MapReduce on Hadoop says 'Output file already exists'

Question

I ran a wordcount example using Mapreduce the first time, and it worked. Then, I stopped the cluster, started it back in a while, and followed the same procedure.

Showed this error:

10P:/$  hadoop jar /usr/local/hadoop/share/hadoop/mapreduce/hadoop-mapreduce-examples-2.6.0.jar wordcount /user/test/tester /user/output
15/08/05 00:16:04 INFO Configuration.deprecation: session.id is deprecated. Instead, use dfs.metrics.session-id
15/08/05 00:16:04 INFO jvm.JvmMetrics: Initializing JVM Metrics with processName=JobTracker, sessionId=
org.apache.hadoop.mapred.FileAlreadyExistsException: **Output directory hdfs://localhost:54310/user/output already exists**
    at org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:146)
    at org.apache.hadoop.mapreduce.JobSubmitter.checkSpecs(JobSubmitter.java:562)
    at org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:432)
    at org.apache.hadoop.mapreduce.Job$10.run(Job.java:1296)
    at org.apache.hadoop.mapreduce.Job$10.run(Job.java:1293)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
    at org.apache.hadoop.mapreduce.Job.submit(Job.java:1293)
    at org.apache.hadoop.mapreduce.Job.waitForCompletion(Job.java:1314)
    at org.apache.hadoop.examples.WordCount.main(WordCount.java:87)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:71)
    at org.apache.hadoop.util.ProgramDriver.run(ProgramDriver.java:144)
    at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:74)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.apache.hadoop.util.RunJar.run(RunJar.java:221)
    at org.apache.hadoop.util.RunJar.main(RunJar.java:136)

Answer 1

hdfs://localhost:54310/user/output

Delete the output directory before running the job.

i.e execute the following command:

hadoop fs -rm -r /user/output 

before running the job.

Answer 2

Add the following code snippet in your configuration class.

    // Delete output if exists
    FileSystem hdfs = FileSystem.get(conf);
    if (hdfs.exists(outputDir))
      hdfs.delete(outputDir, true);

    // Execute job
    int code = job.waitForCompletion(true) ? 0 : 1;
    System.exit(code);