apache/flink · error · IllegalArgumentException

Cannot use custom partitioner for a non-grouped GroupReduce

Error message

Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)

What it means

Error "Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)" thrown in apache/flink.

Source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java:185

     * Creates a non-grouped reduce data flow operator (all-reduce).
     *
     * @param udf The class representing the parameterless user-defined function.
     * @param operatorInfo The type information, describing input and output types of the reduce
     *     function.
     * @param name The name of the operator (for logging and messages).
     */
    public ReduceOperatorBase(
            Class<? extends FT> udf, UnaryOperatorInformation<T, T> operatorInfo, String name) {
        super(new UserCodeClassWrapper<FT>(udf), operatorInfo, name);
    }

    // --------------------------------------------------------------------------------------------

    public void setCustomPartitioner(Partitioner<?> customPartitioner) {
        if (customPartitioner != null) {
            int[] keys = getKeyColumns(0);
            if (keys == null || keys.length == 0) {
                throw new IllegalArgumentException(
                        "Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)");
            }
            if (keys.length > 1) {
                throw new IllegalArgumentException(
                        "Cannot use the key partitioner for composite keys (more than one key field)");
            }
        }
        this.customPartitioner = customPartitioner;
    }

    public Partitioner<?> getCustomPartitioner() {
        return customPartitioner;
    }

    // --------------------------------------------------------------------------------------------

    @Override
    protected List<T> executeOnCollections(

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Address the cause reported by the error message: Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)
2. Verify the inputs, configuration values, and classpath/dependency setup related to this operation, then retry.

Example fix

Correct the condition described ("Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce)") and rerun the job or command.

When it happens

Trigger: Triggered at runtime when the operation fails because: Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce).

Common situations: Commonly caused by misconfiguration, missing dependencies or files, unsupported types or operations, or invalid user input leading to: Cannot use custom partitioner for a non-grouped GroupReduce (AllGroupReduce).

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/b067c0bb8f2bdef1. Report an issue: GitHub.