dvohra commented on a change in pull request #402: New/virtualtables
URL: https://github.com/apache/cassandra/pull/402#discussion_r379567768

 File path: doc/source/new/virtualtables.rst
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+Virtual Tables
+Apache Cassandra 4.0 implements virtual tables (`CASSANDRA-7622
+A virtual table is a table that is backed by an API instead of data explicitly 
managed and stored as SSTables. Apache Cassandra 4.0 implements a virtual 
keyspace interface for virtual tables. Virtual tables are specific to each 
+A virtual table could have several uses including:
+- Expose JMX data/metrics through CQL
+- Expose YAML configuration information
+- Expose other MBean data
+How  are Virtual Tables different from regular tables?
+Virtual tables and virtual keyspaces are quite different from regular tables 
and keyspaces respectively such as:
+- Virtual tables are read-only 
+- Virtual tables are not replicated
+- Virtual tables have no associated SSTables
+- Consistency level of the queries sent virtual tables are ignored
+- Virtual tables are managed by Cassandra and a user cannot run  DDL to create 
new virtual tables or DML to modify existing virtual tables
+- Virtual tables are created in special keyspaces and not just any keyspace
+- All existing virtual tables use LocalPartitioner
+Virtual Keyspaces
+Apache Cassandra 4.0 has added two new keyspaces for virtual tables: 
``system_virtual_schema`` and ``system_views``. Run the following command to 
list the keyspaces.
+ system_schema  system       system_distributed  system_virtual_schema
+ system_auth      system_traces       system_views
+The ``system_virtual_schema keyspace`` contains schema information on virtual 
tables. The ``system_views`` keyspace contains the actual virtual tables. The 
virtual keyspaces metadata is not exposed through ``DESCRIBE`` statement, which 
returns an error message:
+ cqlsh> DESCRIBE KEYSPACE system_views
+ 'NoneType' object has no attribute 'export_for_schema'
+ cqlsh> DESCRIBE KEYSPACE system_virtual_schema;
+ 'NoneType' object has no attribute 'export_for_schema'
+ cqlsh>
+Virtual Table Limitations
+Virtual tables and virtual keyspaces have some limitations such as:
+- Cannot alter virtual keyspaces
+- Cannot drop virtual keyspaces
+- Cannot alter virtual tables
+- Cannot drop virtual tables
+- Cannot truncate virtual tables
+- Expiring columns are not supported by virtual tables
+- Conditional updates are not supported by virtual tables
+- Cannot create tables in virtual keyspaces
+- Cannot perform any operations against virtual keyspace
+- Secondary indexes are not supported on virtual tables
+- Cannot create functions in virtual keyspaces
+- Cannot create types in virtual keyspaces
+- Materialized views are not supported on virtual tables
+- Virtual tables don't support DELETE statements
 Review comment:
   I have added earlier that some of these are "likely to change". Best to keep 
the detail as currently supported. And when any of these change the page could 
be updated.

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