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• Eventually, hive will provide some utilities to copy a table/partition from the primary cluster to the secondary clusters.
  In the first cut, the user needs to do this operation outside hive (one simple way to do so, is distcp the partition from the
  primary to the secondary cluster, and then update the metadata directly - via the thrift api).

is only allowed in this cluster for table C1. but need to allow exceptions
here 3) new partitions are only allowed to be created in C1. 4) all
data changes to T1 happened in the primary cluster should be
replicated to other clusters if there are any secondary clusters. but
there should be a conf to disable it as there are some exception
situations.

We are planning to make hive run across multiple data centers (physical clusters). We prefer to use hive metastore to provide a
unified namespace.

Tables/partitions can exist in more than one cluster. And one cluster
is defined as a primary cluster. A primary cluster is a table level
property. A table T1's primary cluster is C1 meaning :1) C1 contains
all data that is available in all other clusters. 2) write is only
allowed in this cluster for table C1. but need to allow exceptions
here 3) new partitions are only allowed to be created in C1. 4) all
data changes to T1 happened in the primary cluster should be
replicated to other clusters if there are any secondary clusters. but
there should be a conf to disable it as there are some exception
situations.

The first thing that needs to be done is to make hive metastore have a
concept of cluster. And that also means all thrift communication calls
to metastore need to provide a cluster parameter. So we have there
options here:
1) add a cluster parameter to existing thrift interfaces
or
2) add new interfaces which do exactly the same set of functionalities
as old ones but using a different name (use _on_cluster suffifx
maybe?) and have a cluster parameter
or
3) overwrite database name for the purpose of cluster name. And allow
a table co-exist in multiple databases. But that require to promote
table to top level citizen, and degrade database. For example, "show
tables" used to scan all tables in current db, but now need to scan
all tables in all databases.

We would like to get more ideas about which one to choose, and we are
definitely open to other alternatives that we missed here.In order to support the above, hive metastore needs to be enhanced to have the concept of a cluster.
The existing thrift API's will continue to work as if the user is trying to access the default cluster.
New APIs will be added which take the cluster as a new parameter. Almost all the existing APIs will be
enhanced to support this. The behavior will be the same as if, the user issued the command 'USE CLUSTER <CLUSTERNAME>