question: public void addFailedKey ( KeyCacheObject key , Throwable e ) { if ( failedKeys == null ) failedKeys = new ArrayList < > ( ) ; failedKeys . add ( key ) ; if ( err == null ) err = new IgniteCheckedException ( "Failed to update keys on primary node." ) ; err . addSuppressed ( e ) ; }
target:  Adds key to collection of failed keys.