apache/beam · error · IllegalStateException

the left side CEP operation is not legal: ${class}

Error message

the left side CEP operation is not legal: ${class}

What it means

In NFA.evalLeftSideCondition, the input CEPOperation must be a CEPOperator (RexCall-derived) to be reduced; if it is some other CEPOperation subclass (e.g. an unexpected literal or field-ref wrapper) the else branch throws IllegalStateException with the operation's class name.

Source

Thrown at sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/nfa/NFA.java:526

        return inputEvent.toCEPLiteral((CEPFieldRef) inputOperation);
      } else if (inputOperation.getClass() == CEPCall.class) {
        CEPCall call = (CEPCall) inputOperation;
        CEPOperator operator = call.getOperator();
        List<CEPOperation> operands = call.getOperands();
        switch (operator.getCepKind()) {
          case LAST:
            return last(
                operands.get(0), evalLeftSideCondition(operands.get(1), inputEvent), inputEvent);
          case PLUS:
            return plus(
                evalLeftSideCondition(operands.get(0), inputEvent),
                evalLeftSideCondition(operands.get(1), inputEvent));
          default:
            throw new UnsupportedOperationException(
                "the function is not supported for now: " + operator.getCepKind().toString());
        }
      } else {
        throw new IllegalStateException(
            "the left side CEP operation is not legal: " + inputOperation.getClass().toString());
      }
    }

    // evaluates the condition value (right) given an input event
    private CEPLiteral evalRightSideCondition(CEPOperation inputOperation, Event inputEvent) {
      if (inputOperation instanceof CEPLiteral) {
        return (CEPLiteral) inputOperation;
      } else if (inputOperation.getClass() == CEPCall.class) {
        CEPCall call = (CEPCall) inputOperation;
        CEPOperator operator = call.getOperator();
        List<CEPOperation> operands = call.getOperands();
        switch (operator.getCepKind()) {
          case PLUS:
            return plus(
                evalRightSideCondition(operands.get(0), inputEvent),
                evalRightSideCondition(operands.get(1), inputEvent));
          case PREV:

View on GitHub (pinned to 12126d8942)

Solutions

1. Inspect the named class in the message and simplify the DEFINE expression to standard field-vs-literal comparisons.
2. Use only supported patterns: field refs, literals, and supported operators on each side of the comparison.
3. File/patch an issue in CEPOperation.of() if a valid RexNode shape is being converted to the wrong CEPOperation type.
4. Upgrade Beam — newer versions broaden CEP expression support.

Example fix

// before
DEFINE A AS CASE WHEN A.x > 0 THEN 1 ELSE 0 END > 0
// after
DEFINE A AS A.x > 0

if (!(inputOperation instanceof CEPOperator || inputOperation instanceof CEPLiteral || inputOperation instanceof CEPFieldRef)) {
  throw new IllegalArgumentException("Unexpected CEPOperation class: " + inputOperation.getClass());
}

boolean isKnownCepOperation(CEPOperation op) { return op instanceof CEPOperator || op instanceof CEPLiteral || op instanceof CEPFieldRef; }

try { return evalLeftSideCondition(op, event); } catch (IllegalStateException e) { throw new QueryTranslationException("Malformed CEP operation in DEFINE", e); }

When it happens

Trigger: The left operand of a DEFINE comparison resolves to a CEPOperation that is neither CEPLiteral, CEPFieldRef, nor CEPOperator — typically an unexpected expression shape produced during translation of the MATCH_RECOGNIZE query.

Common situations: Complex/nested expressions in DEFINE that the CEPOperation converter mapped to an unexpected class; internal bugs or version mismatches between Calcite translation and the NFA evaluator.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• the comparator is not supported: ${comparator}
• The other object should be an instance of CEPLiteral
• The other CEPLiteral should have type , given:
• RexNode not supported: ${className}
• the function in Measures is not recognized.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/eb50189f2eb392c4. Report an issue: GitHub.