apache/cassandra · error · InvalidRequestException

Unable to find masking function for

Error message

Unable to find masking function for %s, no declared function matches the signature %s

What it means

Thrown by ColumnMask.findMaskingFunction when a MASKED WITH function reference cannot be resolved to any declared function matching the given name and argument types. FunctionResolver.get returned null, meaning no native or user-defined function exists with that signature. Cassandra refuses to attach the masking policy because there is no function to apply.

Solutions

1. Verify the exact masking function name and argument signature against the built-ins in org.apache.cassandra.cql3.functions.masking (MaskingFunction, HashMaskingFunction, etc.) and fix the MASKED WITH clause.
2. Qualify the function with its keyspace (keyspace.funcName) if it is a user-defined masking function and check it exists via SELECT from system_schema.functions.
3. Confirm the argument count and types match a declared overload; use DESCRIBE FUNCTION or cqlsh to inspect available signatures.

Example fix

// before
CREATE TABLE users (email text MASKED WITH bogus_mask(text));
// after
CREATE TABLE users (email text MASKED WITH mask_null(text));

// check function exists before DDL
Row r = session.execute("SELECT function_name FROM system_schema.functions WHERE keyspace_name=? AND function_name=?", ks, fn).one();
if (r == null) throw new IllegalStateException("masking function not found: " + fn);

When it happens

Trigger: Executing CREATE TABLE ... col type MASKED WITH <name>(args) or ALTER TABLE ... MASKED WITH, where the function name is misspelled, the function does not exist in the keyspace, or the argument types/count do not match any declared overload.

Common situations: Typo in the masking function name (e.g. mask_null vs mask_null()), referencing a UDF that was created in a different keyspace without qualification, passing wrong argument arity to a built-in masking function, or running against a cluster where dynamic data masking (CE-3.0 feature) functions are unavailable.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• Aggregate function cannot be used for masking table columns
• Ambiguous call to function
• appendAll() can only be called on non-frozen collections
• Function ' ' is still referenced by column masks in tables
• Invalid call to function

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/36e250eef18d4cde. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/functions/masking/ColumnMask.java:245

        }

        public ColumnMask prepare(String keyspace, String table, ColumnIdentifier column, AbstractType<?> type, UserFunctions functions)
        {
            ScalarFunction function = findMaskingFunction(keyspace, table, column, type, functions);
            ByteBuffer[] partialArguments = preparePartialArguments(keyspace, function);
            return new ColumnMask(function, partialArguments);
        }

        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. " +

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