07e70e4 Jan 25, 2016
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;; This walkthrough introduces the core concepts of core.async.
;; The clojure.core.async namespace contains the public API.
(require '[clojure.core.async :as async :refer :all])
;; Data is transmitted on queue-like channels. By default channels
;; are unbuffered (0-length) - they require producer and consumer to
;; rendezvous for the transfer of a value through the channel.
;; Use `chan` to make an unbuffered channel:
;; Pass a number to create a channel with a fixed buffer:
(chan 10)
;; `close!` a channel to stop accepting puts. Remaining values are still
;; available to take. Drained channels return nil on take. Nils may
;; not be sent over a channel explicitly!
(let [c (chan)]
(close! c))
;; In ordinary threads, we use `>!!` (blocking put) and `<!!`
;; (blocking take) to communicate via channels.
(let [c (chan 10)]
(>!! c "hello")
(assert (= "hello" (<!! c)))
(close! c))
;; Because these are blocking calls, if we try to put on an
;; unbuffered channel, we will block the main thread. We can use
;; `thread` (like `future`) to execute a body in a pool thread and
;; return a channel with the result. Here we launch a background task
;; to put "hello" on a channel, then read that value in the current thread.
(let [c (chan)]
(thread (>!! c "hello"))
(assert (= "hello" (<!! c)))
(close! c))
;; The `go` macro asynchronously executes its body in a special pool
;; of threads. Channel operations that would block will pause
;; execution instead, blocking no threads. This mechanism encapsulates
;; the inversion of control that is external in event/callback
;; systems. Inside `go` blocks, we use `>!` (put) and `<!` (take).
;; Here we convert our prior channel example to use go blocks:
(let [c (chan)]
(go (>! c "hello"))
(assert (= "hello" (<!! (go (<! c)))))
(close! c))
;; Instead of the explicit thread and blocking call, we use a go block
;; for the producer. The consumer uses a go block to take, then
;; returns a result channel, from which we do a blocking take.
;;;; ALTS
;; One killer feature for channels over queues is the ability to wait
;; on many channels at the same time (like a socket select). This is
;; done with `alts!!` (ordinary threads) or `alts!` in go blocks.
;; We can create a background thread with alts that combines inputs on
;; either of two channels. `alts!!` takes a set of operations
;; to perform - either a channel to take from or a [channel value] to put
;; and returns the value (nil for put) and channel that succeeded:
(let [c1 (chan)
c2 (chan)]
(thread (while true
(let [[v ch] (alts!! [c1 c2])]
(println "Read" v "from" ch))))
(>!! c1 "hi")
(>!! c2 "there"))
;; Prints (on stdout, possibly not visible at your repl):
;; Read hi from #<ManyToManyChannel ...>
;; Read there from #<ManyToManyChannel ...>
;; We can use alts! to do the same thing with go blocks:
(let [c1 (chan)
c2 (chan)]
(go (while true
(let [[v ch] (alts! [c1 c2])]
(println "Read" v "from" ch))))
(go (>! c1 "hi"))
(go (>! c2 "there")))
;; Since go blocks are lightweight processes not bound to threads, we
;; can have LOTS of them! Here we create 1000 go blocks that say hi on
;; 1000 channels. We use alts!! to read them as they're ready.
(let [n 1000
cs (repeatedly n chan)
begin (System/currentTimeMillis)]
(doseq [c cs] (go (>! c "hi")))
(dotimes [i n]
(let [[v c] (alts!! cs)]
(assert (= "hi" v))))
(println "Read" n "msgs in" (- (System/currentTimeMillis) begin) "ms"))
;; `timeout` creates a channel that waits for a specified ms, then closes:
(let [t (timeout 100)
begin (System/currentTimeMillis)]
(<!! t)
(println "Waited" (- (System/currentTimeMillis) begin)))
;; We can combine timeout with `alts!` to do timed channel waits.
;; Here we wait for 100 ms for a value to arrive on the channel, then
;; give up:
(let [c (chan)
begin (System/currentTimeMillis)]
(alts!! [c (timeout 100)])
(println "Gave up after" (- (System/currentTimeMillis) begin)))
;; ALT
;; todo
;; Channels can also use custom buffers that have different policies
;; for the "full" case. Two useful examples are provided in the API.
;; Use `dropping-buffer` to drop newest values when the buffer is full:
(chan (dropping-buffer 10))
;; Use `sliding-buffer` to drop oldest values when the buffer is full:
(chan (sliding-buffer 10))