When you have multi-impls of a interface,you can distinguish them with the group.
<dubbo:service group="feedback" interface="com.xxx.IndexService" />
<dubbo:service group="member" interface="com.xxx.IndexService" />
<dubbo:reference id="feedbackIndexService" group="feedback" interface="com.xxx.IndexService" />
<dubbo:reference id="memberIndexService" group="member" interface="com.xxx.IndewxService" />
Any group:
<dubbo:reference id="barService" interface="com.foo.BarService" group="*" />
group="*" is supported after version 2.2.0, always select only one available group of implementations to invoke.