Virtual Tables

Apache Cassandra 4.0 implements virtual tables (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.