问题
What does the distribute cache actually mean? Having a file in distributed cache means that is it available in every datanode and hence there will be no internode communication for that data, or does it mean that the file is in memory in every node? If not, by what means can I have a file in memory for the entire job? Can this be done both for map-reduce, as well as for a UDF..
(In particular there is some configuration data, comparatively small that I would like to keep in memory as a UDF applies on hive query...? )
Thanks and regards, Dhruv Kapur.
回答1:
DistributedCache is a facility provided by the Map-Reduce framework to cache files needed by applications. Once you cache a file for your job, hadoop framework will make it available on each and every data nodes (in file system, not in memory) where you map/reduce tasks are running. Then you can access the cache file as local file in your Mapper Or Reducer job. Now you can easily read the cache file and populate some collection (e.g Array, Hashmap etc.) in your code.
Refer https://hadoop.apache.org/docs/r2.6.1/api/org/apache/hadoop/filecache/DistributedCache.html
Let me know if still you have some questions.
You can read the cache file as local file in your UDF code. After reading the file using JAVA APIs just populate any collection (In memory).
Refere URL http://www.lichun.cc/blog/2013/06/use-a-lookup-hashmap-in-hive-script/
-Ashish
来源：https://stackoverflow.com/questions/23751702/confusion-about-distributed-cache-in-hadoop