Connection pooling


The driver communicates with Cassandra over TCP, using the Cassandra binary protocol. This protocol is asynchronous, which allows each TCP connection to handle multiple simultaneous requests:

  • when a query gets executed, a stream id gets assigned to it. It is a unique identifier on the current connection;
  • the driver writes a request containing the stream id and the query on the connection, and then proceeds without waiting for the response (if you’re using the asynchronous API, this is when the driver will send you back a ResultSetFuture). Once the request has been written to the connection, we say that it is in flight;
  • at some point, Cassandra will send back a response on the connection. This response also contains the stream id, which allows the driver to trigger a callback that will complete the corresponding query (this is the point where your ResultSetFuture will get completed).

You don’t need to manage connections yourself. You simply interact with a Session object, which takes care of it.

For each Session, there is one connection pool per connected host (a host is connected when it is up and not ignored by the load balancing policy).

The number of connections per pool is configurable (this will be described in the next section). The number of stream ids depends on the native protocol version:

  • protocol v2 or below: 128 stream ids per connection.
  • protocol v3 or above: up to 32768 stream ids per connection.
Text Diagram

Configuring the connection pool

Connections pools are configured with a PoolingOptions object, which is global to a Cluster instance. You can pass that object when building the cluster:

PoolingOptions poolingOptions = new PoolingOptions();
// customize options...

Cluster cluster = Cluster.builder()

Most options can also be changed at runtime. If you don’t have a reference to the PoolingOptions instance, here’s how you can get it:

PoolingOptions poolingOptions = cluster.getConfiguration().getPoolingOptions();
// customize options...

Pool size

Connection pools have a variable size, which gets adjusted automatically depending on the current load. There will always be at least a core number of connections, and at most a max number. These values can be configured independently by host distance (the distance is determined by your LoadBalancingPolicy, and will generally indicate whether a host is in the same datacenter or not).

    .setCoreConnectionsPerHost(HostDistance.LOCAL,  4)
    .setMaxConnectionsPerHost( HostDistance.LOCAL, 10)
    .setCoreConnectionsPerHost(HostDistance.REMOTE, 2)
    .setMaxConnectionsPerHost( HostDistance.REMOTE, 4);

For convenience, core and max can be set simultaneously:

    .setConnectionsPerHost(HostDistance.LOCAL,  4, 10)
    .setConnectionsPerHost(HostDistance.REMOTE, 2, 4);

The default settings are:

  • protocol v2:
    • LOCAL hosts: core = 2, max = 8
    • REMOTE hosts: core = 1, max = 2
  • protocol v3:
    • LOCAL hosts: core = max = 1
    • REMOTE hosts: core = max = 1

PoolingOptions.setNewConnectionThreshold determines the threshold that triggers the creation of a new connection when the pool is not at its maximum capacity. In general, you shouldn’t need to change its default value.

Dynamic resizing

If core != max, the pool will resize automatically to adjust to the current activity on the host.

When activity goes up and there are n connections with n < max, the driver will add a connection when the number of concurrent requests is more than (n - 1) * 128 + PoolingOptions.setNewConnectionThreshold (in layman’s terms, when all but the last connection are full and the last connection is above the threshold).

When activity goes down, the driver will “trash” connections if the maximum number of requests in a 10 second time period can be satisfied by less than the number of connections opened. Trashed connections are kept open but do not accept new requests. After a given timeout (defined by PoolingOptions.setIdleTimeoutSeconds), trashed connections are closed and removed. If during that idle period activity increases again, those connections will be resurrected back into the active pool and reused. The main intent of that is to not constantly recreate connections if activity changes quickly over an interval.

Simultaneous requests per connection

PoolingOptions.setMaxRequestsPerConnection allows you to throttle the number of concurrent requests per connection.

With protocol v2, there is no reason to throttle. It is set to 128 (the max) and you should not change it.

With protocol v3, it is set to 1024 for LOCAL hosts, and 256 for REMOTE hosts. These low defaults were chosen so that the default configuration for protocol v2 and v3 allow the same total number of simultaneous requests (to avoid bad surprises when clients migrate from v2 to v3). You can raise this threshold, or even set it to the max:

    .setMaxRequestsPerConnection(HostDistance.LOCAL, 32768)
    .setMaxRequestsPerConnection(HostDistance.REMOTE, 2000);

Just keep in mind that high values will give clients more bandwidth and therefore put more pressure on your cluster. This might require some tuning, especially if you have many clients.


If connections stay idle for too long, they might be dropped by intermediate network devices (routers, firewalls…). Normally, TCP keepalive should take care of this; but tweaking low-level keepalive settings might be impractical in some environments.

The driver provides application-side keepalive in the form of a connection heartbeat: when a connection has been idle for a given amount of time, the driver will simulate activity by writing a dummy request to it.

This feature is enabled by default. The default heartbeat interval is 30 seconds, it can be customized with the following method:


If it gets changed at runtime, only connections created after that will use the new interval. Most users will want to do this at startup.

The heartbeat interval should be set higher than SocketOptions.readTimeoutMillis: the read timeout is the maximum time that the driver waits for a regular query to complete, therefore the connection should not be considered idle before it has elapsed.

To disable heartbeat, set the interval to 0.

Implementation note: the dummy request sent by heartbeat is an OPTIONS message.

Acquisition timeout

When the driver tries to send a request to a host, it will first try to acquire a connection from this host’s pool. If the pool is busy (i.e. all connections are already handling their maximum number of in flight requests), the client thread will block for a while, until a connection becomes available (note that this will block even if you’re using the asynchronous API, like Session.executeAsync).

The time that the driver blocks is controlled by PoolingOptions.setPoolTimeoutMillis. If there is still no connection available after this timeout, the driver will try the next host.

For some applications, blocking is not acceptable, and it is preferable to fail fast if the request cannot be fulfilled. If that’s your case, set the pool timeout to 0. If all hosts are busy, you will get a NoHostAvailableException (if you look at the exception’s details, you will see a java.util.concurrent.TimeoutException for each host).

Monitoring and tuning the pool

The easiest way to monitor pool usage is with Session.getState. Here’s a simple example that will print the number of open connections, active requests, and maximum capacity for each host, every 5 seconds:

final LoadBalancingPolicy loadBalancingPolicy =
final PoolingOptions poolingOptions =

ScheduledExecutorService scheduled =
scheduled.scheduleAtFixedRate(new Runnable() {
    public void run() {
        Session.State state = session.getState();
        for (Host host : state.getConnectedHosts()) {
            HostDistance distance = loadBalancingPolicy.distance(host);
            int connections = state.getOpenConnections(host);
            int inFlightQueries = state.getInFlightQueries(host);
            System.out.printf("%s connections=%d, current load=%d, max
                host, connections, inFlightQueries,
                connections *
}, 5, 5, TimeUnit.SECONDS);

In real life, you’ll probably want something more sophisticated, like exposing a JMX MBean or sending the data to your favorite monitoring tool.

If you find that the current load stays close or equal to the maximum load at all time, it’s a sign that your connection pools are saturated and you should raise the max connections per host, or max requests per connection (protocol v3).

If you’re using protocol v2 and the load is often less than core * 128, your pools are underused and you could get away with less core connections.

Tuning protocol v3 for very high throughputs

As mentioned above, the default pool size for protocol v3 is core = max = 1. This means all requests to a given node will share a single connection, and therefore a single Netty I/O thread.

There is a corner case where this I/O thread can max out its CPU core and become a bottleneck in the driver; in our benchmarks, this happened with a single-node cluster and a high throughput (approximately 80K requests / second).

It’s unlikely that you’ll run into this issue: in most real-world deployments, the driver connects to more than one node, so the load will spread across more I/O threads. However if you suspect that you experience the issue, here’s what to look out for:

  • the driver throughput plateaus but the process does not appear to max out any system resource (in particular, overall CPU usage is well below 100%);
  • one of the driver’s I/O threads maxes out its CPU core. You can see that with a profiler, or OS-level tools like pidstat -tu on Linux. I/O threads are called <cluster_name>-nio-worker-<n>, unless you’re injecting your own EventLoopGroup with NettyOptions.

The solution is to add more connections per node. To ensure that additional connections get created before you run into the bottleneck, either: