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A multi-node service connection pool

Cueball is a library for "playing pool" -- managing a pool of connections to a multi-node service. This implementation of cueball is inspired by the original Node.js implementation of cueball that is used by many of Joyent's services and software components. The rust implementation relies on two primary traits in order to manage a set of connections across a set nodes providing a service. These are the Resolver trait and the Connection trait.


A resolver is responsible for locating all of the nodes or backends available within a logical service, obtaining their IP address and port information (or whatever is required to connect to them) and tracking them. This is normally a service discovery client of some form. An example of this would be a DNS-based Resolver implementation that uses DNS SRV records as a form of service discovery mechanism to find backends.


In cueball, a connection is not necessarily just a TCP socket. It can be anything that provides some kind of logical connection to a service, as long as it obeys a similar interface to a socket.

This is intended to allow users of the API to represent a "connection" as an application or session layer concept. For example, it could be useful to construct a pool of connections to an LDAP server that perform a bind operation (authenticate) before they are considered connected.

In addition to a Resolver and Connection implementation cueball users also provide the cueball connection pool with a function to establish a connection to the desired service. The trait bounds established by the cueball connection pool for this function are as follows:

FnMut(&Backend) -> C + Send + 'static
where C: Connection

The requirement is a function that takes a reference to a Backend from a resolver and returns some instance of a Connection.

The purpose of this function is to provide a way to capture application level configuration information required to establish a connection to a service. e.g. A database connection might require application-specific configuration such as a database name or user name in order to establish a connection.


As Backends for a service come and go the connection pool rebalances the configured number of connections (max_connections) across the available set of Backends. Rebalancing occurs when a Resolver notifies the connection pool that a new backend has been added or that an existing backend has been removed. The connection pool rebalances the connections in response to one of these events in order to maintain an even distribution of the connections among the available backends.

The connection pool uses a configurable delay when a message is received from the Resolver prior to performing the actual rebalancing. This delay is to account for situations where multiple messages might be sent by the Resolver in a very short span of time and allows the connection pool to be more efficient in rebalancing the connections. The default rebalancing delay time is 100 milliseconds.

Rebalancing can cause the connection pool to temporarily exceed the maximum number of connections configured for the pool. If the Resolver notifies the connection pool that a backend is removed, but connections for that backend are still in use the connection count may exceed the maximum until those connections are returned to the connection pool and discarded.


Decoherence in cueball is used to mean a periodic random shuffling of the order of connections in the connection pool. The goal of decoherence is to avoid undesirable patterns that could emerge in the lifetime of the connection pool. For example suppose that a service has three backends, A, B, and C and the connection pool has a maximum connection count of nine. The initial connection distribution might look as follows:

1 2 3 4 5 6 7 8 9

The cueball connection pool uses a queue internally to store the connections. Given that connections from the pool may be claimed for nonuniform periods of time it is possible that the queue could arrive at the following state from its initial state:

1 4 7 2 5 8 3 6 9

This situation is not ideal because the same backend must handle multiple consecutive requests while the other backends are idle. The ideal for cueball is to have an even distribution of work among the backends not just with respect to connection count, but also with respect to to request distribution over time. Now admittedly the above example is an extreme case and the pattern could quickly resolve itself based on the workload, but there is no guarantee that would be the case. The goal of the periodic decoherence shuffle in cueball is to disrupt these sorts of patterns that might arise and persist for an extended period.

There is one configuration option related to decoherence: decoherence_interval. The decoherence_interval represents the length of the period of the decoherence shuffle in seconds. If no value is specified for this in the ConnectionPoolOptions struct the default value is 300 seconds.


Use of cueball for connection management requires both an implementation of the Resolver trait and an implementation of the Connection trait. Implementers of the Resolver trait provide information to the connection pool about the nodes availble to provide a given service. The Connection trait defines a behavior for establishing and closing a connection to a particular service.

Here is an example that uses a hypothetical Resolver and Connection implementation to create a cueball connection pool.

use std::thread;
use std::net::{IpAddr, Ipv4Addr, SocketAddr};
use std::sync::{Arc, Barrier, Mutex};
use std::sync::mpsc::Sender;
use std::{thread, time};

use slog::{Drain, Logger, info, o};

use cueball::backend;
use cueball::backend::{Backend, BackendAddress, BackendPort};
use cueball::connection::Connection;
use cueball::connection_pool::ConnectionPool;
use cueball::connection_pool::types::ConnectionPoolOptions;
use cueball::error::Error;
use cueball::resolver::{BackendAddedMsg, BackendMsg, Resolver};

fn main() {
    let plain = slog_term::PlainSyncDecorator::new(std::io::stdout());
    let log = Logger::root(
        o!("build-id" => "0.1.0")

    let be1 = (IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 55555);
    let be2 = (IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 55556);
    let be3 = (IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 55557);

    let resolver = FakeResolver::new(vec![be1, be2, be3]);

    let pool_opts = ConnectionPoolOptions::<FakeResolver> {
        max_connections: 15,
        claim_timeout: Some(1000)
        resolver: resolver,
        log: log.clone(),
        decoherence_interval: None,

    let pool = ConnectionPool::<DummyConnection, FakeResolver>::new(pool_opts);

    for _ in 0..10 {
        let pool = pool.clone();
        thread::spawn(move || {
            let conn = pool.claim()?;
            // Do stuff here
            // The connection is returned to the pool when it falls out of scope.

There are several implementations of the Resolver and Connection traits that may be useful to anyone looking to get started with cueball:

Resolver trait implementer

Connection trait implementer

Minimum Supported Rust Version

The current minimum supported rust veresion is 1.39.