核心数据库层选型Oracle ExtendRAC模式实现双活和oracle RAC+ADG模式实现准双活有哪些优势和劣势？

Oracle Extend RAC 是RAC的距离延伸，其优势在于常规的多节点A-A模式运行。oracle RAC+ADG按字面理解应该是本地RAC，远端ADG，那么常规情况下，远端能做到只读。但Oracle Extend RAC这种架构对延迟的高要求，其距离小于100公里，因此能够覆盖机房/建筑级别的灾害，但对于地区级（超过100公里）、逻辑级的灾害，例如地震/洪灾/人为误操作造成的灾害则无法覆盖。官方给出的文档描述如下：
Disasters such as earthquakes, hurricanes, and regional floods may affect a greater area. Oracle RAC on Extended Distance Clusters does not rotect from human errors or corruptions in the shared storage, either, as an Oracle RAC system,even on Extended Distance Clusters,is still a tightly coupled and enclosed system. For comprehensive data protection, including protection against corruptions and regional disasters, Oracle recommends using Oracle Data Guard together with Oracle RAC as building blocks of Oracle’s Maximum Availability Architecture(MAA)