Mastering Time with Clojure core.async

core.async is an exciting new library to organize and rationalize highly concurrent applications in Clojure. Presented at the Portland Java User's Group.

Transcript

1. Mastering time with Clojure core.async Howard M. Lewis Ship TWD Consulting © 2013 Howard M. Lewis Ship
2. (defn request-handler [request] (let [data1 (do-query-1 request) data2 (do-query-2 request) data3 (do-query-3 request)] (send-response data1 data2 data3)))
3. 1 3 2
4. Channel Writer Writer Writer Reader Reader
5. (>!! chan val) (<!! chan) Channel – (chan)
6. Channel Writer Writer Writer Reader Reader
8. Channel – (chan 1) (>!! chan val) (>!! chan val) (<!! chan) (<!! chan)
9. using multiple threads
10. (defn request-handler [request] (let [data1 (do-query-1 request) data2 (do-query-2 request) data3 (do-query-3 request)] (send-response data1 data2 data3)))
11. (defn request-handler [request] (let [query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) data1 (<!! query1) data2 (<!! query2) data3 (<!! query3)] (send-response data1 data2 data3))) Evaluate in other thread, return channel
12. (defn request-handler [request] (let [query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) data1 (<!! query1) data2 (<!! query2) data3 (<!! query3)] (send-response data1 data2 data3))) block thread until value available
13. (thread (do-query-1 request)) (<!! query1) Result – (chan 1) (do-query-1 request)
14. 1 3 2
15. What if request is modified before pooled thread executes? (defn request-handler [request] (let [query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) data1 (<!! query1) data2 (<!! query2) data3 (<!! query3)] (send-response data1 data2 data3)))
16. Answer: Mu Clojure data structures are immutable - free to share across threads
17. freedom from blocking
18. (defn request-handler [request] (let [result (go (let [query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) data1 (<! query1) data2 (<! query2) data3 (<! query3)] (send-response data1 data2 data3)))] (<!! result)))
19. (go (let [… data1 (<! query1) data2 (<! query2) data3 (<! query3)] (send-response data1 data2 data3))) Executes in pooled thread Reads value or parks (defn request-handler [request] (let [result (go …)] (<!! result))) Final result available through channel
20. Blocking
21. Parking
22. 1 3 2
23. (defn request-handler [request] (go (let [query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) data1 (<! query1) data2 (<! query2) data3 (<! query3)] (send-async-response request data1 data2 data3))) nil)
24. Inversion of Control query1 (thread (do-query-1 request)) query2 (thread (do-query-2 request)) query3 (thread (do-query-3 request)) 0 data1 (<! query1) 1 data2 (<! query2) 2 data3 (<! query3) (send-async-response request data1 data2 data3) 3
25. What is an API?
26. public interface RequestProcessor { Response process(Request request); … } (defprotocol RequestProcessor (process! [this request] … ) (ns gigantor.request.processor …) (defn process! [request] …) POST http://gigantor.com/request …
27. Input ➠ Process ➠ Result
28. Synchronous (let [result (process! request)] Asynchronous
29. Synchronous What parameters do I pass to the function? Asynchronous What value do I send to the channel? How & when do I get the result?
30. (defn start-request-processor [request-channel] (go (while true (let [[request response-channel] (<! request-channel) _ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response)))) nil)
31. (defn start-request-processor [request-channel] (go (while true (let [[request response-channel] (<! request-channel) _ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response)))) nil) request channel is a parameter
32. (defn start-request-processor [request-channel] (go (while true (let [[request response-channel] (<! request-channel) _ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response)))) nil) Client sends the request and a channel for the response
33. (defn start-request-processor [request-channel] (go (while true (let [[request response-channel] (<! request-channel) _ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response)))) nil) timeout is part of core.async
34. (defn start-request-processor [request-channel] (go (while true (let [[request response-channel] (<! request-channel) _ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response)))) nil) Send the response where the client specified
35. (defn process-request [request processor-channel] (let [response-channel (chan 1)] (>!! processor-channel [request response-channel]) response-channel))
36. (defn process-request [request processor-channel] (let [response-channel (chan 1)] (>!! processor-channel [request response-channel]) response-channel)) Buffer for 1 response, to keep from blocking/parking
37. (defn process-request [request processor-channel] (let [response-channel (chan 1)] (>!! processor-channel [request response-channel]) response-channel)) The API: which channel, what data
38. 0ms 1,500ms 3,000ms 4,500ms 6,000ms 7,500ms 9,000ms 10,500ms 12,000ms 13,500ms 15,000ms 1 2 5 10 25 Total Processing Time Number of processing go blocks 100 Requests 250 Requests Caution: Very Not Scientific! ⚠
39. How deep a buffer on the channel? How many channel processors? Is the channel buffer lossy or blocking? Timeout the operation if not fast enough?
40. beyond simply concurrent
41. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown))))
42. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown)))) First async request
43. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown)))) Second async request
44. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown)))) Parks until one operation has completed
45. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown)))) alts! returns the value and the channel that provided the value
46. (defn get-stock-price [ticker-name] (go (let [disk-response-channel (chan 1) _ (>! disk-cache-channel [ticker-name disk-response-channel]) api-response-channel (chan 1) _ (!> api-channel [ticker-name api-response-channel]) [stock-value from-channel] (alts! [disk-response-channel api-response-channel (timeout 200)])] (if stock-value (:price stock-value) :unknown)))) stock-value is nil if timeout
47. Channel Writer Writer Writer Reader Reader Reader Reader Live Restarts
48. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true))))
49. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Passed in from "supervisor"
50. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Handle a request, recur to handle the next
51. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Like alts!, but evaluates matching expression
52. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Exit the loop
53. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Destructure the value taken from channel
54. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Could be a go block, or function that uses a go block
55. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Handle the next request
56. (defn start-request-processor [request-channel poison-pill-channel] (go (loop [] (alt! poison-pill-channel nil request-channel ([[request response-channel]] (let [_ (<! (timeout 50)) response {:data (* 2 (:data request))}] (>! response-channel response) (recur))) :priority true)))) Option: select 1st clause if both ready (otherwise random)
57. notes and gotchas
58. ClojureScript
59. thread pools (thread) ➠ async-thread-macro-# cached thread pool threads discarded after 60 seconds (go …) etc ➠ async-dispatch-# fixed thread pool, 2 * # of processors + 42 All daemon threads
60. thread pools (go …) blocks should never block (only park) Use (thread …) for code that might block: file system access, database access
61. java.lang.AssertionError: Assert failed: No more than 1024 pending puts are allowed on a single channel. Consider using a windowed buffer. (< (.size puts) impl/MAX-QUEUE-SIZE) at clojure.core.async.impl.channels.ManyToManyChannel.put_BANG_(channels.clj: 95) at clojure.core.async.impl.ioc_macros$put_BANG_.invoke(ioc_macros.clj: 824) at poc.async_alts$get_answer $fn__1764$state_machine__1239__auto____1765.invoke(async_alts.clj:23) at clojure.core.async.impl.ioc_macros $run_state_machine.invoke(ioc_macros.clj:812) at poc.async_alts$get_answer$fn__1764.invoke(async_alts.clj:23) at clojure.lang.AFn.run(AFn.java:24) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java: 1145) at java.util.concurrent.ThreadPoolExecutor $Worker.run(ThreadPoolExecutor.java:615) at java.lang.Thread.run(Thread.java:724)
62. Testing Easy to provide mock channel handlers alts! may be harder to deal with
63. wrap up
64. https://github.com/clojure/core.async
65. http://swannodette.github.io/ David Nolen
66. http://howardlewisship.com
67. Image Credits http://www.flickr.com/photos/8704943@N07/3491779722 - magro_kr http://www.flickr.com/photos/39779537@N05/4589192110 - Lucian Bickerton http://www.flickr.com/photos/38102495@N00/394602448 - jasoneppink http://www.flickr.com/photos/27874172@N03/2682162358/ - Peter Gorges
68. Q & A