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Transparent, non-invasive RPC by clojure and for clojure

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"Superman is a slacker."

slacker is a simple RPC framework for Clojure based on aleph and carbonite. I forked carbonite because slacker requires it to work on clojure 1.2.

slacker is growing.

RPC vs Remote Eval

Before slacker, the clojure world uses a remote eval approach for remoting (nREPL, portal). Comparing to remote eval, RPC (especially slacker) has some pros and cons:


  • slacker uses direct function call, which is much faster than eval (about 100x)
  • with slacker, only selected functions are exposed, instead of the whole java environment when using eval. So it's much securer and generally you don't need a sandbox (like clojail) for slacker.


  • Eval approach provides full features of clojure, you can use high-order functions and lazy arguments. Due to the limitation of serialization, slacker has its difficulty to support these features.



:dependencies [[info.sunng/slacker "0.2.0-SNAPSHOT"]]

Getting Started

Slacker will expose all your public functions under a given namespace.

(ns slapi)
(defn timestamp []

;; ...more functions

To expose slapi, use:

(use 'slacker.server)
(start-slacker-server (the-ns 'slapi) 2104)

On the client side, define a facade for the remote function:

(use 'slacker.client)
(def sc (slackerc "localhost" 2104))
(defremote sc timestamp)

Client Connection Pool

Slacker also supports connection pool in client API, which enables high concurrent communication.

To create a connection pool, use slackerc-pool instead of slackerc.

You can configure the pool with following options:

  • :max-active
  • :exhausted-action
  • :max-wait
  • :max-idle

For the meaning of each option, check the javadoc of commons-pool.

Options in defremote

You are specify the remote function name when the name is occupied in current namespace

(defremote sc remote-time
  :remote-name "timestamp")

If you add an :async flag to defremote, then the facade will be asynchronous which returns a promise when you call it. You should deref it by yourself to get the return value.

(defremote timestamp :async true)

You can also assign a callback for an async facade.

(defremote timestamp :callback #(println %))

Serializing custom types

By default, most clojure data types are registered in carbonite. (As kryo requires you to register a class before you can serialize its instances.) However, you may have additional types to transport between client and server. To add your own types, you should register custom serializers on both server side and client side. Run this before you start server or client:

(use '[slacker.serialization])
(register-serializers some-serializers)

Carbonite has some detailed docs on how to create your own serializers.

JSON Serialization

Other than binary format, you can also use JSON for serialization. JSON is a text based format which is more friendly to human beings. It may be useful for debugging, or communicating with external applications.

Configure slacker client to use JSON:

(def sc (slackerc "localhost" 2104 :content-type :json))

Turn on the debug option, you will see all the JSON data transported between client and server:

(use 'slacker.common)
(binding [slacker.common/*debug* true]
  (inc-m 100))


    dbg:: [100]
    dbg:: 700

One thing you should note is the representation of keyword in JSON. Keywords and strings are both encoded as JSON string in transport. But while decoding, all map keys will be decoded to keyword, and all other strings will be decoded to clojure string. This may lead to inconsistency of your clojure data structure between server and client. Try to avoid this by carefully design your data structure or just using carbonite(default and recommended).


Some performance tests was executed while I'm developing slacker.

A simple test client is here. With the client, as tested on HP DL360 (dual 6 core X5650, 2.66GHz), a single client (50 connections, 50 threads) performed 500000 synchronous calls in 48862 msecs (TPS is about 10232).

Some formal performance benchmark is coming soon.


Copyright (C) 2011 Sun Ning

Distributed under the Eclipse Public License, the same as Clojure.

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