Native protocol layer

The native protocol layer encodes protocol messages into binary, before they are sent over the network.

This part of the code lives in its own project: native-protocol. We extracted it to make it reusable (Simulacron also uses it).

The protocol specifications are available in native-protocol/src/main/resources. These files originally come from Cassandra, we copy them over for easy access. Note that, if the latest version is a beta (this is the case for v5 at the time of writing – September 2019), the specification might not be up to date. Always compare with the latest revision in cassandra/doc.

For a broad overview of how protocol types are used in the driver, let’s step through an example:

  • the user calls session.execute() with a SimpleStatement. The protocol message for a non-prepared request is QUERY;
  • CqlRequestHandler uses Conversions.toMessage to convert the statement into a c.d.o.protocol.internal.request.Query;
  • InflightHandler.write assigns a stream id to that message, and wraps it into a c.d.o.protocol.internal.Frame;
  • FrameEncoder uses c.d.o.protocol.internal.FrameCodec to convert the frame to binary.

(All types prefixed with c.d.o.protocol.internal belong to the native-protocol project.)

A similar process happens on the response path: decode the incoming binary payload into a protocol message, then convert the message into higher-level driver objects: ResultSet, ExecutionInfo, etc.

Native protocol types


Every protocol message is identified by an opcode, and has a corresponding Message subclass.

A Frame wraps a message to add metadata, such as the protocol version and stream id.

Text Diagram

All value classes are immutable, but for efficiency they don’t make defensive copies of their fields. If these fields are mutable (for example collections), they shouldn’t be modified after creating a message instance.

The code makes very few assumptions about how the messages will be used. Data is often represented in the most simple way. For example, ProtocolConstants uses simple integer constants to represent protocol codes (enums wouldn’t work at that level, because we need to add new codes in the DSE driver); the driver generally rewraps them in more type-safe structures before exposing them to higher-level layers.


For every message, there is a corresponding Message.Codec for encoding and decoding. A FrameCodec relies on a set of message codecs, for one or more protocol versions. Given an incoming frame, it looks up the right message codec to use, based on the protocol version and opcode. Optionally, it compresses frame bodies with a Compressor.

Text Diagram

Most of the time, you’ll want to use the full set of message codecs for a given protocol version. CodecGroup provides a convenient way to register multiple codecs at once. The project provides default implementations for all supported protocol version, both for clients like the driver (e.g. encode QUERY, decode RESULT), or servers like Simulacron (decode QUERY encode RESULT).

Text Diagram

The native protocol layer is agnostic to the actual binary representation. In the driver, this happens to be a Netty ByteBuf, but the encoding logic doesn’t need to be aware of that. This is expressed by the type parameter B in FrameCodec<B>. PrimitiveCodec<B> abstracts the basic primitives to work with a B: how to create an instance, read and write data to it, etc.

public interface PrimitiveCodec<B> {
  B allocate(int size);
  int readInt(B source);
  void writeInt(int i, B dest);

Everything else builds upon those primitives. By just switching the PrimitiveCodec implementation, the whole protocol layer could be reused with a different type, such as byte[].

In summary, to initialize a FrameCodec, you need:

  • a PrimitiveCodec;
  • a Compressor (optional);
  • one or more CodecGroups.

Integration in the driver

The driver initializes its FrameCodec in DefaultDriverContext.buildFrameCodec().

  • the primitive codec is ByteBufPrimitiveCodec, which implements the basic primitives for Netty’s ByteBuf;
  • the compressor comes from DefaultDriverContext.buildCompressor(), which determines the implementation from the configuration;
  • it is built with FrameCodec.defaultClient, which is a shortcut to use the default client groups: ProtocolV3ClientCodecs, ProtocolV4ClientCodecs and ProtocolV5ClientCodecs.

Extension points

The default frame codec can be replaced by extending the context to override buildFrameCodec. This can be used to add or remove a protocol version, or replace a particular codec.

If protocol versions change, ProtocolVersionRegistry will likely be affected as well.

Also, depending on the nature of the protocol changes, the driver’s request processors might require some adjustments: either replace them, or introduce separate ones (possibly with new executeXxx() methods on a custom session interface).