apache/kafka · error · IllegalArgumentException

Unexpected config source {source}

Error message

Unexpected config source {source}

What it means

An IllegalArgumentException thrown in the switch that maps protocol ConfigSource values to ConfigEntry.ConfigSource. It fires only on the default case, meaning the broker returned a ConfigSource code the client does not recognize. This is almost always a client/broker version mismatch rather than user error.

Source

Thrown at clients/src/main/java/org/apache/kafka/clients/admin/KafkaAdminClient.java:2948

                configSource = ConfigEntry.ConfigSource.DYNAMIC_TOPIC_CONFIG;
                break;
            case DYNAMIC_BROKER_CONFIG:
                configSource = ConfigEntry.ConfigSource.DYNAMIC_BROKER_CONFIG;
                break;
            case DYNAMIC_DEFAULT_BROKER_CONFIG:
                configSource = ConfigEntry.ConfigSource.DYNAMIC_DEFAULT_BROKER_CONFIG;
                break;
            case STATIC_BROKER_CONFIG:
                configSource = ConfigEntry.ConfigSource.STATIC_BROKER_CONFIG;
                break;
            case DYNAMIC_BROKER_LOGGER_CONFIG:
                configSource = ConfigEntry.ConfigSource.DYNAMIC_BROKER_LOGGER_CONFIG;
                break;
            case DEFAULT_CONFIG:
                configSource = ConfigEntry.ConfigSource.DEFAULT_CONFIG;
                break;
            default:
                throw new IllegalArgumentException("Unexpected config source " + source);
        }
        return configSource;
    }

    @Override
    public AlterConfigsResult incrementalAlterConfigs(Map<ConfigResource, Collection<AlterConfigOp>> configs,
                                                      final AlterConfigsOptions options) {
        final Map<ConfigResource, KafkaFutureImpl<Void>> allFutures = new HashMap<>();
        // BROKER_LOGGER requests always go to a specific, constant broker or controller node.
        //
        // BROKER resource changes for a specific (non-default) resource go to either that specific
        // node (if using bootstrap.servers), or directly to the active controller (if using
        // bootstrap.controllers)
        //
        // All other requests go to the least loaded broker (if using bootstrap.servers) or the
        // active controller (if using bootstrap.controllers)
        final Collection<ConfigResource> unifiedRequestResources = new ArrayList<>();

View on GitHub (pinned to 996fb4585a)

Solutions

1. Upgrade the client to match or exceed the broker version so it knows the new ConfigSource codes.
2. If you cannot upgrade, avoid the config operation that surfaces the unknown source, or filter response entries defensively.
3. Report the unknown source code to narrow down which broker version introduced it.

Example fix

// before
ConfigResource r = new ConfigResource(Type.BROKER, "1");
admin.describeConfigs(Collections.singleton(r)).all().get(); // older client, newer broker

// after: upgrade client dependency
// implementation 'org.apache.kafka:kafka-clients:<newer-or-equal-version>'

// no client-side mitigation; ensure client version >= broker version

try {
    admin.describeConfigs(Collections.singleton(r)).all().get();
} catch (ExecutionException e) {
    if (e.getCause() instanceof IllegalArgumentException
        && e.getCause().getMessage().contains("Unexpected config source")) {
        // upgrade kafka-clients to match the broker, then retry
    }
}

When it happens

Trigger: Calling Admin.describeConfigs or alterConfigs against a broker that returns a ConfigSource enum value newer than the client knows about, then mapping that response through incrementalAlterConfigs/describeConfigs handling.

Common situations: Newer broker adding a config source type (e.g. a new dynamic scope) while the client is on an older version; downgrade scenarios where the client is newer than the broker and a code was removed; custom/patched brokers emitting non-standard codes.

Related errors

• Buffer underflow while parsing response for request with hea
• The response is unrelated to Sasl request since its correlat
• The upgradeType flag should be set to SAFE_DOWNGRADE or UNSA
• Cannot specify a negative version level.
• TransactionalId `{transactionalId}` was not included in the

AI-assisted analysis of apache/kafka@996fb4585a (2026-08-11). Data as JSON: /api/errors/9ade3bf494c69c21. Report an issue: GitHub.