apache/cassandra · error · InvalidRequestException

Not-masking function

Error message

Not-masking function %s cannot be used for masking table columns

What it means

ColumnMask.findMaskingFunction rejects resolved native functions that are not instances of MaskingFunction. Only the built-in masking functions (mask_null, mask_default, mask_inner, mask_outer, hash) may be attached to columns; other native scalars like now(), toTimestamp(), or uuid() are disallowed.

Solutions

1. Use one of the built-in masking functions: mask_null, mask_default, mask_inner, mask_outer, or hash.
2. Create a scalar UDF in the same keyspace as the table and reference that instead.

Example fix

// before
ALTER TABLE users ALTER created_at MASKED WITH now();
// after
ALTER TABLE users ALTER created_at MASKED WITH mask_default(timestamp);

Set<String> nativeMasks = Set.of("mask_null","mask_default","mask_inner","mask_outer","hash");
if (fnKeyspace == null && !nativeMasks.contains(fnName.toLowerCase()))
  throw new IllegalArgumentException(fnName + " is not a masking function");

When it happens

Trigger: MASKED WITH now() or any non-masking native function in CREATE TABLE / ALTER TABLE masked-column DDL.

Common situations: Developer tries to use an ordinary native scalar function as a masker, unaware that the whitelist is limited to MaskingFunction implementations (plus user-defined functions in the table's keyspace).

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Aggregate function cannot be used for masking table columns
• Altering field types is no longer supported
• Cannot use CREATE TABLE LIKE on a materialized view
• Cannot use CREATE TABLE LIKE on an index table
• Frozen UDT column does not support field deletions

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/dbadf424af155154. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/functions/masking/ColumnMask.java:253

        private ScalarFunction findMaskingFunction(String keyspace, String table, ColumnIdentifier column, AbstractType<?> type, UserFunctions functions)
        {
            List<AssignmentTestable> args = new ArrayList<>(rawPartialArguments.size() + 1);
            args.add(type);
            args.addAll(rawPartialArguments);

            Function function = FunctionResolver.get(keyspace, name, args, keyspace, table, type, functions);

            if (function == null)
                throw invalidRequest("Unable to find masking function for %s, " +
                                     "no declared function matches the signature %s",
                                     column, this);

            if (function.isAggregate())
                throw invalidRequest("Aggregate function %s cannot be used for masking table columns", this);

            if (function.isNative() && !(function instanceof MaskingFunction))
                throw invalidRequest("Not-masking function %s cannot be used for masking table columns", this);

            if (!function.isNative() && !function.name().keyspace.equals(keyspace))
                throw invalidRequest("Masking function %s doesn't belong to the same keyspace as the table %s.%s",
                                     this, keyspace, table);

            CQL3Type returnType = function.returnType().asCQL3Type();
            CQL3Type expectedType = type.asCQL3Type();
            if (!returnType.equals(expectedType))
                throw invalidRequest("Masking function %s return type is %s. " +
                                     "This is different to the type of the masked column %s of type %s. " +
                                     "Masking functions can only be attached to table columns " +
                                     "if they return the same data type as the masked column.",
                                     this, returnType, column, expectedType);

            return (ScalarFunction) function;
        }

        private ByteBuffer[] preparePartialArguments(String keyspace, ScalarFunction function)

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