@Override
    public Void visitModifyBackendClause(ModifyBackendClause clause, ConnectContext context) {
        if (clause.getBackendHostPort() == null) {
            checkModifyHostClause(clause.getSrcHost(), clause.getDestHost());
        } else {
            SystemInfoService.validateHostAndPort(clause.getBackendHostPort(), false);
            analyzeBackendProperties(clause.getProperties());
        }
        return null;
    }
@Test(expected = SemanticException.class)
    public void testVisitModifyBackendClauseException() {
        AlterSystemStmtAnalyzer visitor = new AlterSystemStmtAnalyzer();
        ModifyBackendClause clause = new ModifyBackendClause("127.0.0.2", "127.0.0.1");
        visitor.visitModifyBackendClause(clause, null);
    }