Netty pipeline

With the protocol layer in place, the next step is to build the logic for a single server connection.

We use Netty for network I/O (to learn more about Netty, this book is an excellent resource).

Text Diagram

Each Cassandra connection is based on a Netty Channel. We wrap it into our own DriverChannel, that exposes higher-level operations. ChannelFactory is the entry point for other driver components; it handles protocol negotiation for the first channel.

A Netty channel has a pipeline, that contains a sequence of handlers. As a request is sent, it goes through the pipeline top to bottom; each successive handler processes the input, and passes the result to the next handler. Incoming responses go the other way.

Our pipeline is configured with the following handlers:


The implementation is provided by Netty (all the others handlers are custom implementations).

Internally, handler instances are provided by SslHandlerFactory. At the user-facing level, this is abstracted behind SslEngineFactory, based on Java’s default SSL implementation.

See also the Extension points section below.

FrameEncoder and FrameDecoder

This is where we integrate the protocol layer, as explained here.

Unlike the other pipeline stages, we use separate handlers for incoming and outgoing messages.


The heartbeat is a background request sent on inactive connections (no reads since x seconds), to make sure that they are still alive, and prevent them from being dropped by a firewall. This is similar to TCP_KeepAlive, but we provide an application-side alternative because users don’t always have full control over their network configuration.

HeartbeatHandler is based on Netty’s built-in IdleStateHandler, so there’s not much in there apart from the details of the control request.


This handler is where most of the connection logic resides. It is responsible for:

  • writing regular requests:
    • find an available stream id;
    • store the ResponseCallback provided by the client under that id;
    • when the response comes in, retrieve the callback and complete it;
  • cancelling a request;
  • switching the connection to a new keyspace (if a USE statement was executed through the session);
  • handling shutdown: gracefully (allow all request to complete), or forcefully (error out all requests).

The two most important methods are:

  • write(ChannelHandlerContext, Object, ChannelPromise): processes outgoing messages. We accept different types of messages, because cancellation and shutdown also use that path. See DriverChannel, which abstracts those details.
  • channelRead: processes incoming responses.

Netty handlers are confined to the channel’s event loop (a.k.a I/O thread). Therefore the code doesn’t have to be concurrent, fields can be non-volatile and methods are guaranteed not to race with each other.

In particular, a big difference from driver 3 is that stream ids are assigned within the event loop, instead of from client code before writing to the channel (see also connection pooling). StreamIdGenerator is not thread-safe.

All communication between the handler and the outside world must be done through messages or channel events. There are 3 exceptions to this rule: getAvailableIds, getInflight and getOrphanIds, which are based on volatile fields. They are all used for metrics, and getAvailableIds is also used to balance the load over connections to the same node (see ChannelSet).


This handler manages the protocol initialization sequence on a newly established connection (see the STARTUP message in the protocol specification).

Most of the logic resides in InitRequest.onResponse, which acts as a simple state machine based on the last request sent.

There is also a bit of custom code to ensure that the channel is not made available to clients before the protocol is ready. This is abstracted in the parent class ConnectInitHandler.

Once the initialization is complete, ProtocolInitHandler removes itself from the pipeline.

Extension points


The advanced.netty section in the configuration exposes a few high-level options.

For more elaborate customizations, you can extend the context to plug in a custom NettyOptions implementation. This allows you to do things such as:

  • reusing existing event loops;
  • using Netty’s native Epoll transport;
  • adding custom handlers to the pipeline.


The user-facing API (advanced.ssl-engine-factory in the configuration, or SessionBuilder.withSslContext / SessionBuilder.withSslEngineFactory) only supports Java’s default SSL implementation.

The driver can also work with Netty’s native integration with OpenSSL or boringssl. This requires a bit of custom development against the internal API: