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Subject [GitHub] [cassandra] dvohra commented on a change in pull request #402: New/virtualtables
Date Fri, 14 Feb 2020 18:03:53 GMT
dvohra commented on a change in pull request #402: New/virtualtables

 File path: doc/source/new/virtualtables.rst
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+.. Licensed to the Apache Software Foundation (ASF) under one
+.. or more contributor license agreements.  See the NOTICE file
+.. distributed with this work for additional information
+.. regarding copyright ownership.  The ASF licenses this file
+.. to you under the Apache License, Version 2.0 (the
+.. "License"); you may not use this file except in compliance
+.. with the License.  You may obtain a copy of the License at
+.. Unless required by applicable law or agreed to in writing, software
+.. distributed under the License is distributed on an "AS IS" BASIS,
+.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+.. See the License for the specific language governing permissions and
+.. limitations under the License.
+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 node. 
+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
+- Cannot CREATE TRIGGER against a virtual table
+- Conditional BATCH statements cannot include mutations for virtual tables
+- Cannot include a virtual table statement in a logged batch
+- Mutations for virtual and regular tables cannot exist in the same batch
+- Conditional BATCH statements cannot include mutations for virtual tables
+- Cannot create aggregates in virtual keyspaces
+Listing and Describing Virtual Tables
+Virtual tables in a virtual keyspace may however be listed with ``DESC TABLES``.  The ``system_views``
virtual keyspace tables include the following:
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