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local min_idle_connections = 4;
local max_idle_connections = 8;

function connect_server()
    local least_idle_conns_ndx = 0;
    local least_idle_conns = 0;
   
    --寻找一个空闲连接数最小的MySQL进行连接 
    for i = 1, #proxy.global.backends do
        local s = proxy.global.backends[i];
        local pool = s.pool;
        local cur_idle = pool.users[""].cur_idle_connections;

        --如果完全没有空闲连接，则直接连接 
        if cur_idle == 0 then
            proxy.connection.backend_ndx = i;
            return
        end
       
        --否则寻找空闲连接数最小的进行连接 
        if least_idle_conns_ndx == 0 or
           ( cur_idle < min_idle_connections and
             cur_idle < least_idle_conns ) then
            least_idle_conns_ndx = i;
            if s.idling_connections then
                least_idle_conns = s.idling_connections;
            else
                least_idle_conns = 0;
            end
        end
    end

    --如果找到可以空闲连接数未满的MySQL则进行连接
    if least_idle_conns_ndx > 0 then
        proxy.connection.backend_ndx = least_idle_conns_ndx;
    end
    if proxy.connection.backend_ndx > 0 then
        local s = proxy.global.backends[proxy.connection.backend_ndx];
        local pool = s.pool;
        local cur_idle = pool.users[""].cur_idle_connections;
        if cur_idle >= min_idle_connections then
            return proxy.PROXY_IGNORE_RESULT;
        end
    end
end

function read_query(packet)
    local cmd = commands.parse(packet)
        --忽略退出命令
        if packet:byte() == proxy.COM_QUIT then
            proxy.response = {
                type = proxy.MYSQLD_PACKET_ERR,
                errmsg = "ignored the COM_QUIT"
            };
            return proxy.PROXY_SEND_RESULT;
        end

        --这里默认选择连接1
        proxy.connection.backend_ndx = 1;
        if packet:byte() == proxy.COM_QUERY then
            --解析表名
            local tokens = tokenizer.tokenize(cmd.query);
            local table = "";
            for i = 1, #tokens do
                if tokens[i].token_name == "TK_LITERAL" then
                    table = tokens[i].text;
                    break;
                end
            end

            --如果表名是t2则选择连接2
            if table == "t2" then
                proxy.connection.backend_ndx = 2;
            end
        end

    local s = proxy.connection.server;
    local c = proxy.connection.client;
    --如果数据库不一致则自动进行切换
    --这里append方法的第一个参数为query的自定义id
    --在read_query_result方法中会用到
    if cmd.type ~= proxy.COM_INIT_DB and
        c.default_db and c.default_db ~= s.default_db then
        proxy.queries:append(2, string.char(proxy.COM_INIT_DB) .. c.default_db, { resultset_is_needed = true });
    end
    proxy.queries:append(1, packet, { resultset_is_needed = true });

    return proxy.PROXY_SEND_QUERY;
end

function read_query_result(inj)
    --如果是自动切换数据库的返回结果则进行忽略，在read_query中将切换数据库的query的自定义id设置为2
    if inj.id ~= 1 then
       return proxy.PROXY_IGNORE_RESULT;
    end

    proxy.connection.backend_ndx = 0;
end