A WeakMap where values (not keys) are weak references?

Michael Kriegel michael.kriegel at actifsource.com
Thu Mar 30 09:22:05 UTC 2017


BTW are the security concerns mentioned by Erik Arvidsson in this thread 
(http://mozilla.6506.n7.nabble.com/What-is-the-status-of-Weak-References-tp271746p271750.html) 
still present?

Wouldn't a solution be, to make the weak references of one actor dispose 
as soon as this actor has no strong references to the object (no matter 
whether there are other actors having strong references to that same 
object)?

Also the last commit in https://github.com/tc39/proposal-weakrefs was 9 
months ago - is there any way to find information on he status of that? 
Is there still anyone working on it?


On 30.03.2017 11:09, Michael Kriegel wrote:
>
> Totally agree. WeakRef is what we want and WeakValueMap is what I want 
> to build with it. Sugar would be if the standard provides "an 
> out-of-the-box WeakValueMap *as well*".
>
> Automatic removal of entries after they became obsolete would be 
> necessary - be it integrated or via a callback mechanism (i guess the 
> executor is sth. like that.)
>
>
> On 30.03.2017 10:45, T.J. Crowder wrote:
>> Right -- but it's really weak references you're after, right? Because 
>> with weak references, a Map can be a WeakValueMap, but other 
>> structures are also possible. With that proposal's `executor` 
>> feature, the map can even be proactive removing entries for objects 
>> that have become only weakly reachable.
>>
>> I'd like to see weak references (perhaps not quite that exact 
>> proposal, but along those lines), not just a WeakValueMap. I've only 
>> needed them rarely (your example is a good one), but when I've needed 
>> them, they were pretty much the only option... I don't mind an 
>> out-of-the-box WeakValueMap *as well*, I just don't want it *instead*.
>>
>> -- T.J. Crowder
>>
>> On Thu, Mar 30, 2017 at 9:21 AM, Michael Kriegel 
>> <michael.kriegel at actifsource.com 
>> <mailto:michael.kriegel at actifsource.com>> wrote:
>>
>>     I am also still very interested in Weak references / a weak map,
>>     where the values are held weakly. Here is my example: I want to
>>     easily build caches, which lose their values, when there are no
>>     other references to them. Example pseudo code (assuming server
>>     request answers synchronously for simplicity):
>>
>>     var Cache = new WeakValueMap();
>>
>>     function request(Key) {
>>
>>         var CachedResult = Cache.get(Key);
>>
>>         if (CachedResult) return CachedResult;
>>
>>         var UnprocessedResult = doServerRequest(Key);
>>
>>         var BigProcessedObject =
>>     doVeryHeavyComputingTask(UnprocessedResult);
>>
>>         Cache.set(Key,BigProcessedObject);
>>
>>         return BigProcessedObject;
>>
>>     }
>>
>>     Thoughts:
>>
>>     - doHeavyComputingTask takes a long time, so it should be done
>>     only once per key (until it is uncached)
>>
>>     - BigProcessedObject is really big, it shall be in memory at most
>>     once at any time
>>
>>     - When the last reference to a BigProcessedObject is lost, it
>>     shall be removed from cache to save memory
>>
>>     - removal could be delayed by holding a reference to a
>>     BigProcessedObject and releasing it using a setTimeout-Timer.
>>
>>     Without WeakValueMap I have to build my own reference counting,
>>     so everyone who requests the object for a Key has to "unrequest"
>>     it again, else it would provoke a memory leak. This implies, that
>>     the life cycle of an object is tracked, so each object, which has
>>     a reference to a BigProcessedObject needs something like a
>>     destructor. As there is no destructor in JS you have to write a
>>     method and manually call it. Thus each object needs to have a
>>     single known owner or reference counting/tracking has to be done
>>     for that object, too. This propagates through your whole program.
>>     Finally as a developer you end up building your own garbage
>>     collector, which:
>>
>>     - is error prone
>>
>>     - consumes extra memory, as there are already all structures
>>     present for garbage collecting in the JS engine, just it is not
>>     usable
>>
>>     - takes up lots of time for development and debugging
>>
>>     So it is a/the classical example case for
>>     https://github.com/tc39/proposal-weakrefs
>>     <https://github.com/tc39/proposal-weakrefs>
>>
>>
>>     On 21.03.2017 08:34, T.J. Crowder wrote:
>>>     What are your use cases for it?
>>>
>>>     Rather than tying it to being a `Map`, I'd prefer to see
>>>     something like Java's [`WeakReference`][1]:  You could store the
>>>     `WeakReference`s in a `Map` if you wanted strongly-referenced
>>>     keys with weakly-referenced values.
>>>
>>>     -- T.J. Crowder
>>>
>>>     [1]:
>>>     http://docs.oracle.com/javase/8/docs/api/java/lang/ref/WeakReference.html
>>>     <http://docs.oracle.com/javase/8/docs/api/java/lang/ref/WeakReference.html>
>>>
>>>     On Tue, Mar 21, 2017 at 6:15 AM, /#!/JoePea <joe at trusktr.io
>>>     <mailto:joe at trusktr.io>> wrote:
>>>
>>>         Hey all,
>>>
>>>         I'd like to have something like a WeakMap where the keys can
>>>         be primitives and the values are weakly-held Objects, so if
>>>         there are no more references to any of the Object values
>>>         that the entry gets removed from the map.
>>>
>>>         For example, it might look like this:
>>>
>>>         ```
>>>         {
>>>         ["foo"] => SomeObject,
>>>         ["
>>>         ​bar​
>>>         "] =>
>>>         ​OtherObject​
>>>         ,
>>>         }
>>>         ```
>>>
>>>         where if there are no more references to `OtherObject`, then
>>>         `['bar'] => OtherObject` is removed from the map.
>>>
>>>         Usage would be very similar to WeakMap, like
>>>
>>>         ```
>>>         let m = new ReversedWeakMap
>>>
>>>         m.add('foo', SomeObject)
>>>         m.add('
>>>         ​bar​
>>>         ',
>>>         ​OtherObject​
>>>         )
>>>         ​console.log(m.get('bar')) // OtherObject
>>>
>>>         ... time passes, no more references to OtherObject,
>>>         OtherObject is collected ...
>>>
>>>         console.log(m.get('bar'))​ // undefined
>>>
>>>         ```
>>>
>>>         I thought of using WeakMap values as keys, and vice versa,
>>>         but it seems to be impossible.
>>>
>>>         I guess maybe it is difficult to add this natively because
>>>         it would mean that GC needs to be completely deterministic
>>>         as far as JS programs go. For example:
>>>
>>>         ```js
>>>         let m = new ReversedWeakMap
>>>
>>>         function main() {
>>>         m.set('foo', {})
>>>         console.log(m.get('foo')) // {}
>>>         }()
>>>
>>>         main()
>>>
>>>         // GC would have to be guaranteed to have happened at this
>>>         point.
>>>
>>>         console.log(m.get('foo')) // undefined
>>>
>>>         ```
>>>
>>>
>>>         */#/!//*JoePea
>>>
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>>
>>     -- 
>>     Michael Kriegel • Head of R&D • Actifsource AG • Haldenstrasse 1 • CH-6340 Baar •www.actifsource.com <http://www.actifsource.com>  •+41 56 250 40 02 <tel:+41%2056%20250%2040%2002>
>>
>>     _______________________________________________ es-discuss
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>>
> -- 
> Michael Kriegel • Head of R&D • Actifsource AG • Haldenstrasse 1 • CH-6340 Baar •www.actifsource.com  • +41 56 250 40 02
>
> _______________________________________________
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-- 
Michael Kriegel • Head of R&D • Actifsource AG • Haldenstrasse 1 • CH-6340 Baar • www.actifsource.com • +41 56 250 40 02
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