apache/hadoop · error · IllegalArgumentException

Name output ' ' has not been defined as multi

Error message

Name output '{}' has not been defined as multi

What it means

A named output in MultipleOutputs is either single (one file per task, registered with addNamedOutput) or multi (an unbounded set of files named name_multiName, registered with addMultiNamedOutput, which sets mo.namedOutput.<name>.multi=true). getCollector(name, multiName, reporter) throws this IllegalArgumentException when multiName is not null but the channel was registered as single. The framework refuses to guess which file family you meant.

Solutions

1. Register the channel as multi: MultipleOutputs.addMultiNamedOutput(conf, "seq", SequenceFileOutputFormat.class, keyClass, valueClass)
2. If you really want a single file, call getCollector("seq", reporter) without the multiName argument
3. Keep the single/multi decision in the same place as the registration so driver and task code cannot disagree

Example fix

// before: single registration + multi call -> IllegalArgumentException
MultipleOutputs.addNamedOutput(conf, "seq", SequenceFileOutputFormat.class, LongWritable.class, Text.class);
mos.getCollector("seq", "A", reporter);

// after: multi registration matches the multi call
MultipleOutputs.addMultiNamedOutput(conf, "seq", SequenceFileOutputFormat.class, LongWritable.class, Text.class);
mos.getCollector("seq", "A", reporter);

// check the mode before deciding the call shape
if (MultipleOutputs.isMultiNamedOutput(conf, "seq")) {
  collector = mos.getCollector("seq", multiName, reporter);
} else {
  collector = mos.getCollector("seq", reporter);
}

static boolean isMulti(JobConf conf, String name) {
  return MultipleOutputs.isMultiNamedOutput(conf, name);
}

When it happens

Trigger: Driver registers MultipleOutputs.addNamedOutput(conf, "seq", ...) (single), then task code calls mos.getCollector("seq", "A", reporter) with a second-level name. Typical when the channel was originally multi and someone 'simplified' the driver call to addNamedOutput, or when example code for multi outputs was mixed with a single-output driver.

Common situations: Sharding output by key/category (getCollector("seq", keyCategory, reporter)) where the driver author didn't realize the multi variant needs addMultiNamedOutput; upgrading old job templates that only registered single outputs.

Related errors

• Cannot recover task output for first attempt...
• Counters are enabled, Reporter cannot be NULL
• getShuffleFinishTime() not supported for MapTask
• Name cannot be have a
• Name cannot be have a

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/ef264a0883134bb9. Report an issue: GitHub.

Appendix: source

Thrown at hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/lib/MultipleOutputs.java:529

   * @param multiName   the multi name part
   * @param reporter    the reporter
   * @return the output collector for the given named output
   * @throws IOException thrown if output collector could not be created
   */
  @SuppressWarnings({"unchecked"})
  public OutputCollector getCollector(String namedOutput, String multiName,
                                      Reporter reporter)
    throws IOException {

    checkNamedOutputName(namedOutput);
    if (!namedOutputs.contains(namedOutput)) {
      throw new IllegalArgumentException("Undefined named output '" +
        namedOutput + "'");
    }
    boolean multi = isMultiNamedOutput(conf, namedOutput);

    if (!multi && multiName != null) {
      throw new IllegalArgumentException("Name output '" + namedOutput +
        "' has not been defined as multi");
    }
    if (multi) {
      checkTokenName(multiName);
    }

    String baseFileName = (multi) ? namedOutput + "_" + multiName : namedOutput;

    final RecordWriter writer =
      getRecordWriter(namedOutput, baseFileName, reporter);

    return new OutputCollector() {

      @SuppressWarnings({"unchecked"})
      public void collect(Object key, Object value) throws IOException {
        writer.write(key, value);
      }

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