shawnding opened a new pull request #679: [WIP] [ISSUE #672] Add namespace 
metadata 
URL: https://github.com/apache/incubator-iceberg/pull/679
 
 
   **Create a namespace metadata file  and save it into** 
_${WAREHOUSE_BASE}/namespace_metadata_  
   **Then :** 
   1. The commands of `list namespace` or `list table` read the namespace 
metadata file. Reading from the file could have better performance than listing 
directories on file system, especially  on Object Store, like S3。
   2. Move the functions of namespace manager out of catalog. Hive catalog 
could also make good use of the namespace manager to support multi part 
namespace.
   3. It could help to manage the Iceberg tables and namespaces without the 
help from a center meta store service like Hive MetaStore.
    
   **TODO: To guarantee the atomicity of create/drop table,  which includes the 
operations on both 'metadata.json' and 'namespace_metadata.json'**
   
   The followings demonstrate the content of 'namespace_metadata.json'
   ```
   {
     "format-version" : 1,
     "uuid" : "628e40f3-93dc-4b4f-bd00-dee78d7073d1",
     "location" : "/data/tmp/namespace_metadata",
     "last-updated-ms" : 1575278464343,
     "namespaces" : [
                      {
                         "namespace" : "default",
                          "last-updated-ms" : 1575278464343,
                          "tables" : ["tbl1", "tbl2" ],
                          "properties" : { 
                             "owner" : "apache" 
                           }
                      }, 
                      {
                            "namespace" : "db.n2.n3",
                             "last-updated-ms" : 1575278464343,
                             "tables" : [ ],
                             "properties" : { }
                      } 
       ]
   }
   ```
   
   

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