question: public synchronized void addFailedKeys ( Collection < KeyCacheObject > keys , Throwable e , GridCacheContext ctx ) { if ( failedKeys == null ) failedKeys = new ArrayList < > ( keys . size ( ) ) ; failedKeys . addAll ( keys ) ; if ( err == null ) err = new IgniteCheckedException ( "Failed to update keys on primary node." ) ; err . addSuppressed ( e ) ; }
target:  Adds keys to collection of failed keys.