nutch 分布式索引（爬虫)
其实，全网抓取比intranet区别再于，
前者提供了较为多的urls入口，
没有使用crawl-urlfilter.txt 中并没有限制哪些url ，(如果没有使用crawl命令）
并通过逐步处理的方式得以可按的局面；
在1.3，还有此区别，
如默认的fetcher.parse是false，使得每次fetch后必须有一个parse step，刚开始老是看不懂为什么tutorial中这样做。。
其次是，此版本不再有crawl-urlfiter.txt，而是用regex-urlfilter.txt替换。
在recrawl时的区别见nutch 数据增量更新
其实这个过程可以说是nutch对hadoop的利用的最深体会，我是这样认为的。想想看，当初，hadoop是内嵌在Nutch中，作为其中的一个功能模块。现在版本的nutch将hadop分离出来，但在分布式抓取时又得将它（配置文件，jar等）放回Nutch下。刚开始时老想nutch怎样结合hadoop进行分布式抓取；但分布式搜索还是有些不一样的，因为后者即使也是分布式，但它利用的hdfs对nutch是透明的。
install processes:
a.configure hadoop to run on cluster mode;
b.put all the config files belong hadoop(master and slaves) to conf dir of nutch(s) respectively;
c.execute the crawl command (SHOULD use individual commands to do INSTEAD OF 'craw',as 'crawl' is used for intranet usually)
here are the jobs belong this step:
| Available Jobs | ||||
| Job tracker Host Name | Job tracker Start time | Job Id | Name | User |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0001 | inject crawl-url | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0002 | crawldb crawl/dist/crawldb | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0003 | generate: select from crawl/dist/crawldb | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0004 | generate: partition crawl/dist/segments/2011110720 | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0005 | fetch crawl/dist/segments/20111107205746 | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0006 | crawldb crawl/dist/crawldb(update db actually ) |
hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0007 | linkdb crawl/dist/linkdb | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0008 | index-lucene crawl/dist/indexes | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0009 | dedup 1: urls by time | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0010 | dedup 2: content by hash | hadoop |
| master | Mon Nov 07 20:50:54 CST 2011 | job_201111072050_0011 | dedup 3: delete from index(es) |
hadoop |
* the jobs above with same color is ONE step beong the crawl command;
* job 2 :将sortjob結果作为输入(与已有的current数据合并)，生成新的crawldb;所以可以有重复的urls,在reduce中会去重 ?
* job 4:由于存在多台crawlers,所以需要利用partition来划分urls(by host by default),来避免重复让一台机来抓取 ；
here is the output of resulst:
hadoop@leibnitz-laptop:/xxxxxxxxx$ hadoop fs -lsr crawl/dist/
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00000
-rw-r--r-- 2 hadoop supergroup 6240 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00000/data
-rw-r--r-- 2 hadoop supergroup 215 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00001
-rw-r--r-- 2 hadoop supergroup 7779 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00001/data
-rw-r--r-- 2 hadoop supergroup 218 2011-11-07 21:00 /user/hadoop/crawl/dist/crawldb/current/part-00001/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:07 /user/hadoop/crawl/dist/index
-rw-r--r-- 2 hadoop supergroup 369 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.fdt
-rw-r--r-- 2 hadoop supergroup 20 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.fdx
-rw-r--r-- 2 hadoop supergroup 71 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.fnm
-rw-r--r-- 2 hadoop supergroup 1836 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.frq
-rw-r--r-- 2 hadoop supergroup 14 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.nrm
-rw-r--r-- 2 hadoop supergroup 4922 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.prx
-rw-r--r-- 2 hadoop supergroup 171 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.tii
-rw-r--r-- 2 hadoop supergroup 11234 2011-11-07 21:07 /user/hadoop/crawl/dist/index/_2.tis
-rw-r--r-- 2 hadoop supergroup 20 2011-11-07 21:07 /user/hadoop/crawl/dist/index/segments.gen
-rw-r--r-- 2 hadoop supergroup 284 2011-11-07 21:07 /user/hadoop/crawl/dist/index/segments_2
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000
-rw-r--r-- 2 hadoop supergroup 223 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.fdt
-rw-r--r-- 2 hadoop supergroup 12 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.fdx
-rw-r--r-- 2 hadoop supergroup 71 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.fnm
-rw-r--r-- 2 hadoop supergroup 991 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.frq
-rw-r--r-- 2 hadoop supergroup 9 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.nrm
-rw-r--r-- 2 hadoop supergroup 2813 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.prx
-rw-r--r-- 2 hadoop supergroup 100 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.tii
-rw-r--r-- 2 hadoop supergroup 5169 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/_0.tis
-rw-r--r-- 2 hadoop supergroup 0 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/index.done
-rw-r--r-- 2 hadoop supergroup 20 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/segments.gen
-rw-r--r-- 2 hadoop supergroup 240 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00000/segments_2
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001
-rw-r--r-- 2 hadoop supergroup 150 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.fdt
-rw-r--r-- 2 hadoop supergroup 12 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.fdx
-rw-r--r-- 2 hadoop supergroup 71 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.fnm
-rw-r--r-- 2 hadoop supergroup 845 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.frq
-rw-r--r-- 2 hadoop supergroup 9 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.nrm
-rw-r--r-- 2 hadoop supergroup 2109 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.prx
-rw-r--r-- 2 hadoop supergroup 106 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.tii
-rw-r--r-- 2 hadoop supergroup 6226 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/_0.tis
-rw-r--r-- 2 hadoop supergroup 0 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/index.done
-rw-r--r-- 2 hadoop supergroup 20 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/segments.gen
-rw-r--r-- 2 hadoop supergroup 240 2011-11-07 21:04 /user/hadoop/crawl/dist/indexes/part-00001/segments_2
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00000
-rw-r--r-- 2 hadoop supergroup 8131 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00000/data
-rw-r--r-- 2 hadoop supergroup 215 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00001
-rw-r--r-- 2 hadoop supergroup 11240 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00001/data
-rw-r--r-- 2 hadoop supergroup 218 2011-11-07 21:01 /user/hadoop/crawl/dist/linkdb/current/part-00001/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00000
-rw-r--r-- 2 hadoop supergroup 13958 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00000/data
-rw-r--r-- 2 hadoop supergroup 213 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00001
-rw-r--r-- 2 hadoop supergroup 6908 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00001/data
-rw-r--r-- 2 hadoop supergroup 224 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/content/part-00001/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00000
-rw-r--r-- 2 hadoop supergroup 255 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00000/data
-rw-r--r-- 2 hadoop supergroup 213 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00001
-rw-r--r-- 2 hadoop supergroup 266 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00001/data
-rw-r--r-- 2 hadoop supergroup 224 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_fetch/part-00001/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:58 /user/hadoop/crawl/dist/segments/20111107205746/crawl_generate
-rw-r--r-- 2 hadoop supergroup 255 2011-11-07 20:58 /user/hadoop/crawl/dist/segments/20111107205746/crawl_generate/part-00000
-rw-r--r-- 2 hadoop supergroup 86 2011-11-07 20:58 /user/hadoop/crawl/dist/segments/20111107205746/crawl_generate/part-00001
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_parse
-rw-r--r-- 2 hadoop supergroup 6819 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_parse/part-00000
-rw-r--r-- 2 hadoop supergroup 8302 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/crawl_parse/part-00001
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00000
-rw-r--r-- 2 hadoop supergroup 2995 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00000/data
-rw-r--r-- 2 hadoop supergroup 213 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00001
-rw-r--r-- 2 hadoop supergroup 1917 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00001/data
-rw-r--r-- 2 hadoop supergroup 224 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_data/part-00001/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00000
-rw-r--r-- 2 hadoop supergroup 3669 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00000/data
-rw-r--r-- 2 hadoop supergroup 213 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00000/index
drwxr-xr-x - hadoop supergroup 0 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00001
-rw-r--r-- 2 hadoop supergroup 2770 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00001/data
-rw-r--r-- 2 hadoop supergroup 224 2011-11-07 20:59 /user/hadoop/crawl/dist/segments/20111107205746/parse_text/part-00001/index
从以上分析可得，除了merged index外，其它目录都存在两份－对应两台crawlers.
利用这两份索引 ，就可以实现分布式搜索了。
剩下问题：为什么网上介绍的分步方式都没有使用dedup命令？
从 nutch 数据增量更新 上可知， 分布式抓取也应该使用这个dedup命令。
see also