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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 metrics through CQL
- Expose YAML configuration information

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, but it is likely to change
- Virtual tables are not replicated
- Virtual tables are local only and non distributed
- 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``. Since a virtual table is not replicated the partitioner sorts in order of     partition   keys instead of by their hash.
- Making advanced queries with ``ALLOW FILTERING`` and aggregation functions may be used with virtual tables even though in normal  tables we   don't recommend it

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.

Virtual Table Limitations
^^^^^^^^^^^^^^^^^^^^^^^^^

Virtual tables and virtual keyspaces have some limitations initially though some of these could change such as:

- Cannot alter or drop virtual keyspaces or 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
- Cannot create aggregates in virtual keyspaces; but may run aggregate functions on select

Listing and Describing Virtual Tables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Virtual tables in a virtual keyspace may be listed with ``DESC TABLES``.  The ``system_views`` virtual keyspace tables include the following:

::

 cqlsh> USE system_views;
 cqlsh:system_views> DESC TABLES;
 coordinator_scans   clients             tombstones_scanned  internode_inbound
 disk_usage          sstable_tasks       live_scanned        caches
 local_writes        max_partition_size  local_reads
 coordinator_writes  internode_outbound  thread_pools
 local_scans         coordinator_reads   settings

Some of the salient virtual tables in ``system_views`` virtual keyspace are described in Table 1.

Table 1 : Virtual Tables in system_views

+------------------+---------------------------------------------------+
|Virtual Table     | Description                                       |
+------------------+---------------------------------------------------+
| clients          |Lists information about all connected clients.     |
+------------------+---------------------------------------------------+
| disk_usage       |Disk usage including disk_space, keyspace_name,    |
|                  |and table_name by system keyspaces.                |
+------------------+---------------------------------------------------+
| local_writes     |A table metric for local writes                    |
|                  |including count, keyspace_name,                    |
|                  |max, median, per_second, and                       |
|                  |table_name.                                        |
+------------------+---------------------------------------------------+
| caches           |Displays the general cache information including   |
|                  |cache name, capacity_bytes, entry_count, hit_count,|
|                  |hit_ratio double, recent_hit_rate_per_second,      |
|                  |recent_request_rate_per_second, request_count, and |
|                  |size_bytes.                                        |
+------------------+---------------------------------------------------+
| local_reads      |A table metric for  local reads information.       |
+------------------+---------------------------------------------------+
| sstable_tasks    |Lists currently running tasks such as compactions  |
|                  |and upgrades on SSTables.                          |
+------------------+---------------------------------------------------+
|internode_inbound |Lists information about the inbound                |
|                  |internode messaging.                               |
+------------------+---------------------------------------------------+
| thread_pools     |Lists metrics for each thread pool.                |
+------------------+---------------------------------------------------+
| settings         |Displays configuration settings in cassandra.yaml. |
+------------------+---------------------------------------------------+
|max_partition_size|A table metric for maximum partition size.         |
+------------------+---------------------------------------------------+
|internode_outbound|Information about the outbound internode messaging.|
|                  |                                                   |
+------------------+---------------------------------------------------+

We shall discuss some of the virtual tables in more detail next.

Clients Virtual Table
*********************

The ``clients`` virtual table lists all active connections (connected clients) including their ip address, port, connection stage, driver name, driver version, hostname, protocol version, request count, ssl enabled, ssl protocol and user name:

::

 cqlsh:system_views> select * from system_views.clients;
  address   | port  | connection_stage | driver_name | driver_version | hostname  | protocol_version | request_count | ssl_cipher_suite | ssl_enabled | ssl_protocol | username
 -----------+-------+------------------+-------------+----------------+-----------+------------------+---------------+------------------+-------------+--------------+-----------
  127.0.0.1 | 50628 |            ready |        null |           null | localhost |                4 |            55 |             null |       False |         null | anonymous
  127.0.0.1 | 50630 |            ready |        null |           null | localhost |                4 |            70 |             null |       False |         null | anonymous

 (2 rows)

Some examples of how ``clients`` can be used are:

- To find applications using old incompatible versions of   drivers before upgrading and with ``nodetool enableoldprotocolversions`` and  ``nodetool disableoldprotocolversions`` during upgrades.
- To identify clients sending too many requests.
- To find if SSL is enabled during the migration to and from   ssl.


The virtual tables may be described with ``DESCRIBE`` statement. The DDL listed however cannot be run to create a virtual table. As an example describe the ``system_views.clients`` virtual table:

::

  cqlsh:system_views> DESC TABLE system_views.clients;
 CREATE TABLE system_views.clients (
    address inet,
    connection_stage text,
    driver_name text,
    driver_version text,
    hostname text,
    port int,
    protocol_version int,
    request_count bigint,
    ssl_cipher_suite text,
    ssl_enabled boolean,
    ssl_protocol text,
    username text,
    PRIMARY KEY (address, port)) WITH CLUSTERING ORDER BY (port ASC)
    AND compaction = {'class': 'None'}
    AND compression = {};

Caches Virtual Table
********************
The ``caches`` virtual table lists information about the  caches. The four caches presently created are chunks, counters, keys and rows. A query on the ``caches`` virtual table returns the following details:

::

 cqlsh:system_views> SELECT * FROM system_views.caches;
 name     | capacity_bytes | entry_count | hit_count | hit_ratio | recent_hit_rate_per_second | recent_request_rate_per_second | request_count | size_bytes
 ---------+----------------+-------------+-----------+-----------+----------------------------+--------------------------------+---------------+------------
   chunks |      229638144 |          29 |       166 |      0.83 |                          5 |                              6 |           200 |     475136
 counters |       26214400 |           0 |         0 |       NaN |                          0 |                              0 |             0 |          0
     keys |       52428800 |          14 |       124 |  0.873239 |                          4 |                              4 |           142 |       1248
     rows |              0 |           0 |         0 |       NaN |                          0 |                              0 |             0 |          0

 (4 rows)

Settings Virtual Table
**********************
The ``settings`` table  is rather useful and lists all the current configuration settings from the ``cassandra.yaml``.  The encryption options are overridden to hide the sensitive truststore information or passwords.  The configuration settings however cannot be set using DML  on the virtual table presently:
::

 cqlsh:system_views> SELECT * FROM system_views.settings;

 name                                 | value
 -------------------------------------+--------------------
   allocate_tokens_for_keyspace       | null
   audit_logging_options_enabled      | false
   auto_snapshot                      | true
   automatic_sstable_upgrade          | false
   cluster_name                       | Test Cluster
   enable_transient_replication       | false
   hinted_handoff_enabled             | true
   hints_directory                    | /home/ec2-user/cassandra/data/hints
   incremental_backups                | false
   initial_token                      | null
                            ...
                            ...
                            ...
   rpc_address                        | localhost
   ssl_storage_port                   | 7001
   start_native_transport             | true
   storage_port                       | 7000
   stream_entire_sstables             | true
   (224 rows)


The ``settings`` table can be really useful if yaml file has been changed since startup and don't know running configuration, or to find if they have been modified via jmx/nodetool or virtual tables.


Thread Pools Virtual Table
**************************

The ``thread_pools`` table lists information about all thread pools. Thread pool information includes active tasks, active tasks limit, blocked tasks, blocked tasks all time,  completed tasks, and pending tasks. A query on the ``thread_pools`` returns following details:

::

 cqlsh:system_views> select * from system_views.thread_pools;

 name                         | active_tasks | active_tasks_limit | blocked_tasks | blocked_tasks_all_time | completed_tasks | pending_tasks
 ------------------------------+--------------+--------------------+---------------+------------------------+-----------------+---------------
             AntiEntropyStage |            0 |                  1 |             0 |                      0 |               0 |             0
         CacheCleanupExecutor |            0 |                  1 |             0 |                      0 |               0 |             0
           CompactionExecutor |            0 |                  2 |             0 |                      0 |             881 |             0
         CounterMutationStage |            0 |                 32 |             0 |                      0 |               0 |             0
                  GossipStage |            0 |                  1 |             0 |                      0 |               0 |             0
              HintsDispatcher |            0 |                  2 |             0 |                      0 |               0 |             0
        InternalResponseStage |            0 |                  2 |             0 |                      0 |               0 |             0
          MemtableFlushWriter |            0 |                  2 |             0 |                      0 |               1 |             0
            MemtablePostFlush |            0 |                  1 |             0 |                      0 |               2 |             0
        MemtableReclaimMemory |            0 |                  1 |             0 |                      0 |               1 |             0
               MigrationStage |            0 |                  1 |             0 |                      0 |               0 |             0
                    MiscStage |            0 |                  1 |             0 |                      0 |               0 |             0
                MutationStage |            0 |                 32 |             0 |                      0 |               0 |             0
    Native-Transport-Requests |            1 |                128 |             0 |                      0 |             130 |             0
       PendingRangeCalculator |            0 |                  1 |             0 |                      0 |               1 |             0
 PerDiskMemtableFlushWriter_0 |            0 |                  2 |             0 |                      0 |               1 |             0
                    ReadStage |            0 |                 32 |             0 |                      0 |              13 |             0
                  Repair-Task |            0 |         2147483647 |             0 |                      0 |               0 |             0
         RequestResponseStage |            0 |                  2 |             0 |                      0 |               0 |             0
                      Sampler |            0 |                  1 |             0 |                      0 |               0 |             0
     SecondaryIndexManagement |            0 |                  1 |             0 |                      0 |               0 |             0
           ValidationExecutor |            0 |         2147483647 |             0 |                      0 |               0 |             0
            ViewBuildExecutor |            0 |                  1 |             0 |                      0 |               0 |             0
            ViewMutationStage |            0 |                 32 |             0 |                      0 |               0 |             0

(24 rows)

Internode Inbound Messaging Virtual Table
*****************************************

The ``internode_inbound``  virtual table is for the internode inbound messaging. Initially no internode inbound messaging may get listed. In addition to the address, port, datacenter and rack information includes  corrupt frames recovered, corrupt frames unrecovered, error bytes, error count, expired bytes, expired count, processed bytes, processed count, received bytes, received count, scheduled bytes, scheduled count, throttled count, throttled nanos, using bytes, using reserve bytes. A query on the ``internode_inbound`` returns following details:

::

 cqlsh:system_views> SELECT * FROM system_views.internode_inbound;
 address | port | dc | rack | corrupt_frames_recovered | corrupt_frames_unrecovered |
 error_bytes | error_count | expired_bytes | expired_count | processed_bytes |
 processed_count | received_bytes | received_count | scheduled_bytes | scheduled_count | throttled_count | throttled_nanos | using_bytes | using_reserve_bytes
 ---------+------+----+------+--------------------------+----------------------------+-
 ----------
 (0 rows)

SSTables Tasks Virtual Table
****************************

The ``sstable_tasks`` could be used to get information about running tasks. It lists following columns:

::

  cqlsh:system_views> SELECT * FROM sstable_tasks;
  keyspace_name | table_name | task_id                              | kind       | progress | total    | unit
  ---------------+------------+--------------------------------------+------------+----------+----------+-------
         basic |      wide2 | c3909740-cdf7-11e9-a8ed-0f03de2d9ae1 | compaction | 60418761 | 70882110 | bytes
         basic |      wide2 | c7556770-cdf7-11e9-a8ed-0f03de2d9ae1 | compaction |  2995623 | 40314679 | bytes


As another example, to find how much time is remaining for SSTable tasks, use the following query:

::

  SELECT total - progress AS remaining
  FROM system_views.sstable_tasks;

Other Virtual Tables
********************

Some examples of using other virtual tables are as follows.

Find tables with most disk usage:

::

  cqlsh> SELECT * FROM disk_usage WHERE mebibytes > 1 ALLOW FILTERING;

  keyspace_name | table_name | mebibytes
  ---------------+------------+-----------
     keyspace1 |  standard1 |       288
    tlp_stress |   keyvalue |      3211

Find queries on table/s with greatest read latency:

::

  cqlsh> SELECT * FROM  local_read_latency WHERE per_second > 1 ALLOW FILTERING;

  keyspace_name | table_name | p50th_ms | p99th_ms | count    | max_ms  | per_second
  ---------------+------------+----------+----------+----------+---------+------------
    tlp_stress |   keyvalue |    0.043 |    0.152 | 49785158 | 186.563 |  11418.356


The system_virtual_schema keyspace
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``system_virtual_schema`` keyspace has three tables: ``keyspaces``,  ``columns`` and  ``tables`` for the virtual keyspace definitions, virtual table definitions, and virtual column definitions  respectively. It is used by Cassandra internally and a user would not need to access it directly.