Accessing files in hadoop distributed cache

Question

I want to use the distributed cache to allow my mappers to access data. In main, I\'m using the command

DistributedCache.addCacheFile(new URI(\"/user/peter/cac         


        

Answer 1

Problem here was that I was doing the following:

Configuration conf = new Configuration();
Job job = new Job(conf, "wordcount");
DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);

Since the Job constructor makes an internal copy of the conf instance, adding the cache file afterwards doesn't affect things. Instead, I should do this:

Configuration conf = new Configuration();
DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);
Job job = new Job(conf, "wordcount");

And now it works. Thanks to Harsh on hadoop user list for the help.

Answer 2

Configuration conf = new Configuration();  
Job job = new Job(conf, "wordcount");
DistributedCache.addCacheFile(new URI("/userpetercacheFiletestCache1"),job.getConfiguration());

You can also do it in this way.

Answer 3

This version of code ( which is slightly different from the above mentioned constructs) has always worked for me.

//in main(String [] args)
Job job = new Job(conf,"Word Count"); 
...
DistributedCache.addCacheFile(new URI(/user/peter/cacheFile/testCache1), job.getConfiguration());

I didnt see the complete setup() function in Mapper code

public void setup(Context context) throws IOException, InterruptedException {

    Configuration conf = context.getConfiguration();
    FileSystem fs = FileSystem.getLocal(conf);

    Path[] dataFile = DistributedCache.getLocalCacheFiles(conf);

    // [0] because we added just one file.
    BufferedReader cacheReader = new BufferedReader(new InputStreamReader(fs.open(dataFile[0])));
    // now one can use BufferedReader's readLine() to read data

}

Answer 4

Once the Job is assigned to with a configuration object, ie Configuration conf = new Configuration();

Job job = new Job(conf, "wordcount");

And then if deal with attributes of conf as shown below, eg

conf.set("demiliter","|");

or

DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);

Such changes would not be reflected in a pseudo cluster or cluster how ever it would work with local environment.