Getting Started

Installation

Binary packages are available (CentOS, Ubuntu, and Windows) or the driver can be built from source.

Connecting

#include <cassandra.h>
#include <stdio.h>

int main() {
  /* Setup and connect to cluster */
  CassCluster* cluster = cass_cluster_new();
  CassSession* session = cass_session_new();

  /* Add contact points */
  cass_cluster_set_contact_points(cluster, "127.0.0.1");

  /* Provide the cluster object as configuration to connect the session */
  CassFuture* connect_future = cass_session_connect(session, cluster);

  /* This operation will block until the result is ready */
  CassError rc = cass_future_error_code(connect_future);

  printf("Connect result: %s\n", cass_error_desc(rc));

  /* Run queries... */

  cass_future_free(connect_future);
  cass_session_free(session);
  cass_cluster_free(cluster);

  return 0;
}

To connect a session, a CassCluster object will need to be created and configured. The minimal configuration needed to connect is a list of contact points. The contact points are used to initialize the driver and it will automatically discover the rest of the nodes in your cluster.

Perfomance Tip: Include more than one contact point to be robust against node failures.

Futures

The driver is designed so that no operation will force an application to block. Operations that would normally cause the application to block, such as connecting to a cluster or running a query, instead return a CassFuture object that can be waited on, polled, or used to register a callback.

NOTE: The API can also be used synchronously by waiting on or immediately attempting to get the result from a future.

Executing Queries

Queries are executed using CassStatement objects. Statements encapsulate the query string and the query parameters. Query parameters are not supported by earlier versions of Cassandra (1.2 and below) and values need to be inlined in the query string itself.

/* Create a statement with zero parameters */
CassStatement* statement
  = cass_statement_new("INSERT INTO example (key, value) VALUES ('abc', 123)", 0);

CassFuture* query_future = cass_session_execute(session, statement);

/* Statement objects can be freed immediately after being executed */
cass_statement_free(statement);

/* This will block until the query has finished */
CassError rc = cass_future_error_code(query_future);

printf("Query result: %s\n", cass_error_desc(rc));

cass_future_free(query_future);

Parameterized Queries (Positional)

Cassandra 2.0+ supports the use of parameterized queries. This allows the same query string to be executed multiple times with different values; avoiding string manipulation in your application.

Perfomance Tip: If the same query is being reused multiple times, prepared statements should be used to optimize performance.

/* There are two bind variables in the query string */
CassStatement* statement
  = cass_statement_new("INSERT INTO example (key, value) VALUES (?, ?)", 2);

/* Bind the values using the indices of the bind variables */
cass_statement_bind_string(statement, 0, "abc");
cass_statement_bind_int32(statement, 1, 123);

CassFuture* query_future = cass_session_execute(session, statement);

/* Statement objects can be freed immediately after being executed */
cass_statement_free(statement);

/* This will block until the query has finished */
CassError rc = cass_future_error_code(query_future);

printf("Query result: %s\n", cass_error_desc(rc));

cass_future_free(query_future);

Handling Query Results

A single row can be retrieved using the convenience function cass_result_first_row() to get the first row. A CassIterator object may also be used to iterate over the returned row(s).

/* Execute "SELECT * FROM example (key, value) WHERE key = 'abc'" */

/* This will also block until the query returns */
const CassResult* result = cass_future_get_result(future);

/* If there was an error then the result won't be available */
if (result == NULL) {
  /* Handle error */
  cass_future_free(query_future);
  return -1;
}

/* The future can be freed immediately after getting the result object */
cass_future_free(query_future);

/* This can be used to retrieve on the first row of the result */
const CassRow* row = cass_result_first_row(result);

/* Now we can retrieve the column values from the row */
const char* key;
size_t key_length;
/* Get the column value of "key" by name */
cass_value_get_string(cass_row_get_column_by_name(row, "key"), &key, &key_length);

cass_int32_t value;
/* Get the column value of "value" by name */
cass_value_get_int32(cass_row_get_column_by_name(row, "value"), &value);


/* This will free the result as well as the string pointed to by 'key' */
cass_result_free(result);

Architecture

Cluster

The CassCluster object describes a Cassandra cluster’s configuration. The default cluster object is good for most clusters and only requires a single or multiple list of contact points in order to establish a session connection. Once a session is connected using a cluster object its configuration is constant. Modifying the cluster object configuration once a session is established does not alter the session’s configuration.

Session

The CassSession object is used for query execution. Internally, a session object also manages a pool of client connections to Cassandra and uses a load balancing policy to distribute requests across those connections. An application should create a single session object per keyspace as a session object is designed to be created once, reused, and shared by multiple threads within the application. The throughput of a session can be scaled by increasing the number of I/O threads. An I/O thread is used to handle reading and writing query request data to and from Cassandra. The number of I/O threads defaults to one per CPU core, but it can be configured using cass_cluster_set_num_threads_io(). It’s generally better to create a single session with more I/O threads than multiple sessions with a smaller number of I/O threads. More DataStax driver best practices can be found in this post.

Asynchronous I/O

Each I/O thread maintains a small number of connections for each node in the Cassandra cluster and each of those connections can handle several simultaneous requests using pipelining. Asynchronous I/O and pipelining together allow each connection to handle several (up to 128 requests with protocol v1/v2 and 32k with protocol v3/v4) in-flight requests concurrently. This significantly reduces the number of connections required to be open to Cassandra and allows the driver to batch requests destined for the same node.

Thread safety

A CassSession is designed to be used concurrently from multiple threads. CassFuture is also thread safe. Other than these exclusions, in general, functions that might modify an object’s state are NOT thread safe. Objects that are immutable (marked ‘const’) can be read safely by multiple threads.

NOTE: The object/resource free-ing functions (e.g. cass_cluster_free, cass_session_free, … cass_*_free) cannot be called concurrently on the same instance of an object.

Memory handling

Values such as strings (const char*), bytes and decimals (const cass_bytes_t*) point to memory held by the result object. The lifetimes of these values are valid as long as the result object isn’t freed. These values must be copied into application memory if they need to live longer than the result object’s lifetime. Primitive types such as cass_int32_t are copied by the driver because it can be done cheaply without incurring extra allocations.

NOTE: Advancing an iterator invalidates the value it previously returned.

TODO

Here are some features that are missing from the C/C++ driver, but are included with other drivers. The schedule for these features can be found on JIRA.

Compression
Query tracing
Event registration and notification
Callback interfaces for load balancing, authentication, reconnection and retry
Generic SASL authentication interface