=================== Cassandra Connector =================== The Cassandra connector allows querying data stored in Cassandra. Compatibility ------------- Connector is compatible with all Cassandra versions starting from 2.1.5. Configuration ------------- To configure the Cassandra connector, create a catalog properties file ``~/.prestoadmin/catalog/cassandra.properties`` with the following contents, replacing ``host1,host2`` with a comma-separated list of the Cassandra nodes used to discovery the cluster topology: .. code-block:: none connector.name=cassandra cassandra.contact-points=host1,host2 You will also need to set ``cassandra.native-protocol-port`` if your Cassandra nodes are not using the default port (9042). Multiple Cassandra Clusters ^^^^^^^^^^^^^^^^^^^^^^^^^^^ You can have as many catalogs as you need, so if you have additional Cassandra clusters, simply add another properties file to ``~/.prestoadmin/catalog`` with a different name (making sure it ends in ``.properties``). For example, if you name the property file ``sales.properties``, Presto will create a catalog named ``sales`` using the configured connector. Configuration Properties ------------------------ The following configuration properties are available: ================================================== ====================================================================== Property Name Description ================================================== ====================================================================== ``cassandra.contact-points`` Comma-separated list of hosts in a Cassandra cluster. The Cassandra driver will use these contact points to discover cluster topology. At least one Cassandra host is required. ``cassandra.native-protocol-port`` The Cassandra server port running the native client protocol (defaults to ``9042``). ``cassandra.limit-for-partition-key-select`` Limit of rows to read for finding all partition keys. If a Cassandra table has more rows than this value, splits based on token ranges are used instead. Note that for larger values you may need to adjust read timeout for Cassandra. ``cassandra.max-schema-refresh-threads`` Maximum number of schema cache refresh threads. This property corresponds to the maximum number of parallel requests. ``cassandra.schema-cache-ttl`` Maximum time that information about a schema will be cached (defaults to ``1h``). ``cassandra.schema-refresh-interval`` The schema information cache will be refreshed in the background when accessed if the cached data is at least this old (defaults to ``2m``). ``cassandra.consistency-level`` Consistency levels in Cassandra refer to the level of consistency to be used for both read and write operations. More information about consistency levels can be found in the `Cassandra consistency`_ documentation. This property defaults to a consistency level of ``ONE``. Possible values include ``ALL``, ``EACH_QUORUM``, ``QUORUM``, ``LOCAL_QUORUM``, ``ONE``, ``TWO``, ``THREE``, ``LOCAL_ONE``, ``ANY``, ``SERIAL``, ``LOCAL_SERIAL``. ``cassandra.allow-drop-table`` Set to ``true`` to allow dropping Cassandra tables from Presto via :doc:`/sql/drop-table` (defaults to ``false``). ``cassandra.username`` Username used for authentication to the Cassandra cluster. This is a global setting used for all connections, regardless of the user who is connected to Presto. ``cassandra.password`` Password used for authentication to the Cassandra cluster. This is a global setting used for all connections, regardless of the user who is connected to Presto. ================================================== ====================================================================== .. note:: If authorization is enabled, ``cassandra.username`` must have enough permissions to perform ``SELECT`` queries on the ``system.size_estimates`` table. .. _Cassandra consistency: http://www.datastax.com/documentation/cassandra/2.0/cassandra/dml/dml_config_consistency_c.html .. note:: An outdated cache may result in wrong results. For example, a primary key added after the latest refresh will not be visible in the results. If your use case entails frequent addition of data to Cassandra, consider lowering the value of ``cassandra.schema-cache-ttl`` (sometimes even to ``0s``). Be aware that this may cause a significant performance degradation. The following advanced configuration properties are available: ============================================================= ====================================================================== Property Name Description ============================================================= ====================================================================== ``cassandra.fetch-size`` Number of rows fetched at a time in a Cassandra query. ``cassandra.fetch-size-for-partition-key-select`` Number of rows fetched at a time in a Cassandra query that selects partition keys. ``cassandra.partition-size-for-batch-select`` Number of partitions batched together into a single select for a single partion key column table. ``cassandra.split-size`` Number of keys per split when querying Cassandra. ``cassandra.client.read-timeout`` Maximum time the Cassandra driver will wait for an answer to a query from one Cassandra node. Note that the underlying Cassandra driver may retry a query against more than one node in the event of a read timeout. Increasing this may help with queries that use an index. ``cassandra.client.connect-timeout`` Maximum time the Cassandra driver will wait to establish a connection to a Cassandra node. Increasing this may help with heavily loaded Cassandra clusters. ``cassandra.client.so-linger`` Number of seconds to linger on close if unsent data is queued. If set to zero, the socket will be closed immediately. When this option is non-zero, a socket will linger that many seconds for an acknowledgement that all data was written to a peer. This option can be used to avoid consuming sockets on a Cassandra server by immediately closing connections when they are no longer needed. ``cassandra.retry-policy`` Policy used to retry failed requests to Cassandra. This property defaults to ``DEFAULT``. Using ``BACKOFF`` may help when queries fail with *"not enough replicas"*. The other possible values are ``DOWNGRADING_CONSISTENCY`` and ``FALLTHROUGH``. ``cassandra.load-policy.use-dc-aware`` Set to ``true`` to use ``DCAwareRoundRobinPolicy`` (defaults to ``false``). ``cassandra.load-policy.dc-aware.local-dc`` The name of the local datacenter for ``DCAwareRoundRobinPolicy``. ``cassandra.load-policy.dc-aware.used-hosts-per-remote-dc`` Uses the provided number of host per remote datacenter as failover for the local hosts for ``DCAwareRoundRobinPolicy``. ``cassandra.load-policy.dc-aware.allow-remote-dc-for-local`` Set to ``true`` to allow to use hosts of remote datacenter for local consistency level. ``cassandra.load-policy.use-token-aware`` Set to ``true`` to use ``TokenAwarePolicy`` (defaults to ``false``). ``cassandra.load-policy.shuffle-replicas`` Set to ``true`` to use ``TokenAwarePolicy`` with shuffling of replicas (defaults to ``false``). ``cassandra.load-policy.use-white-list`` Set to ``true`` to use ``WhiteListPolicy`` (defaults to ``false``). ``cassandra.load-policy.white-list.addresses`` Comma-separated list of hosts for ``WhiteListPolicy``. ``cassandra.no-host-available-retry-count`` Retry count for ``NoHostAvailableException`` (defaults to ``1``). ``cassandra.speculative-execution.limit`` The number of speculative executions (defaults to ``1``). ``cassandra.speculative-execution.delay`` The delay between each speculative execution (defaults to ``500ms``). ============================================================= ====================================================================== Querying Cassandra Tables ------------------------- The ``users`` table is an example Cassandra table from the Cassandra `Getting Started`_ guide. It can be created along with the ``mykeyspace`` keyspace using Cassandra's cqlsh (CQL interactive terminal): .. _Getting Started: https://wiki.apache.org/cassandra/GettingStarted .. code-block:: none cqlsh> CREATE KEYSPACE mykeyspace ... WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 }; cqlsh> USE mykeyspace; cqlsh:mykeyspace> CREATE TABLE users ( ... user_id int PRIMARY KEY, ... fname text, ... lname text ... ); This table can be described in Presto:: DESCRIBE cassandra.mykeyspace.users; .. code-block:: none Column | Type | Extra | Comment ---------+---------+-------+--------- user_id | bigint | | fname | varchar | | lname | varchar | | (3 rows) This table can then be queried in Presto:: SELECT * FROM cassandra.mykeyspace.users; Data types ---------- The data types mappings are as follows: ================ ====== Cassandra Presto ================ ====== ASCII VARCHAR BIGINT BIGINT BLOB VARBINARY BOOLEAN BOOLEAN DECIMAL DOUBLE DOUBLE DOUBLE FLOAT DOUBLE INET VARCHAR(45) INT INTEGER LIST VARCHAR MAP VARCHAR SET VARCHAR TEXT VARCHAR TIMESTAMP TIMESTAMP TIMEUUID VARCHAR VARCHAR VARCHAR VARIANT VARCHAR ================ ====== Any collection (LIST/MAP/SET) can be designated as FROZEN, and the value is mapped to VARCHAR. Additionally, blobs have the limitation that they cannot be empty. Types not mentioned in the table above are not supported (e.g. tuple or UDT). Partition keys can only be of the following types: | ASCII | TEXT | VARCHAR | BIGINT | BOOLEAN | DOUBLE | INET | INT | FLOAT | DECIMAL | TIMESTAMP | UUID | TIMEUUID Limitations ----------- * ``cassandra.schema-refresh-interval`` cannot be set to the value ``0s``. * Queries without filters containing the partition key result in fetching all partitions. This causes a full scan of the entire data set, therefore it's much slower compared to a similar query with partition key in filter. * ``IN`` list filters are only allowed on index (that is, partition key or clustering key) columns. * Range (``<`` or ``>`` and ``BETWEEN``) filters can be applied only to the partition keys. * Clustering keys are not pushed down. * Aggregates are also not pushed down.