digraph G {
    graph [ rankdir = LR ]
    fontname = "WenQuanYi Zen Hei Mono"
    fontsize = 8

    node [
        fontname = "WenQuanYi Zen Hei Mono"
        fontsize = 8
    ]

    edge [
        fontname = "WenQuanYi Zen Hei Mono"
        fontsize = 8
    ]

    subgraph cluster_interface {
        a [label = "读入用户编号"];
        b [label = "读入图书书号"];
        c [label = "显示：用户不存在或已超过限额"];
        d [label = "显示：此书已被借走"];
        e [label = "显示：完成"];
        label = "用户接口"
    }

    a -> g;
    b -> g;

    c -> h [label = "Y", dir = back]
    d -> i [label = "Y", dir = back]
    e -> j [dir = back]


    subgraph cluster_main {
        f [shape = "point"];
        g [label = "获取信息"];
        h [label = "用户不存在或超额？", shape = "diamond"];
        i [label = "图书已被借走？", shape = "diamond"];
        j [label = "更新数据库信息"];
        k [shape = "doublecircle", style = filled, label = ""];
        label = "借书主流程"
        {rank = same; f; g; h; i; j; k}
    }
     g -> f [dir = back];
     h -> g [dir = back];
     i -> h [label = "N", dir = back];
     j -> i [label = "N", dir = back];
     k -> j [label = "", dir = back];


    subgraph cluster_database {
        user [label = "图书馆用户信息"];
        book [label = "图书馆图书信息"];
        label = "数据库"
    }

    h -> user;
    user -> h;
    h -> book;
    book -> h;

    j -> user;
    j -> book;

}