apache/hadoop · error · IOException
MROutput/MRErrThread failed:
Error message
MROutput/MRErrThread failed:
What it means
Reducer-side twin of the mapper check: inside the reduce loop over values, PipeReducer.reduce() sees outerrThreadsThrowable set (the output or stderr reader thread for the piped reducer died) and throws IOException('MROutput/MRErrThread failed:', outerrThreadsThrowable) after calling mapRedFinished(). The cause chain holds the thread's original exception.
Source
Thrown at hadoop-tools/hadoop-streaming/src/main/java/org/apache/hadoop/streaming/PipeReducer.java:97
this.numOfReduceOutputKeyFields = job_.getInt("stream.num.reduce.output.key.fields", 1);
}
public void reduce(Object key, Iterator values, OutputCollector output,
Reporter reporter) throws IOException {
// init
if (doPipe_ && outThread_ == null) {
startOutputThreads(output, reporter);
}
try {
while (values.hasNext()) {
Writable val = (Writable) values.next();
numRecRead_++;
maybeLogRecord();
if (doPipe_) {
if (outerrThreadsThrowable != null) {
mapRedFinished();
throw new IOException("MROutput/MRErrThread failed:",
outerrThreadsThrowable);
}
inWriter_.writeKey(key);
inWriter_.writeValue(val);
} else {
// "identity reduce"
output.collect(key, val);
}
}
if(doPipe_ && skipping) {
//flush the streams on every record input if running in skip mode
//so that we don't buffer other records surrounding a bad record.
clientOut_.flush();
}
} catch (IOException io) {
// a common reason to get here is failure of the subprocess.
// Document that fact, if possible.
String extraInfo = "";View on GitHub (pinned to 2add963021)
Solutions
- Inspect the wrapped cause in the task log (MROutputThread/MRErrThread stack trace just before this IOException)
- Conform reducer stdout to key\tvalue lines; move logs/warnings to stderr
- Match stream.num.reduce.output.key.fields and any custom reader class to what the tool actually prints
- Test the reducer tool locally: cat input | reducer.sh and verify clean output and exit 0
Defensive patterns
Strategy: try-catch
Validate before calling
// preflight for the reducer tool: local pipeline smoke test // sh -c 'cat sample-group.txt | ./my_reducer.sh' must print clean key\tvalue lines and exit 0
Try / catch
catch IOException from reduce(); inspect the cause chain for the failed reader thread's exception and fix the output-format or tool-crash root cause; retrying the task without changes will fail identically.
Prevention
- Keep reducer stdout strictly key\tvalue; no headers or progress messages
- Align stream.num.reduce.output.key.fields and custom reader classes with actual output
- Handle partial input and empty groups gracefully in reducer scripts
- Correlate with the subprocess exit-code error: thread failures often follow a tool crash
When it happens
Trigger: With -reducer as an external tool, the thread reading reducer stdout or stderr fails mid-reduce — output that violates the expected format, pipe breakage when the reducer tool exits early, or custom stream.reduce.output.reader.class parse errors — and the next iteration of the value loop aborts.
Common situations: Reducer scripts emitting headers/debug lines to stdout, key-field count misconfiguration (stream.num.reduce.output.key.fields), reducers crashing partway through a value stream, or aggregator scripts buffering incorrectly and dying; wrapped cause in logs identifies which thread and why.
Related errors
- MROutput/MRErrThread failed:
- hasNext failed
- next value iterator failed
- iterate past last value
- next value iterator interrupted
AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22).
Data as JSON: /api/errors/aadc96a93d7fd4eb.
Report an issue: GitHub.