apache/cassandra · warning

Ignoring codec because it collides with previously…

Error message

Ignoring codec {} because it collides with previously registered codec {}

What it means

CodecRegistry refuses (with a warning, not an exception) to register a custom TypeCodec that accepts the same CQL type and Java type as one of the driver's built-in codecs; the new codec is ignored and the registry is returned unchanged. BUILT_IN_CODECS always win over user codecs.

Solutions

1. Register a codec only for mappings not already built-in (check TypeCodec primitives/TypeCodec.enumIntCodec etc.)
2. If custom behavior is needed for a built-in mapping, bypass CodecRegistry and pass the codec explicitly to statement setters/getters
3. Remove the redundant registration call

Example fix

// before
registry.register(TypeCodec.blob()); // collides with built-in
// after
// no registration needed; built-in codec is used automatically

// Only register codecs for non-builtin mappings
function needsRegistration(codec) {
  const builtin = TypeCodec.primitiveMappings();
  return !builtin.some(b => b.accepts(codec.getCqlType()) && b.accepts(codec.getJavaType()));
}
if (needsRegistration(myCodec)) registry.register(myCodec);

When it happens

Trigger: Calling codecRegistry.register(newCodec) where newCodec.getCqlType() and getJavaType() are both accepted by an existing built-in codec, e.g. registering a custom codec for (int <-> Integer).

Common situations: Users re-registering a codec for a primitive mapping that the Java driver already handles; copy-pasted codec tutorials for types already supported; driver-version upgrades that made a previously custom mapping built-in.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Codec not found for requested operation
• Ignoring codec because it collides with previously…
• Cannot parse 32-bits int value from
• Cannot parse 64-bits double value from
• Cannot parse 64-bits long value from

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/231946d229be4bdb. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/functions/types/CodecRegistry.java:460

    /**
     * Register the given codec with this registry.
     *
     * <p>This method will log a warning and ignore the codec if it collides with a previously
     * registered one. Note that this check is not done in a completely thread-safe manner; codecs
     * should typically be registered at application startup, not in a highly concurrent context (if a
     * race condition occurs, the worst possible outcome is that no warning gets logged, and the codec
     * gets registered but will never actually be used).
     *
     * @param newCodec The codec to add to the registry.
     * @return this CodecRegistry (for method chaining).
     */
    public CodecRegistry register(TypeCodec<?> newCodec)
    {
        for (TypeCodec<?> oldCodec : BUILT_IN_CODECS)
        {
            if (oldCodec.accepts(newCodec.getCqlType()) && oldCodec.accepts(newCodec.getJavaType()))
            {
                logger.warn(
                "Ignoring codec {} because it collides with previously registered codec {}",
                newCodec,
                oldCodec);
                return this;
            }
        }
        for (TypeCodec<?> oldCodec : codecs)
        {
            if (oldCodec.accepts(newCodec.getCqlType()) && oldCodec.accepts(newCodec.getJavaType()))
            {
                logger.warn(
                "Ignoring codec {} because it collides with previously registered codec {}",
                newCodec,
                oldCodec);
                return this;
            }
        }
        CacheKey key = new CacheKey(newCodec.getCqlType(), newCodec.getJavaType());

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