages	O
with	O
relevant	O
ontological	O
concepts	O
for	O
the	O
selection	O
and	O
categorization	O
purposes	O
.	O
</s>
<s>
Najork	O
and	O
Weiner	O
show	O
that	O
breadth-first	B-Algorithm
crawling	O
,	O
starting	O
from	O
popular	O
seed	O
pages	O
,	O
leads	O
to	O
collecting	O
large-PageRank	O
pages	O
early	O
in	O
the	O
crawl	O
.	O
</s>
<s>
A	O
kind	O
of	O
semantic	O
focused	B-Protocol
crawler	I-Protocol
,	O
making	O
use	O
of	O
the	O
idea	O
of	O
reinforcement	O
learning	O
has	O
been	O
introduced	O
by	O
Meusel	O
et	O
al	O
.	O
</s>
<s>
using	O
online-based	O
classification	O
algorithms	O
in	O
combination	O
with	O
a	O
bandit-based	O
selection	O
strategy	O
to	O
efficiently	O
crawl	O
pages	O
with	O
markup	O
languages	O
like	O
RDFa	B-General_Concept
,	O
Microformats	B-General_Concept
,	O
and	O
Microdata	B-General_Concept
.	O
</s>
<s>
The	O
performance	O
of	O
a	O
focused	B-Protocol
crawler	I-Protocol
depends	O
on	O
the	O
richness	O
of	O
links	O
in	O
the	O
specific	O
topic	O
being	O
searched	O
,	O
and	O
focused	B-Protocol
crawling	I-Protocol
usually	O
relies	O
on	O
a	O
general	O
web	B-Application
search	I-Application
engine	I-Application
for	O
providing	O
starting	O
points	O
.	O
</s>
<s>
Davison	O
presented	O
studies	O
on	O
Web	O
links	O
and	O
text	O
that	O
explain	O
why	O
focused	B-Protocol
crawling	I-Protocol
succeeds	O
on	O
broad	O
topics	O
;	O
similar	O
studies	O
were	O
presented	O
by	O
Chakrabarti	O
et	O
al	O
.	O
</s>
<s>
Seed	O
selection	O
can	O
be	O
important	O
for	O
focused	B-Protocol
crawlers	I-Protocol
and	O
significantly	O
influence	O
the	O
crawling	O
efficiency	O
.	O
</s>
<s>
These	O
high	O
quality	O
seeds	O
should	O
be	O
selected	O
based	O
on	O
a	O
list	O
of	O
URL	O
candidates	O
which	O
are	O
accumulated	O
over	O
a	O
sufficient	O
long	O
period	O
of	O
general	O
web	B-Application
crawling	I-Application
.	O
</s>
