mysql 读写分离(基础篇)
MySQL Proxy最强大的一项功能是实现“读写分离(Read/Write Splitting)”。
基本的原理是让主数据库处理事务性查询,而从数据库处理SELECT查询。数据库复制被用来把事务性查询导致的变更同步到集群中的从数据库。Jan Kneschke在《MySQL Proxy learns R/W Splitting》中详细的介绍了这种技巧以及连接池问题:
为了实现读写分离我们需要连接池。我们仅在已打开了到一个后端的一条经过认证的连接的情况下,才切换到该后端。MySQL协议首先进行握手。当进入到查询/返回结果的阶段再认证新连接就太晚了。我们必须保证拥有足够的打开的连接才能保持运作正常。
实现读写分离的LUA脚本:
-- 读写分离
--
-- 发送所有的非事务性Select到一个从数据库
代码如下:
if is_in_transaction == 0 and
packet:byte() == proxy.COM_QUERY and
packet:sub(2, 7) == "SELECT" then
local max_conns = -1
local max_conns_ndx = 0
for i = 1, #proxy.servers do
local s = proxy.servers[i]
-- 需要选择一个拥有空闲连接的从数据库
if s.type == proxy.BACKEND_TYPE_RO and
s.idling_connections > 0 then
if max_conns == -1 or
s.connected_clients max_conns = s.connected_clients
max_conns_ndx = i
end
end
end
-- 至此,我们找到了一个拥有空闲连接的从数据库
if max_conns_ndx > 0 then
proxy.connection.backend_ndx = max_conns_ndx
end
else
-- 发送到主数据库
end
return proxy.PROXY_SEND_QUERY
注释:此技巧还可以用来实现其他的数据分布策略,例如分片(Sharding)。

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