digraph backend_abstraction {
    rankdir=TB;
    fontname="Helvetica";
    node [fontname="Helvetica", fontsize=10, style=filled, shape=box];
    edge [fontname="Helvetica", fontsize=9];

    label="Backend Abstraction Layer";
    labelloc=t;
    fontsize=14;
    compound=true;

    subgraph cluster_consumers {
        label="Consumers (schema-agnostic operations)";
        style=filled;
        color="#d5e8d5";
        fillcolor="#f0fff0";

        writers [label="Schema Writers\n(region, kernel_dispatch, ...)", fillcolor="#c0e0c0"];
        readers [label="Schema Readers\n(reader_impl)", fillcolor="#c0e0c0"];
    }

    subgraph cluster_statements {
        label="Statement Layer (schema-specific SQL)";
        style=filled;
        color="#e5ddf0";
        fillcolor="#f5f0ff";

        insert_stmts [label="insert_statements<Backend>\n(schema v3)", fillcolor="#ddd0f0"];
        read_stmts [label="read_statements\n(schema v3)", fillcolor="#ddd0f0"];
        insert_stmts_v4 [label="insert_statements<Backend>\n(schema v4 — future)", fillcolor="#e8e0f0", style="filled,dashed"];
        read_stmts_v4 [label="read_statements\n(schema v4 — future)", fillcolor="#e8e0f0", style="filled,dashed"];
    }

    subgraph cluster_builders {
        label="Query Builders (shared utilities)";
        style=filled;
        color="#f5f5e0";
        fillcolor="#fffff0";

        insert_builder [label="table_insert_query\n(fluent INSERT builder)", fillcolor="#f0f0c0"];
        select_builder [label="table_select_query\n(fluent SELECT builder)", fillcolor="#f0f0c0"];
    }

    subgraph cluster_backend {
        label="Backend Layer (database engine)";
        style=filled;
        color="#f5e0e0";
        fillcolor="#fff5f5";

        sqlite [label="sqlite_backend\n\nprepared_insert_statement<Values...>\nresult_set<T>\ntransaction_guard\nstatement_handle\n\ncreate_write_statement_executor()\ncreate_read_statement_executor()\nbegin_transaction()\nflush()", fillcolor="#f0d0d0"];
    }

    subgraph cluster_db {
        label="";
        style=invis;

        database [label="SQLite Database\n(in-memory or on-disk)", shape=cylinder, fillcolor="#e0e0e0"];
    }

    writers -> insert_stmts [label="  calls statements  "];
    readers -> read_stmts [label="  calls statements  "];

    insert_stmts -> insert_builder [label="  builds SQL  ", style=dashed, color=gray50];
    read_stmts -> select_builder [label="  builds SQL  ", style=dashed, color=gray50];
    insert_stmts_v4 -> insert_builder [style=dashed, color=gray50];
    read_stmts_v4 -> select_builder [style=dashed, color=gray50];

    insert_stmts -> sqlite [label="  create_write_statement_executor  ", color="#a06020", fontcolor="#a06020"];
    read_stmts -> sqlite [label="  create_read_statement_executor  ", color="#a06020", fontcolor="#a06020"];
    insert_stmts_v4 -> sqlite [style=dashed, color=gray50];
    read_stmts_v4 -> sqlite [style=dashed, color=gray50];

    writers -> sqlite [label="  begin_transaction()  ", color="#2060a0", fontcolor="#2060a0"];

    sqlite -> database [label="  SQLite C API  "];
}