alibaba/canal · error · IllegalArgumentException

!! Unknown BLOB packlen =

Error message

!! Unknown BLOB packlen = 

What it means

Thrown while decoding a MYSQL_TYPE_BLOB column family. The metadata field (meta, named packlen here) encodes the length-prefix size: 1=TINYBLOB, 2=BLOB, 3=MEDIUMBLOB, 4=LONGBLOB. The switch handles 1-4 only; any other value is invalid because MySQL blob length prefixes are always 1-4 bytes. Hitting the default means the Table_map event carried a corrupt or unrecognized blob metadata word.

Source

Thrown at dbsync/src/main/java/com/taobao/tddl/dbsync/binlog/event/RowsLogBuffer.java:983

                        byte[] binary = new byte[len24];
                        buffer.fillBytes(binary, 0, len24);
                        value = binary;
                        javaType = Types.LONGVARBINARY;
                        length = len24;
                        break;
                    }
                    case 4: {
                        /* LONGBLOB/LONGTEXT */
                        final int len32 = (int) buffer.getUint32();
                        byte[] binary = new byte[len32];
                        buffer.fillBytes(binary, 0, len32);
                        value = binary;
                        javaType = Types.LONGVARBINARY;
                        length = len32;
                        break;
                    }
                    default:
                        throw new IllegalArgumentException("!! Unknown BLOB packlen = " + meta);
                }
                break;
            }
            case LogEvent.MYSQL_TYPE_VARCHAR:
            case LogEvent.MYSQL_TYPE_VAR_STRING: {
                /*
                 * Except for the data length calculation, MYSQL_TYPE_VARCHAR,
                 * MYSQL_TYPE_VAR_STRING and MYSQL_TYPE_STRING are handled the same way.
                 */
                len = meta;
                if (len < 256) {
                    len = buffer.getUint8();
                } else {
                    len = buffer.getUint16();
                }

                if (isBinary) {
                    // fixed issue #66 ,binary类型在binlog中为var_string

View on GitHub (pinned to 87be50e876)

Solutions

1. Verify the BLOB/TEXT column type on the master and that Canal's table-map cache matches.
2. Run mysqlbinlog --hexdump at the failing position to confirm metadata integrity.
3. Upgrade Canal to the version matching the master.
4. Filter the table if the blob column is not needed.

boolean isValidBlobPacklen(int meta) {
    return meta >= 1 && meta <= 4; // TINY/BLOB/MEDIUM/LONG
}

try {
    buffer.fetchValue(name, index, type, meta, isBinary);
} catch (IllegalArgumentException e) {
    if (e.getMessage().startsWith("!! Unknown BLOB packlen")) {
        logger.warn("Corrupt BLOB packlen meta={} in tableId={}, skipping column", meta, tableId);
    } else throw e;
}

When it happens

Trigger: A Table_map_event declares a BLOB column with a packlen meta value of 0 or > 4. Caused by corruption or a source that mis-encodes blob metadata.

Common situations: Binlog corruption; replicating from a non-MySQL-compatible source; partial binlog read truncating the table-map event so meta is misread.

Related errors

• !! Don't know how to handle column type=%d meta=%d (%04X)
• !! Unknown Bit len =
• !! Unknown ENUM packlen =
• !! Unknown Set len =
• !! Unknown JSON packlen =

AI-assisted analysis of alibaba/canal@87be50e876 (2026-08-14). Data as JSON: /api/errors/0c72c81d185b33ed. Report an issue: GitHub.