apache/flink · error · IllegalArgumentException

Cannot use the key partitioner for composite keys (more than

Error message

Cannot use the key partitioner for composite keys (more than one key field)

What it means

Error "Cannot use the key partitioner for composite keys (more than one key field)" thrown in apache/flink.

Source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java:189

     *     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(
            List<T> inputData, RuntimeContext ctx, ExecutionConfig executionConfig)
            throws Exception {
        // make sure we can handle empty inputs
        if (inputData.isEmpty()) {

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Address the cause reported by the error message: Cannot use the key partitioner for composite keys (more than one key field)
2. Verify the inputs, configuration values, and classpath/dependency setup related to this operation, then retry.

Example fix

Correct the condition described ("Cannot use the key partitioner for composite keys (more than one key field)") and rerun the job or command.

When it happens

Trigger: Triggered at runtime when the operation fails because: Cannot use the key partitioner for composite keys (more than one key field).

Common situations: Commonly caused by misconfiguration, missing dependencies or files, unsupported types or operations, or invalid user input leading to: Cannot use the key partitioner for composite keys (more than one key field).

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/4da5c65d40548a05. Report an issue: GitHub.