[GitHub] [cassandra] clohfink commented on a change in pull request #402: New/virtualtables

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clohfink commented on a change in pull request #402: New/virtualtables
URL: https://github.com/apache/cassandra/pull/402#discussion_r379489690
 
 

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 File path: doc/source/new/virtualtables.rst
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+
+Virtual Tables
+--------------
+
+Apache Cassandra 4.0 implements virtual tables (`CASSANDRA-7622
+<https://issues.apache.org/jira/browse/CASSANDRA-7622>`_).
+
+Definition
+^^^^^^^^^^
+
+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 node. 
+
+Objective
+^^^^^^^^^
+
+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.
+
+::
+
+ cqlsh> DESC 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:
 
 Review comment:
   Thats due to a bug in the driver that is fixed upstream and will soon be resolved and in trunk, DESCRIBE works on both virtual keyspaces and virtual tables - its just broken in trunk due to using older driver see https://issues.apache.org/jira/browse/CASSANDRA-14872

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