[GitHub] [incubator-pegasus] empiredan commented on issue #865: Feature: support to update the replication factor of each table

[GitBox <gi...@apache.org>]

empiredan commented on issue #865:
URL: https://github.com/apache/incubator-pegasus/issues/865#issuecomment-1006584218


   > Good enhancement~
   > 
   > I have some questions about updating table replica count:
   > 
   > 1. There're also cluster-level config called `max_replica_count`, will it be confict if cluster `max_replica_count` is not equal to table `max_replica_count`? Will important features work such as user read/write, learn, load balance, rolling_update?
   > 2. Table `app_info` file will store in each replica disk (_dir/.app_info), if `max_replica_count` is updated, this file should also be updated, how to update it?
   > 3. In your description, `cure` (executed each 10s) will delete redunant replica or add lacking replica. Could you please add some manual test results?
   > 
   > Expecting your reply~
   
   Ok, let's discuss one by one:
   1.  Does cluster-level config mean `max_allowed_replica_count` in https://github.com/XiaoMi/rdsn/pull/963 ? Actually `max_replica_count` is the RF, and `max_allowed_replica_count` is a the upper bound to which an RF can be set. Or we should rename `max_replica_count` as replication_factor to avoid misunderstanding ?
   2. Good question ! This implementation has't written into `.app_info`, this is a bug ! To guarantee the atomicity, I think first the request `set_replica_count` can be replicated to each replica; Received the request, each replica writes it into `slog`, and then writes the new `max_replica_count` into `.app_info`. Even if a replica server fails during the process, the request will not be lost.
   3. As is described in this issue, increasing `max_replica_count` has been tested sufficiently, however decreasing `max_replica_count` has not been tested. I'll test decreasing later to see if redundant replica can be processed correctly.


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