I reckon if that worked, there would be no need for memoize anyway, but I
don't think swap! will allow for it. I'm far from an expert on swap! or
atoms, but several swap!s may be run simultaneously on a single atom (and
swap! may re-run the swapping function if the atom has been changed since
the swapping function was invoked). In other words, if two swap!s are
invoked at about the same time for the same key (which is not currently in
the cache), both invocations may be given the same value of the cache at
that moment, which will lead to the value being re-calculated in both
invocations - and even if calc-value is memoized, the same thing may occur
in the memoized function.
As I said, I'm far from an expert, so I wrote a small test that shows that
calc-value may indeed be called more than once (core.async here is simply
to ensure that each invocation of calc-value is printed on its own line):
(def out-ch (chan 100))
(go-loop []
(println (<! out-ch))
(recur))
(defn calc-value [x]
(put! out-ch x)
(* x x))
(def cache (atom {}))
(def lookup
(let [calc-value* (memoize calc-value)]
(fn [cache key]
((swap! cache (fn [cache']
(if (contains? cache' key)
cache'
(assoc cache' key (calc-value* key)))))
key))))
=> (dotimes [_ 1000]
(future (lookup cache (rand-int 20))))
11
12
16
1
nil
6
15
17
18
14
2
5
19
10
0
7
*9*
*4*
*4*
*13*
*9*
*13*
Den måndagen den 1:e september 2014 kl. 21:32:01 UTC+2 skrev Alex Baranosky:
>
> I believe you can replace:
> (when-not (contains? @cache k)
> (swap! cache assoc k (calc-value* k))))
>
> with:
> (swap! cache (fn [cache']
> (if (contains? cache' k)
> cache'
> (assoc cache' k (calc-value* k)))))
>
> Note: I haven't run this code
>
>
> On Mon, Sep 1, 2014 at 2:20 AM, Colin Fleming <colin.ma...@gmail.com
> <javascript:>> wrote:
>
>> Hi Thomas,
>>
>> Normally I'd agree with you, but in my case it actually works quite well
>> since I don't need to expire or worry about sizing. This is for caching
>> objects on IntelliJ parse tree elements, and the element with its cached
>> values is thrown away every time the document is parsed, so these are
>> pretty transient caches. In this particular case the calculation isn't too
>> heavyweight either so recalculating it in the unlikely event of contention
>> isn't a big deal. In fact, this discussion and the related thinking has
>> made me realise that my original solution was actually sufficient for my
>> somewhat strange use case, which is nice :-). For more typical server side
>> caching, I agree that Guava would be a great solution.
>>
>> Cheers,
>> Colin
>>
>>
>> On 1 September 2014 20:54, Thomas Heller <th.h...@gmail.com <javascript:>
>> > wrote:
>>
>>> As much as I like Clojure and atoms, I do not think they are a good fit
>>> for caching. Not only is it impossible to address the concurrency issues
>>> related to multiple threads loading the same object, but you also have to
>>> do expiration and size management yourself. Immutability doesn't help much
>>> for caching either. There is core.cache that does some bits but I probably
>>> would recommend using something like CacheBuilder from the guava libs:
>>>
>>> See
>>> https://code.google.com/p/guava-libraries/
>>>
>>> http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/cache/CacheBuilder.html
>>>
>>> Its Java but the Clojure<->Java interop is so good that it doesn't
>>> matter much.
>>>
>>> On Saturday, August 30, 2014 7:27:05 AM UTC+2, Colin Fleming wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I want to use a map to cache values based on a key. I'm planning to use
>>>> an atom for this. My basic operation is "give me the value for this key" -
>>>> if the value exists in the map then that value should be returned,
>>>> otherwise a new value should be calculated, inserted in the map and then
>>>> returned. My plan is to implement something like the following:
>>>>
>>>>
>>>> (defn ensure [cache key] (if (contains? cache key) cache (assoc
>>>> cache key (calc-value key))))(let [value (get (swap! cache ensure key)
>>>> key)] ... do my thing with value ...)
>>>>
>>>>
>>>> So 'ensure' ensures that the cache contains the value for key, the
>>>> swap! operation returns the cache with the value and then I get it out.
>>>> This works but feels a little clumsy, is there a better way to do this?
>>>>
>>>> Also, looking at the Atom source code, I see that this will cause a CAS
>>>> operation even if the value returned from swap! is identical to the
>>>> original value. It seems like a reasonable optimisation would be to check
>>>> if the values are identical and not update if so - is there a reason this
>>>> might not be a good idea?
>>>>
>>>> Thanks,
>>>> Colin
>>>>
>>> --
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To post to this group, send email to clo...@googlegroups.com
>>> <javascript:>
>>> Note that posts from new members are moderated - please be patient with
>>> your first post.
>>> To unsubscribe from this group, send email to
>>> clojure+u...@googlegroups.com <javascript:>
>>> For more options, visit this group at
>>> http://groups.google.com/group/clojure?hl=en
>>> ---
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To unsubscribe from this group and stop receiving emails from it, send
>>> an email to clojure+u...@googlegroups.com <javascript:>.
>>> For more options, visit https://groups.google.com/d/optout.
>>>
>>
>> --
>> You received this message because you are subscribed to the Google
>> Groups "Clojure" group.
>> To post to this group, send email to clo...@googlegroups.com
>> <javascript:>
>> Note that posts from new members are moderated - please be patient with
>> your first post.
>> To unsubscribe from this group, send email to
>> clojure+u...@googlegroups.com <javascript:>
>> For more options, visit this group at
>> http://groups.google.com/group/clojure?hl=en
>> ---
>> You received this message because you are subscribed to the Google Groups
>> "Clojure" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to clojure+u...@googlegroups.com <javascript:>.
>> For more options, visit https://groups.google.com/d/optout.
>>
>
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
