question: public KafkaCluster usingDirectory ( File dataDir ) { if ( running ) throw new IllegalStateException ( "Unable to add a broker when the cluster is already running" ) ; if ( dataDir != null && dataDir . exists ( ) && ! dataDir . isDirectory ( ) && ! dataDir . canWrite ( ) && ! dataDir . canRead ( ) ) { throw new IllegalArgumentException ( "The directory must be readable and writable" ) ; } this . dataDir = dataDir ; return this ; }
target:  Set the parent directory where the brokers logs and server's logs and snapshots will be kept.