Connection pooling in Cassandra drivers

Drivers maintain a pool of connections to each node selected by the load balancing policy. By default, the driver instance creates one connection to each of the local datacenter hosts in the default load balancing policies.

Connection pools are accessed asynchronously, and multiple requests can be submitted on a single connection simultaneously. For most Hyper-Converged Database (HCD) workloads, DataStax recommends one long-lived connection from the driver to each host server (HCD cluster/database).

You can use your driver’s default connection pool settings, or you can customize connection pool settings such as the number of connections per host and the maximum number of concurrent requests per connection. Available settings vary by driver, and these settings can be constrained by underlying CQL/Cassandra limitations. For example, the binary protocol (Cassandra Native Protocol) allows no more than 32,768 concurrent requests per connection.

Connection pools and initial contact points

Connection pooling is separate from the initial contact points. Initial contact points are supplied to the driver’s root object (typically a cluster), and those contact points are used to establish the control connection required to discover the cluster topology only.

The driver discovers all the nodes after a successful connection to the cluster. If no contact points are available, then the connection fails and the root object cannot be created.

When you create a root object to connect to HCD, DataStax recommends that you specify multiple contact points from your cluster. This improves resilience and minimizes the chance of connection failures due to unavailable contact points. If your cluster is deployed to multiple availability zones, specify contact points from different zones.

Configure connection pools

C/C++ driver

Use CassCluster options, such as cass_cluster_set_core_connections_per_host and cass_cluster_set_max_concurrent_requests_threshold.

C# driver

See Connection pooling.

Go driver

Use PoolConfig.

Java driver

See the documentation for your version of the Java driver:

Node.js driver

See the documentation for your version of the Node.js driver:

Python driver

For the Python driver, you can configure hosts and connection pools with cassandra.pool. However, you cannot change the number of connections per host due to single thread/Global Interpreter Lock (GIL) in Python. For more information, see the documentation for your version of the Python driver:

Inspect connected nodes

You can check which nodes your driver sessions connect to over time.

Logging node metadata can help with application development requirements like performance monitoring, troubleshooting, and cost optimization. For implementation details and more information, see the documentation for your driver:

C/C++ driver

See Host State Changes and CassSession.

C# driver

See Retrieving metadata.

Go driver

Each instance of CqlSession maintains an internal connection pool to many Cassandra nodes from the cluster. However, the GoCQL driver doesn’t provide a direct way to get information about the nodes a client is connected to at a given time.

Java driver

Each instance of CqlSession maintains an internal connection pool to many Cassandra nodes from the cluster.

You can use session.getMetadata().getNodes() to get information about the specific nodes that a given client is connected to at a certain point in time:

Example: Capture node metadata logs

import org.apache.commons.lang3.StringUtils;
private static String prettyPrintConnectedNodes(CqlSession session) {
    StringBuilder message = new StringBuilder();
    message.append("Connected Cassandra nodes:\n");
    message.append("Endpoint                       | Host ID                              | Location                  | State        | Connections\n");
    message.append("-------------------------------+--------------------------------------+---------------------------+--------------+------------\n");
    session.getMetadata().getNodes().forEach((hostId, node) -> {
        message
            .append(printColumn(node.getEndPoint().toString(), 30)).append(" | ")
            .append(printColumn(node.getHostId(), 36)).append(" | ")
            .append(printColumn(node.getDatacenter() + (node.getRack() != null ? " (" + node.getRack() + ")" : ""), 25)).append(" | ")
            .append(printColumn(node.getState(), 12)).append(" | ")
            .append(printColumn(node.getOpenConnections() + (node.isReconnecting() ? " (reconn)" : ""), 12))
            .append("\n");
    });
    return message.toString();
}
private static String printColumn(Object value, int length) {
    return StringUtils.rightPad(StringUtils.abbreviate(value != null ? value.toString() : "NULL", length), length);
}

Example: Node metadata output

Connected Cassandra nodes:
Endpoint       | Host ID                               | Location           | State | Connections
---------------+---------------------------------------+--------------------+-------+--
/127.0.0.1:9044 | eaadabc5-dc40-4b81-8230-436901eba91d | datacenter1 (rack1) | UP   | 2
/127.0.0.1:9043 | 109d8339-54f3-4fd7-9706-8dd746800136 | datacenter1 (rack1) | UP   | 1
/127.0.0.1:9045 | 04b4f617-b5e3-47dc-8f04-117ea63c2e57 | datacenter1 (rack1) | UP   | 1

For more information, see the documentation for your version of the Java driver:

Node.js driver

See the documentation for your version of the Node.js driver:

Python driver

Each instance of Session maintains an internal connection pool to many Cassandra nodes from the cluster.

In cassandra.pool, the Host class provides attributes that you can use to get information about the nodes a client is connected to at a given time. For example, the following code snippet logs information like host_id, endpoint, datacenter, and rack:

def log_connected_node(session: cluster.Session):
    log = logging.getLogger(__name__)
    row_format = "{:>30}|{:>40}|{:>25}|{:>12}"
    log.debug(row_format.format('Endpoint', 'Host ID', 'Location', 'State'))
    log.debug(row_format.format('-' * 30, '-' * 40, '-' * 25, '-' * 12))
    for host in session.hosts:
        log.debug(row_format.format(str(host.endpoint),
                                    str(host.host_id),
                                    host.datacenter + '(' + host.rack + ')',
                                    "UP" if host.is_up else "DOWN"))

Connection pooling in Cassandra drivers

Connection pools and initial contact points

Configure connection pools

Inspect connected nodes

See also

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