nutch-site.xml

在nutch2.2.1中,有两份配置文件:nutch-default.xml与nutch-site.xml。

其中前者是nutch自带的默认属性,一般情况下不要修改。

如果需要修改默认属性,可以在nutch-site.xml中增加一个同名的属性,并修改其值。nutch-site.xml中的属性值会覆盖nutch-default.xml中的值。

1、db.ignore.external.links

若为true,则只抓取本域名内的网页,忽略外部链接。

可以在 regex-urlfilter.txt中增加过滤器达到同样效果,但如果过滤器过多,如几千个,则会大大影响nutch的性能。

<property>
<name>db.ignore.external.links</name>
<value>true</value>
<description>If true, outlinks leading from a page to external hosts
will be ignored. This is an effective way to limit the crawl to include
only initially injected hosts, without creating complex URLFilters.
</description>
</property>

2、fetcher.parse

能否在抓取的同时进行解释:可以,但不 建议这样做。

<property>
<name>fetcher.parse</name>
<value>false</value>
<description>If true, fetcher will parse content. NOTE: previous releases would
default to true. Since 2.0 this is set to false as a safer default.</description>
</property>

官方解释

N.B. In a parsing fetcher, outlinks are processed in the reduce phase (at least when outlinks are followed). If a fetcher's reducer stalls you may run out of memory or disk space,
usually after a very long reduce job. Behaviour typical to this is usually
observed in this situation.

In summary, if it is possible, users are advised not to use a parsing fetcher as it is heavy on IO and often leads to the above outcome.

3、db.max.outlinks.per.page

默认情况下,Nutch只抓取某个网页的100个外部链接,导致部分链接无法抓取。若要改变此情况,可以修改此配置项。

<property>
<name>db.max.outlinks.per.page</name>
<value>100</value>
<description>The maximum number of outlinks that we'll process for a page. If this value is nonnegative (>=0), at most db.max.outlinks.per.page outlinks will be processed for a page; otherwise, all outlinks will be processed.
</description>
</property>

官方说明如下:http://wiki.apache.org/nutch/FAQ/

Nutch doesn't crawl relative URLs? Some pages are not indexed but my regex file and everything else is okay - what is going on?

The crawl tool has a default limitation of 100 outlinks of one page that are being fetched. To overcome this limitation change thedb.max.outlinks.per.page property to a higher
value or simply -1 (unlimited).

file: conf/nutch-default.xml

 <property>
<name>db.max.outlinks.per.page</name>
<value>-1</value>
<description>The maximum number of outlinks that we'll process for a page.
If this value is nonnegative (>=0), at most db.max.outlinks.per.page outlinks
will be processed for a page; otherwise, all outlinks will be processed.
</description>
</property>

see also: http://www.mail-archive.com/nutch-user@lucene.apache.org/msg08665.html

4、file.content.limit   http.content.limit  ftp.content.limit

默认情况下,nutch只抓取网页的前65536个字节,之后的内容将被丢弃。

但对于某些大型网站,首页的内容远远不止65536个字节,甚至前面65536个字节里面均是一些布局信息,并没有任何的超链接。

因此修改默认值如下:

<property>
<name>file.content.limit</name>
<value>-1</value>
<description>The length limit for downloaded content using the file
protocol, in bytes. If this value is nonnegative (>=0), content longer
than it will be truncated; otherwise, no truncation at all. Do not
confuse this setting with the http.content.limit setting.
</description>
</property> <property>
<name>http.content.limit</name>
<value>-1</value>
<description>The length limit for downloaded content using the http
protocol, in bytes. If this value is nonnegative (>=0), content longer
than it will be truncated; otherwise, no truncation at all. Do not
confuse this setting with the file.content.limit setting.
</description>
</property> <property>
<name>ftp.content.limit</name>
<value>-1</value>
<description>The length limit for downloaded content, in bytes.
If this value is nonnegative (>=0), content longer than it will be truncated;
otherwise, no truncation at all.
Caution: classical ftp RFCs never defines partial transfer and, in fact,
some ftp servers out there do not handle client side forced close-down very
well. Our implementation tries its best to handle such situations smoothly.
</description>
</property>

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