Clojure’s core.async is a library that enables Clojure developers to build async programs. In this post we briefly introduce the principles behind core.async, namely CSP.


Clojure’s core.async is based on the concept of Communicating Sequential Processes (CSP), an idea first presented by Robert Hoare in 1978. The impetus behind CSP was that processes need a way to communicate and synchronize with each other, and doing so via shared memory and locks is error-prone. (Besides CSP, actors are the other popular paradigm to address this problem.)

CSP instead suggests that processes communicate with each other via a mechanism called a channel. Processes can put a message onto a channel or they can take messages from a channel. In its simplest form, the putting process waits until the receiving process is ready to take, and vice versa. There can be multiple writers and readers on a channel, and neither knows about the other.

Another idea from CSP is the ability to choose from a variety of alternate inputs, or outputs. A thread can take the first value from multiple channels, whichever is ready first (or, a channel can put to one of many channels, whichever is ready to receive first).

Clojure’s core.async builds on these general ideas, with a few nuances. First, the processes in reality are threads, or an even lighter-weight construct that generally imitates the idea of a thread (referred to as go blocks, or IOC threads).

The other notable feature is that Clojure enables channel buffering, with the requirement that the buffers be bounded.


Below is a simple example that utilizes the following constructs from core.async:

  • (chan) returns an unbuffered channel
  • (>!! ch message) puts a given message on a channel. Blocks until a receiver is ready
  • (<!! ch) takes a channel and returns the next message from the channel. Blocks until a message is delivered to the channel
  • (thread & body) takes a body and executes the body on a separate thread

Simple put-receive

(defn- put-task [c]
  (Thread/sleep 1000)
  (println "Putting message on channel")
  (>!! c "test message"))

(defn example []
  (let [c (chan)]
    (thread (put-task c))
    (println "Started thread")
    (->> (<!! c)
      (str "Received message from channel: ")
    (close! c)))

user=> (example)
"Started thread"
"Putting message on channel"
"Received message from channel: test message"