apache/cassandra · error · MarshalException

Cannot find comparator for component

Error message

Cannot find comparator for component 

What it means

After decoding the header, DynamicCompositeType must resolve a concrete comparator: either the named class (bytes) or an alias byte (header & 0xFF). If the alias is not present in the type's alias map (e.g. aliases defined as 'a->IntType' and data uses 'z') or the named class cannot be resolved to a comparator, this MarshalException is thrown. It means the data references a comparator the type does not know about.

Source

Thrown at src/java/org/apache/cassandra/db/marshal/DynamicCompositeType.java:520

            {
                // ByteBufferUtil.string failed.
                // Log it here and we'll further throw an exception below since comparator == null
                logger.error("Failed when decoding the byte buffer in ByteBufferUtil.string()", ce);
            }
            catch (Exception e)
            {
                // parse failed.
                // Log it here and we'll further throw an exception below since comparator == null
                logger.error("Failed to parse value string \"{}\" with exception:", valueStr, e);
            }
        }
        else
        {
            comparator = aliases.get((byte)(header & 0xFF));
        }

        if (comparator == null)
            throw new MarshalException("Cannot find comparator for component " + i);
        else
            return comparator;
    }

    public ByteBuffer decompose(Object... objects)
    {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean isCompatibleWith(AbstractType<?> previous)
    {
        if (this == previous)
            return true;

        if (!(previous instanceof DynamicCompositeType))
            return false;

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Restore/add the missing alias to the DynamicCompositeType definition in the schema (e.g. ALTER with aliases covering all bytes present in the data)
2. Identify the alias byte from the error context and ensure the schema maps it to the intended type (e.g. 'z->UUIDType')
3. If the comparator class name path failed, verify the referenced AbstractType class exists in this Cassandra version

Example fix

// before
DynamicCompositeType.getInstance(ascii->UTF8Type)
// after (data contains alias byte 'b')
DynamicCompositeType.getInstance(ascii->UTF8Type, b->Int32Type)

// before writing, confirm every alias byte used is declared in the schema
def checkAliases(typeDef, aliasBytes) { return aliasBytes.every(b -> typeDef.aliases.containsKey(b)); }

try { type.validate(buffer); } catch (MarshalException e) { log.error("Unknown comparator alias/class in composite value", e); }

When it happens

Trigger: Validating a DynamicCompositeType value whose component alias byte has no registered alias, or whose comparator class name doesn't resolve; typically when reading old data after the aliases definition in the schema changed.

Common situations: The DynamicCompositeType alias string in the schema was edited (aliases removed/renamed) after data was written; reading SSTables produced with different alias definitions; hand-built buffers with bogus alias bytes.

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

• category %s not found in %s
• 'Get CIDR groups for IP' operation not supported by %s
• ACCESS TO DATACENTERS operations not supported by AllowAllNe
• frozen<> is only allowed on collections, tuples, and user-de
• Keyspace %s doesn't exist

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/86c542615a15b402. Report an issue: GitHub.