实现ClickHouse的xml配置文件的poco::xml::document对象与kv对象的相互转化。
对kv的定义：
class Item {
public:
Item(std::string _name) : name(_name) {}
Item() {}
std::map<std::string, std::string> attr; // 参数的属性
std::string name; // 配置的参数名
std::string value; // 参数值
};xml文件示例：
<clickhouse>
<listen_host>0.0.0.0</listen_host>
<listen_host>localhost</listen_host>
<global_config>uncompressed_cache_size,remote_servers.shard_0.shard[0]</global_config>
<logger>
<level>trace</level>
<log>/root/chdata/log/clickhouse-server/clickhouse-server.log</log>
<size>100M</size>
</logger>
<uncompressed_cache_size>8589934592</uncompressed_cache_size>
<mark_cache_size>5368709120</mark_cache_size>
<mlock_executable>true</mlock_executable>
<remote_servers>
<shard_0>
<shard c1="5">
<replica>
<host a1="1" a2="2">clickhouse-env-dev-ch</host>
<port>9000</port>
</replica>
<replica>
<host a3="3" a4="4">clickhouse-env-dev-ch2</host>
<port>9000</port>
</replica>
</shard>
<shard>
<replica>
<host b1="1" b2="2">clickhouse-env-runner-0.clickhouse-env-runner</host>
<port>9000</port>
</replica>
<replica>
<host b3="3" b4="4">clickhouse-env-runner-0.clickhouse-env-runner2</host>
<port>9000</port>
</replica>
</shard>
</shard_0>
<shard_1>
<shard>
<replica>
<host>clickhouse-env-runner-0.clickhouse-env-runner</host>
<port>9000</port>
</replica>
</shard>
<shard>
<replica>
<host>clickhouse-env-runner-1.clickhouse-env-runner</host>
<port>9000</port>
</replica>
</shard>
</shard_1>
</remote_servers>
</clickhouse>示例转成的kv对象：
name:value
attr_name:attr_value
listen_host[0]:0.0.0.0
listen_host[1]:localhost
global_config:uncompressed_cache_size,remote_servers.shard_0.shard[0]
logger.level:trace
logger.log:/root/chdata/log/clickhouse-server/clickhouse-server.log
logger.size:100M
uncompressed_cache_size:8589934592
mark_cache_size:5368709120
mlock_executable:true
remote_servers.shard_0.shard[0]:
c1:5
remote_servers.shard_0.shard[0].replica[0].host:clickhouse-env-dev-ch
a1:1
a2:2
remote_servers.shard_0.shard[0].replica[0].port:9000
remote_servers.shard_0.shard[0].replica[1].host:clickhouse-env-dev-ch2
a3:3
a4:4
remote_servers.shard_0.shard[0].replica[1].port:9000
remote_servers.shard_0.shard[1].replica[0].host:clickhouse-env-runner-0.clickhouse-env-runner
b1:1
b2:2
remote_servers.shard_0.shard[1].replica[0].port:9000
remote_servers.shard_0.shard[1].replica[1].host:clickhouse-env-runner-0.clickhouse-env-runner2
b3:3
b4:4
remote_servers.shard_0.shard[1].replica[1].port:9000
remote_servers.shard_1.shard[0].replica.host:clickhouse-env-runner-0.clickhouse-env-runner
remote_servers.shard_1.shard[0].replica.port:9000
remote_servers.shard_1.shard[1].replica.host:clickhouse-env-runner-1.clickhouse-env-runner
remote_servers.shard_1.shard[1].replica.port:9000
此处kv对象的name符合 Poco/Util/XMLConfiguration.h 中对name的格式，通过 has 方法能判断name对应的参数在xml文件中是否存在。
using ConfigurationPtr = Poco::AutoPtr<Poco::Util::AbstractConfiguration>;
if (configuration->has(name)){
...
}通过Poco::XML::Document的 getNodeByPath 方法，能得到 Document 对象中对应 path 的 Node 结点。
name 和 path 对应关系举例：
- name = remote_servers.shard_1.shard[0].replica.host
- path = clickhouse/remote_servers/shard_1/shard[0]/replica/host