# Connection pooling

For each host selected by the load-balancing policy, the driver keeps a core amount of connections open at all times (`GetCoreConnectionsPerHost(HostDistance)`).

If the use of those connections reaches a configurable threshold (`GetMaxSimultaneousRequestsPerConnectionTreshold(HostDistance)`), more connections are created up to the configurable maximum number of connections (`GetMaxConnectionPerHost(HostDistance)`).

The driver uses connections in an asynchronous manner and multiple requests can be submitted on the same connection at the same time without waiting for a response. This means that the driver only needs to maintain a relatively small number of connections to each host. The [PoolingOptions](https://docs.datastax.com/en/drivers/csharp/latest/api/Cassandra.PoolingOptions.html) allows you to control how many connections are kept per host.

Each of these parameters can be separately set for `Local` and `Remote` hosts. For `Ignored` hosts, the default for all those settings is `0` and cannot be changed.

The default amount of connections depends on the Cassandra version of your cluster, because newer versions of Cassandra (2.1 and above) support a higher number of in-flight requests.

For Cassandra 2.1 and above, the default amount of connections per host is:

- Local datacenter: one core connection per host, with two connections as maximum if the simultaneous requests threshold is reached.
- Remote datacenter: one core connection per host (being one also max).

For older Cassandra versions (1.2 and 2.0), the default amount of connections per host are:

- Local datacenter: two core connection per host, with eight connections as maximum if the simultaneous requests threshold is reached.
- Remote datacenter: one core connection per host (being two the maximum).

## Simultaneous requests per connection

The driver limits the amount of concurrent requests per connection to `2048`.

When the limit is reached for all connections to a host, the driver will move to the next host according to the query plan. When the query plan is exhausted, the driver will yield a `NoHostAvailableException` containing `BusyPoolException` instances per each host in the `Errors` property.

You can use `SetMaxRequestsPerConnection()` on `PoolingOptions` to set the limit for the request rate.

## Get status of the connection pools

You can use `GetState()` extension method to get a point-in-time information of the state of the connections pools to each host.

```csharp
ISessionState state = session.GetState();
foreach (var host in state.GetConnectedHosts())
{
    Console.WriteLine($"Host '{host.Address}': " +
                      $"open connections = {state.GetOpenConnections(host)}; " +
                      $"in flight queries = {state.GetInFlightQueries(host)}");
}
```