前言:dbcached 1.0 beta* 在 Memcached 1.2.4 的基础上编写而成,也是我的第一个开源C项目。编写 dbcached 的目的是为了最大限度的发挥 Memcached 内存缓存的优势,便捷地维护 Memcached 服务器节点哈希列表,智能地支持 Memcached 故障转移,同时保证数据的持久化存储。


  协议:New BSD License

  dbcached 是什么?

  ● dbcached 是一款基于 Memcached 和 NMDB 的分布式 key-value 数据库内存缓存系统。

  ● dbcached = Memcached + 持久化存储管理器 + NMDB 客户端接口

  ● Memcached 是一款高性能的,分布式的内存对象缓存系统,用于在动态应用中减少数据库负载,提升访问速度。

  ● NMDB 是一款多协议网络数据库(dbm类)管理器,它由内存缓存和磁盘存储两部分构成,使用 QDBM 或 Berkeley DB 作为后端数据库。

  ● QDBM 是一个管理数据库的例程库,它参照 GDBM 为了下述三点而被开发:更高的处理速度,更小的数据库文件大小,和更简单的API。QDBM 读写速度比 Berkeley DB 要快,详细速度比较见《Report of Benchmark Test》。


  Memcached 和 dbcached 在功能上一样吗?

  ● 兼容:Memcached 能做的,dbcached 都能做。除此之外,dbcached 还将“Memcached、持久化存储管理器、NMDB 客户端接口”在一个程序中结合起来,对任何原有 Memcached 客户端来讲,dbcached 仍旧是个 Memcached 内存对象缓存系统,但是,它的数据可以持久存储到本机或其它服务器上的 QDBM 或 Berkeley DB 数据库中。

  ● 性能:前端 dbcached 的并发处理能力跟 Memcached 相同;后端 NMDB 跟 Memcached 一样,采用了libevent 进行网络IO处理,拥有自己的内存缓存机制,性能不相上下。

  ● 写入:当“dbcached 的 Memcached 部分”接收到一个 set(add/replace/...) 请求并储存 key-value 数据到内存中后,“dbcached 持久化存储管理器”能够将 key-value 数据通过“NMDB 客户端接口”保存到 QDBM 或 Berkeley DB 数据库中。

  ● 速度:如果加上“-z”参数,采用 UDP 协议“只发送不接收”模式将 set(add/replace/...) 命令写入的数据传递给 NMDB 服务器端,对 Memcache 客户端写速度的影响几乎可以忽略不计。在千兆网卡、同一交换机下服务器之间的 UDP 传输丢包率微乎其微。在命中的情况下,读取数据的速度跟普通的 Memcached 无差别,速度一样快。

  ● 读取:当“dbcached 的 Memcached 部分”接收到一个 get(incr/decr/...) 请求后,如果“dbcached 的 Memcached 部分”查询自身的内存缓存未命中,则“dbcached 持久化存储管理器”会通过“NMDB 客户端接口”从 QDBM 或 Berkeley DB 数据库中取出数据,返回给用户,然后储存到 Memcached 内存中。如果有用户再次请求这个 key,则会直接从 Memcached 内存中返回 Value 值。

  ● 持久:使用 dbcached,不用担心 Memcached 服务器死机、重启而导致数据丢失。

  ● 变更:使用 dbcached,即使因为故障转移,添加、减少 Memcached 服务器节点而破坏了“key 信息”与对应“Memcached 服务器”的映射关系也不怕。

  ● 分布:dbcached 和 NMDB 既可以安装在同一台服务器上,也可以安装在不同的服务器上,多台 dbcached 服务器可以对应一台 NMDB 服务器。

  ● 特长:dbcached 对于“读”大于“写”的应用尤其适用。

  1. dbcached

  Installation (安装)
wget http://www.monkey.org/~provos/libevent-1.3e.tar.gz
tar zxvf libevent-1.3e.tar.gz
cd libevent-1.3e/
./configure --prefix=/usr
make && make install
cd ../

wget http://dbcached.googlecode.com/files/dbcached-1.0.beta2.tar.gz
tar zxvf dbcached-1.0.beta2.tar.gz
cd dbcached-1.0.beta2/
./configure --prefix=/usr/local/dbcached --with-libevent=/usr
make && make install
cd ../

  Run as a daemon (作为守护进程运行)
/usr/local/dbcached/bin/memcached -d -m 256 -p 11211 -c 51200 -u nobody -x -y 26010 -z 26010

  ● -x nmdb 服务器的域名或者IP地址,推荐使用IP地址

  ● -y <端口号> nmdb 服务器的TCP端口号 (默认: 26010) 支持 set/delete/... 等写命令 和 get 等读命令

  ● -z <端口号> nmdb 服务器的UDP端口号 (默认: 26010) 只支持 get 等都命令, 当使用 -z 参数时,将使用 UDP 协议代替 TCP 协议执行 set 操作,执行 get 操作时仍然使用 TCP 协议。强烈推荐加上 -z 参数。
  ● 其他参数跟 memcached 1.2.4 完全一样,就不再详细说明。

  ● 如果想让 dbcached 通过 NMDB 保存数据时采用 TCP 协议,去掉 -z 参数即可,例如:(除非因防火墙、NAT穿透等问题导致 UDP 协议不可用,否则不建议使用 TCP 协议)
/usr/local/dbcached/bin/memcached -d -m 256 -p 11211 -c 51200 -u nobody -x -y 26010

  ● 如果想让 dbcached 作为普通的 Memcached 运行,去掉 -x、-y、-z 参数即可,例如:
/usr/local/dbcached/bin/memcached -d -m 256 -p 11211 -c 51200 -u nobody

  2. QDBM & NMDB

  QDBM 和 NMDB 均为原版,可以从它们的官方网站下载最新版本。

  QDBM Installation (安装)
wget http://qdbm.sourceforge.net/qdbm-1.8.77.tar.gz
tar zxvf qdbm-1.8.77.tar.gz
cd qdbm-1.8.77/
./configure --prefix=/usr
make install
cd ../

  NMDB Installation (安装)
wget http://www.monkey.org/~provos/libevent-1.3e.tar.gz
tar zxvf libevent-1.3e.tar.gz
cd libevent-1.3e/
./configure --prefix=/usr
make && make install
cd ../

wget http://auriga.wearlab.de/~alb/nmdb/files/0.21/nmdb-0.21.tar.gz
tar zxvf nmdb-0.21.tar.gz
cd nmdb-0.21/
cd ../

  Run as a daemon (作为守护进程运行)
/usr/local/bin/nmdb -d /var/dbcached.db -t 26010 -T -u 26010 -U -c 1024

  ● -d 数据库路径(这里使用比 Berkeley DB 更快的 QDBM 数据库),例如 /var/dbcached.db

  ● -t TCP 监听端口 (默认:26010)

  ● -T TCP 监听地址 (默认:任何地址)

  ● -u UDP 监听端口 (默认:26010)

  ● -U UDP 监听地址 (默认:任何地址)

  ● -c 最大的缓存对象数目,单位为千 (默认:128)

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