TeensyP

A small, zero-dependency Clojure TCP server that uses Java NIO.

Installation

Add the following dependency to your deps.edn file:

dev.weavejester/teensyp {:mvn/version "0.5.3"}

Or to your Leiningen project file:

[dev.weavejester/teensyp "0.5.3"]

Usage

At minimum, TeensyP requires a port to listen on and a handler function:

(require '[teensyp.server :as tcp])

(tcp/start-server {:port 3000, :handler demo-handler})

Handlers

The handler function has three arities, and defines how the server behaves.

(defn example-handler
  ([socket]
   ;; The 1 argument arity is called when the socket is accepted.
   ;; The return value is the session state.
   {:read-bytes 0})
  ([state socket ^java.nio.ByteBuffer buffer]
   ;; The 3 argument arity is called when the socket receives data from
   ;; the client, contained in a ByteBuffer. The return value is the
   ;; updated session state.
   (let [remaining (.remaining buffer)]
     (.position buffer (.limit buffer))  ; fake a read
     (update state :read-bytes + remaining)))
  ([state exception]
   ;; The 2 argument arity is called when the socket is closed. If it
   ;; closed due to an exception, that exception is passed as the
   ;; second argument, otherwise it will be nil.
   (println "Total bytes:" (:read-bytes state))))

The handler maintains a session state, which can be any Clojure data structure, but is usually a map. This state is unique to the socket, and is updated each time the socket is read from by the 3-arity form of the handler.

Uncaught exceptions will close the channel and trigger the 2-arity form of the handler.

Sockets

The socket argument satisfies the teensyp.server/Socket protocol, and represents the current connection between server and client.

• teensyp.server/write - writes a buffer to the channel
• teensyp.server/close - closes the channel
• teensyp.server/pause-reads - pause reads until resumed
• teensyp.server/resume-reads - resume reads

All of these functions are asynchronous, and can be supplied with an optional zero-argument callback function that will be called once they successfully complete.

I/O errors will close the socket and trigger the 2-arity form of the handler.

Guarantees

TeensyP makes several guarantees that apply per socket:

• The 1-arity accept is always called first.
• The 2-arity close is always called last.
• The current handler call for the socket must finish before the next can begin.
• The write queue must be empty before the read arity is called.

Server Options

The start-server function takes a number of options:

Key	Description	Mandatory	Default
:executor	An ExecutorService to use for handler calls
:handler	The handler function	Yes
:port	The port number to listen on	Yes
:read-buffer-size	The size in bytes of the channel read buffer		8K
:write-buffer-size	The size in bytes of the channel write buffer		32K
:write-queue-size	The maximum number of writes that can be queued		64

Documentation

• API Documentation

License

Copyright © 2026 James Reeves

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.