Slide 1
Slide 1 text
Mastering time with
Clojure core.async
Howard M. Lewis Ship
TWD Consulting
© 2013 Howard M. Lewis Ship
Slide 2
Slide 2 text
(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)))
Slide 3
Slide 3 text
1 3
2
Slide 4
Slide 4 text
Channel
Writer
Writer
Writer
Reader
Reader
Slide 5
Slide 5 text
(>!! chan val)
(
Slide 6
Slide 6 text
Channel
Writer
Writer
Writer
Reader
Reader
Slide 7
Slide 7 text
No content
Slide 8
Slide 8 text
Channel – (chan 1)
(>!! chan val)
(>!! chan val)
(
Slide 9
Slide 9 text
using multiple threads
Slide 10
Slide 10 text
(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)))
Slide 11
Slide 11 text
(defn request-handler [request]
(let [query1 (thread (do-query-1 request))
query2 (thread (do-query-2 request))
query3 (thread (do-query-3 request))
data1 (
Slide 12
Slide 12 text
(defn request-handler [request]
(let [query1 (thread (do-query-1 request))
query2 (thread (do-query-2 request))
query3 (thread (do-query-3 request))
data1 (
Slide 13
Slide 13 text
(thread (do-query-1 request)) (
Slide 14
Slide 14 text
1
3
2
Slide 15
Slide 15 text
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 (
Slide 16
Slide 16 text
Answer: Mu
Clojure data structures
are immutable - free to
share across threads
Slide 17
Slide 17 text
freedom from blocking
Slide 18
Slide 18 text
(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 (
Slide 19
Slide 19 text
(go
(let […
data1 (
Slide 20
Slide 20 text
Blocking
Slide 21
Slide 21 text
Parking
Slide 22
Slide 22 text
1
3
2
Slide 23
Slide 23 text
(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 (
Slide 24
Slide 24 text
Inversion of Control
query1 (thread (do-query-1 request))
query2 (thread (do-query-2 request))
query3 (thread (do-query-3 request))
0
data1 (
Slide 25
Slide 25 text
What is an API?
Slide 26
Slide 26 text
public interface RequestProcessor {
Response process(Request request);
…
}
(defprotocol RequestProcessor
(process! [this request] …
)
(ns gigantor.request.processor …)
(defn process! [request] …)
POST http://gigantor.com/request
…
Slide 27
Slide 27 text
Input ➠
Process ➠
Result
Slide 28
Slide 28 text
Synchronous
(let [result (process! request)]
Asynchronous
Slide 29
Slide 29 text
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?
Slide 30
Slide 30 text
(defn start-request-processor
[request-channel]
(go
(while true
(let [[request response-channel] (! response-channel response))))
nil)
Slide 31
Slide 31 text
(defn start-request-processor
[request-channel]
(go
(while true
(let [[request response-channel] (! response-channel response))))
nil)
request channel is a
parameter
Slide 32
Slide 32 text
(defn start-request-processor
[request-channel]
(go
(while true
(let [[request response-channel] (! response-channel response))))
nil)
Client sends the request and a
channel for the response
Slide 33
Slide 33 text
(defn start-request-processor
[request-channel]
(go
(while true
(let [[request response-channel] (! response-channel response))))
nil)
timeout is part of core.async
Slide 34
Slide 34 text
(defn start-request-processor
[request-channel]
(go
(while true
(let [[request response-channel] (! response-channel response))))
nil)
Send the response where the client
specified
Slide 35
Slide 35 text
(defn process-request
[request processor-channel]
(let [response-channel (chan 1)]
(>!! processor-channel [request response-channel])
response-channel))
Slide 36
Slide 36 text
(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
Slide 37
Slide 37 text
(defn process-request
[request processor-channel]
(let [response-channel (chan 1)]
(>!! processor-channel [request response-channel])
response-channel))
The API: which channel, what data
Slide 38
Slide 38 text
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!
⚠
Slide 39
Slide 39 text
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?
Slide 40
Slide 40 text
beyond simply concurrent
Slide 41
Slide 41 text
(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))))
Slide 42
Slide 42 text
(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
Slide 43
Slide 43 text
(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
Slide 44
Slide 44 text
(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
Slide 45
Slide 45 text
(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
Slide 46
Slide 46 text
(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
Slide 47
Slide 47 text
Channel
Writer
Writer
Writer
Reader
Reader
Reader
Reader
Live Restarts
Slide 48
Slide 48 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Slide 49
Slide 49 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Passed in from "supervisor"
Slide 50
Slide 50 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Handle a request, recur to
handle the next
Slide 51
Slide 51 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Like alts!, but evaluates
matching expression
Slide 52
Slide 52 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Exit the loop
Slide 53
Slide 53 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Destructure the value
taken from channel
Slide 54
Slide 54 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Could be a go block, or
function that uses a go block
Slide 55
Slide 55 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Handle the next request
Slide 56
Slide 56 text
(defn start-request-processor
[request-channel poison-pill-channel]
(go
(loop []
(alt!
poison-pill-channel nil
request-channel ([[request response-channel]]
(let [_ (! response-channel response)
(recur)))
:priority true))))
Option: select 1st clause if
both ready (otherwise random)
Slide 57
Slide 57 text
notes and gotchas
Slide 58
Slide 58 text
ClojureScript
Slide 59
Slide 59 text
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
Slide 60
Slide 60 text
thread pools
(go …) blocks should never block (only park)
Use (thread …) for code that might block:
file system access, database access
Slide 61
Slide 61 text
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)
Slide 62
Slide 62 text
Testing
Easy to provide mock channel handlers
alts! may be harder to deal with
Slide 63
Slide 63 text
wrap up
Slide 64
Slide 64 text
https://github.com/clojure/core.async
Slide 65
Slide 65 text
http://swannodette.github.io/
David Nolen
Slide 66
Slide 66 text
http://howardlewisship.com
Slide 67
Slide 67 text
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
Slide 68
Slide 68 text
Q & A