RGW:Implement cross region replication feature

RGW:Implement cross region replication feature

[yiming xie]

Hi Yehuda and Benjamin:

I want to impl rgw CRR(cross region replication) feature like s3.
I find some limited conditions of s3 CRR。
 1. multi-zonegroup can not replication to the same zonegroup on the same bucket。
  user can not set the logic of replication like this:
   zg1  bucket1 ->  zg3 bucket3
   zg2  bucket2 ->  zg3 bucket3

 2. s3 CRR does not support chain replication. user can set the  logic of replication like this：
 zg1 bucket1 -> zg2 bucket2 -> zg3 bucket3
 when user wrtie the test file to zg1 bucket1，the test file will only be copied from zg1 to zg2, and will not continue from zg2 to zg3

I do not understand why s3 take these strategies. So I can not judge whether these strategies apply to rgw。

When we achieve this feature, whether we need to adopt these strategies?  Tks！


plato