12.2. 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:

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.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 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.

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.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-timeout Retry timeout for NoHostAvailableException (defaults to 1m).
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):

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;

 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

  • 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 a partition key as a 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.
  • Non-equality predicates on clustering keys are not pushed down (only = and IN are pushed down) .