Smalltalk › Pharo › Pharo Smalltalk Developers

Understanding WeakValueDictionary

_‹ Previous Topic Next Topic _›

Classic

List

Threaded

5 messages Options

NorbertHartl

Understanding WeakValueDictionary

While looking at the code in voyage I got to WeakValueDictionary as voyage uses it as object cache. If I looked correctly the WeakValueDictionary is based on equality and not identity. Is that correct? Feels wrong!

Norbert

Igor Stasenko

Re: Understanding WeakValueDictionary

On 17 April 2014 10:26, Norbert Hartl <[hidden email]> wrote:

While looking at the code in voyage I got to WeakValueDictionary as voyage uses it as object cache. If I looked correctly the WeakValueDictionary is based on equality and not identity. Is that correct? Feels wrong!

well, comparison used for keys..

and i agree that for caching, an identity comparison makes much more sense,

since you want it to be fast, and identity comparison is no doubt faster than

equality comparison, especially if keys are complex objects which implement equality based on comparing own fields.

but for that, i think we would need new kind of dictionary, e.g.

WeakValueIdentityKeyDictionary (or some similar name).

Btw, for caching i prefer to use round-robin cache with limited size

(like that cache never grows too big, and instead simply drops/overrides existing entries when it become full).. this helps maintaining cache size under control.

A good example is LRUCache, which is very suitable method to cache objects.. albeit at cost of some extra processing for accessing it.

Norbert

--
Best regards,
Igor Stasenko.

NorbertHartl

Re: Understanding WeakValueDictionary

Am 17.04.2014 um 16:06 schrieb Igor Stasenko <[hidden email]>:

On 17 April 2014 10:26, Norbert Hartl <[hidden email]> wrote:

While looking at the code in voyage I got to WeakValueDictionary as voyage uses it as object cache. If I looked correctly the WeakValueDictionary is based on equality and not identity. Is that correct? Feels wrong!

well, comparison used for keys..
and i agree that for caching, an identity comparison makes much more sense,

since you want it to be fast, and identity comparison is no doubt faster than
equality comparison, especially if keys are complex objects which implement equality based on comparing own fields.

Agreed. But I think that equality based comparison is not only slower but wrong! Implementing an object cache based on equality can easily screw up your graph. You can call it the crappiest kind of become: you’ve never wanted!

but for that, i think we would need new kind of dictionary, e.g.
WeakValueIdentityKeyDictionary (or some similar name).

Agreed.

Btw, for caching i prefer to use round-robin cache with limited size

You mean those things where you can state: „Hey, I’m fast because I exchanged * with + ?“ :)

Norbert

(like that cache never grows too big, and instead simply drops/overrides existing entries when it become full).. this helps maintaining cache size under control.

A good example is LRUCache, which is very suitable method to cache objects.. albeit at cost of some extra processing for accessing it.

Norbert

--
Best regards,
Igor Stasenko.

Jesús Marí

Re: Understanding WeakValueDictionary

I'm trying to make a voyage version for CouchDB and I've copied the VoyageMongoCache class, but I've been experimenting estrange behavior using this class, because of the "equality" of the objects.
What I have done is to change the class of reversedObject from WeakKeyDictionary to WeakIdentityKeyDictionary and the method

at: anOID put: anObject
self compactIfNeeded.
self mutex critical: [
objects at: anOID put: anObject.
reversedObjects at: anObject put: anOID ].

to:
at: anUUID put: anObject

self compactIfNeeded.
self mutex critical: [
reversedObjects at: anObject ifAbsent:[
objects at: anUUID put: anObject.
reversedObjects at: anObject put:anUUID] ].

so now there is only one instance of an object in the cache. May be it could be useful for you?

Sean P. DeNigris

Re: Understanding WeakValueDictionary

Administrator

Apparently never reached the list. Forwarding for Jesús...

Jesús wrote

I'm trying to make a voyage version for CouchDB and I've copied the VoyageMongoCache class, but I've been experimenting estrange behavior using this class, because of the "equality" of the objects.
What I have done is to change the class of reversedObject from WeakKeyDictionary to WeakIdentityKeyDictionary and the method

at: anOID put: anObject
self compactIfNeeded.
self mutex critical: [
objects at: anOID put: anObject.
reversedObjects at: anObject put: anOID ].

to:
at: anUUID put: anObject

self compactIfNeeded.
self mutex critical: [
reversedObjects at: anObject ifAbsent:[
objects at: anUUID put: anObject.
reversedObjects at: anObject put:anUUID] ].

so now there is only one instance of an object in the cache. May be it could be useful for you?

Cheers,
Sean