Coaxial

Socket handling for Scala

Coaxial provides a generic framework for client/server communications over a network, abstracting over protocol differences, including statefulness. Implementations are provided for sending and receiving UDP packets, and connecting and serving TCP connections.

Features

• provides an abstract framework for data transmission through sockets
• supports both stateful (e.g. TCP) and stateless (e.g. UDP) connections
• server and client implementations for UDP, TCP and UNIX domain sockets
• allows servers and clients to process connections as state machines
• uses a safe, functional API

Availability

Getting Started

All terms and types are defined in the coaxial package, which can be imported with:

import coaxial.*

Transmitting data

In general, a client connects to a listing server by calling either transmit (for stateless transmission) or connect (to start a stateful connection) on an endpoint. The endpoint type determines the transmission protocol.

Endpoint types include,

• local UDP ports, e.g. udp"3480"
• local TCP ports, e.g. tcp"2205"
• remote ports, e.g. host"example.com" on tcp"8844" or ip"192.168.1.24" on udp"1280"
• UNIX domain sockets
• any type for which an Addressable or Connectable typeclass instance exists

For example, a simple message could be sent to a local UDP port with,

udp"7327".transmit(t"Hello world!")

This will send a single UDP packet containing the bytes of the string Hello world!, and nothing further. This is useful for "fire-and-forget" messaging.

The payload of the UDP packet is a Text string, but can be any type for which a Transmissible typeclass instance exists, which includes Text, Bytes and anything that has a Spectacular Encoder.

All Addressable endpoints can have data transmitted to them, but some, such as TCP ports, can initiate a bidirectional connection. These are those endpoints for which a Connectable typeclass instance exists. Connectable is a subtype of Addressable.

Starting a stateful connection, with the connect method, requires a bit more work. Its state is user-defined, and can be represented by any type. So a connection requires the following:

• the initial state
• the type of the state, if it can't be inferred from the initial state
• the initial message, in a type wich is Transmissible
• a lambda to interact with the server, interpreting responses, modifying state, and sending further messages

Of these, the lambda is where most of the work is done. It will be executed sequentially for every chunk of data, as Bytes, received from the server, and must choose how to process that data.

The return type of the lambda must be one of the following Control values:

• Reply(message), to respond to the server with a new message on the current connection
• Continue(state), to update the state to a new value, and wait for more data from the server
• Terminate, to abort the connection
• Conclude(message, state), to respond to the server with a final message, and update the state to a new value

Within the body of the lambda, the named contextual value, state, provides the current state.

The return type of connect will be the final state value.

Here is an trivial example:

(host"remote.com" on tcp"1920").connect(t"")(t"DATA\n"): bytes =>
  val text = bytes.as[Text]
  val state2 = state+text
  if state2.length < 100 then Control.Continue(state2)
  else Control.Conclude(t"END\n", state2)

This will connect to TCP port 1920 on host remote.com, sending the initial request message, DATA. When the server (hopefully) responds with some textual data, we convert the bytes to text and append them to the existing state, accessed with the named contextual value, state. If we have less than 100 characters of text, then we continue waiting, updating the state to the new value. But if we have already received more than 100 characters, we conclude the connection by sending a final message, END, to the server, updating the state to its final value. The return value will be at least 100 characters of text.

It should be clear from this example that more complex connection handlers are possible, and can be written in a style that's similar to folding over a stream.

The functional interface of connect, whose lambda requires a Control value to be specified means that it's impossible to forget how to handle the input; the programmer must decide to reply, continue, conclude or terminate the connection, otherwise the code will not compile.

Implementing a Server

Writing a server can be simpler than sending a client request. The listen method is available on any type with a Bindable typeclass instance, but this notably includes:

• UNIX domain sockets
• TCP ports
• UDP ports

We can write a server by calling the listen method on a Bindable type, and specifying, by means of a lambda, how a request should be handled.

Calling listen will start a new server in a separate thread. Any incoming request will dispatch it to the handler lambda. The type of the input will depend on the type of the local endpoint. So a stateful connection will provide an input type which can support an interactive session, whereas a stateless connection will provide just the payload.

The return value of the lambda will also be determined from the endpoint type, as appropriate for stateful and stateless connections.

As an example, a simple UDP server takes input as a UdpPacket and returns a UdpResponse. UdpPacket is a case class with fields: data, the payload bytes; sender, an Ipv4 or Ipv6 source address; and port, the remote UDP port.

UdpResponse is an enumeration, which is either Ignore (if no response is expected to the UDP packet), or Reply(payload) with the bytes to respond with.

So a UDP server which responds to PING messages with PONG, but ignores other messages could be implemented with just:

val server = udp"1722".listen: packet =>
  if packet.as[Text] == t"PING" then Reply(t"PONG") else Ignore

The return value, server, is a SocketService representing the server running in the background, and will be returned as soon as the port has been bound. It may be stopped at any time by calling its stop method, like so:

server.stop()

Status

Coaxial is classified as embryotic. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Coaxial is designed to be small. Its entire source code currently consists of 350 lines of code.

Building

Coaxial will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Coaxial?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Coaxial's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Coaxial and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of coaxial. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Coaxial's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributing

Contributors to Coaxial are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Coaxial easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Author

Coaxial was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Name

Coaxial cable is designed for the transmission of high-frequency data with minimal loss, alluding to the function of transmission of data that this library provides.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

Logo

The logo shows a cross-section of a coaxial connector.

License

Coaxial is copyright © 2025 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.