On Fri, Jun 17, 2011 at 2:39 PM, Nicolas Cellier < nicolas.cellier.aka.nice@gmail.com> wrote:
2011/6/17 Eliot Miranda <eliot.miranda@gmail.com>:
On Fri, Jun 17, 2011 at 1:26 AM, Martin Dias <tinchodias@gmail.com>
wrote:
Hi Eliot, I am very happy to read your mail.
On Wed, Jun 15, 2011 at 3:29 PM, Eliot Miranda <eliot.miranda@gmail.com
wrote:
Hi Martin & Mariano, regarding filtering. Yesterday my colleague Yaron and I
successfully
finished our port of Fuel to Newspeak and are successfully using it to save and restore our data sets; thank you, its a cool framework. We had to implement two extensions, the first of which the ability to save and restore Newspeak classes, which is complex because these are instantiated classes inside instantiated Newspeak modules, not static Smalltalk classes in the Smalltalk dictionary. The second extension is the ability to map specific objects to nil, to prune objects on the way out. I want to discuss this latter extension. In our data set we have a set of references to objects that are logically not persistent and hence not to be saved. I'm sure that this will be a common case. The requirement is for the pickling system to prune certain objects, typically by arranging that when an object graph is pickled, references to the pruned objects are replaced by references to nil. One way of doing this is as described below, by specifiying per-class lists of instance variables whose referents shoudl not be saved. But this can be clumsy; there may be references to objects one wants to prune from e.g. more than one class, in which case one may have to provide multiple lists of the relevant inst vars; there may be references to objects one wants to prune from e.g. collections (e.g. sets and dictionaries) in which case the instance variable list approach just doesn't work. Here are two more general schemes. VFirst, most directly, Fuel could provide two filters, implemented in the default mapper, or the core analyser. One is a set of classes whose instances are not to be saved. Any reference to an instance of a class in the toBePrunedClasses set is saved as nil. The other is a set of instances that are not to be saved, and also any reference to an instance in the toBePruned set is saved as nil. Why have both? It can be convenient and efficient to filter by class (in our case we had many instances of a specific class, all of which should be filtered, and finding them could be time consuming), but filtering by class can be too inflexible, there may indeed be specific instances to exclude (thing for example of part of the object graph that functions as a cache; pruning the specific objects in the cache is the right thing to do; pruning all instances of classes whose instances exist in the cache may prune too much). As an example here's how we implemented pruning. Our system is called Glue, and we start with a mapper for Glue objects, FLGlueMapper: FLMapper subclass: #FLGlueMapper instanceVariableNames: 'prunedObjectClasses newspeakClassesCluster modelClasses' classVariableNames: '' poolDictionaries: '' category: 'Fuel-Core-Mappers' It accepts newspeak objects and filters instances in the prunedObjectsClasses set, and as a side-effect collects certain classes that we need in a manifest: FLGlueMapper>>accepts: anObject "Tells if the received object is handled by this analyzer. We want to hand-off instantiated Newspeak classes to the newspeakClassesCluster, and we want to record other model classes. We want to filter-out instances of any class in prunedObjectClasses." ^anObject isBehavior ifTrue: [(self isInstantiatedNewspeakClass: anObject) ifTrue: [true] ifFalse: [(anObject inheritsFrom: GlueDataObject) ifTrue: [modelClasses add: anObject]. false]] ifFalse: [prunedObjectClasses includes: anObject class] It prunes by mapping instances of the prunedObjectClasses to a special cluster. It can do this in visitObject: since any newspeak objects it is accepting will be visited in its visitClassOrTrait: method (i.e. it's implicit that all arguments to visitObjects: are instances of the prunedObjectsClasses set). FLGlueMapper>>visitObject: anObject analyzer mapAndTrace: anObject to: FLPrunedObjectsCluster instance into: analyzer clustersWithBaselevelObjects FLPrunedObjectsCluster is a specialization of the nil,true,false cluster that maps its objects to nil: FLNilTrueFalseCluster subclass: #FLPrunedObjectsCluster instanceVariableNames: '' classVariableNames: '' poolDictionaries: '' category: 'Fuel-Core-Clusters' FLPrunedObjectsCluster >>serialize: aPrunedObject on: aWriteStream super serialize: nil on: aWriteStream
So this would generalize by the analyser having an e.g. FLPruningMapper as the first mapper, and this having a prunedObjects and a priunedObjectClasses set and going something like this: FLPruningMapper>>accepts: anObject ^(prunedObjects includes: anObject) or: [prunedObjectClasses includes: anObject class] FLPruningMapper >>visitObject: anObject analyzer mapAndTrace: anObject to: FLPrunedObjectsCluster instance into: analyzer clustersWithBaselevelObjects and then one would provide accessors in FLSerialzer and/or FLAnalyser to add objects and classes to the prunedObjects and prunedObjectClasses set. For efficiency one could arrange that the FLPruningMapper was not added to the sequence of mappers unless and until objects or classes were added to the prunedObjects and prunedObjectClasses set.
Excellent. I love the botanical metaphor of pruning! Of course we can include FLPruningMapper and FLPrunedObjectsCluster in Fuel.
We are also interested in pruning objects but not necessarily replacing them by nil, but for another user defined objects. For example proxies. We can extend the pruning stuff for doing that.
That was an idea Yaron came up with. That instead of using fuelIgnoredInstanceVariableNames one uses e.g. Object>>objectToSerialize ^self and then if one wants to prune specific inst vars in MyClass one implements MyClass>>objectToSerialize ^self shallowCopy prepareForSerialization
Hi Eliot,
I'm not convinced by the shallowCopy solution, except for the simple structures. If object graph is complex (have share nodes, loops, ...) then you gonna end up in a replication problem equivalent to the one Fuel is trying to solve.
The assumption is that the analyser would create a maximum of one proxy per object in the graph (default, no proxy) and that it would map objects with proxies to their proxies. So if proxies only nilled out inst vars I don't see a problem. What's attractive about this is that it provides a general solution to a couple of problems, a) how to replace a class of objects by some substitute (e.g. nil), b) how to prune state that needn't be saved. It is also conceptually simple; one just creates a proxy instance; no defining metadata, such as inst var names, and hence the code is always up-to-date (e.g. a class redefine won't automatically uncover renamed inst vars in serialization metadata).
Nicolas
MyClass>>prepareForSerialization instVarIDontWantToSerialize := nil. ^self and for objects one doesn't want to serlalize one implements MyNotToBeSerializedClass>>objectToSerialize ^nil So its more general. But I would pass the analyser in as an argument, which would allow things like MyPerhapsNotToBeSerializedClass>>objectToSerializeIn: anFLAnalyser ^(anFLAnalyser shouldPrune: self) ifFalse: [self] ifTrue: [nil] which would of course be the default in Object: Object>>objectToSerializeIn: anFLAnalyser ^(anFLAnalyser shouldPrune: self) ifFalse:: [self]
I think both Yaron and I feel the Fuel framework is comprehensible and flexible. We enjoyed using it and while we took two passes at coming
up
with the pruning scheme we liked (our first was based on not serializing specific ins vars and was much more complex than our second, based on pruning instances of specific classes) we got there quickly and will very little frustration along the way. Thank you very much.
:-) thank you!
Finally, a couple of things. First, it may be more flexible to
implement
fuelCluster as fuelClusterIn: anFLAnalyser so that if one is trying to override certain parts of the mapping framework an implementation can access the analyser to find existing clusters, e.g. MyClass>>fuelClusterIn: anFLAnalyser ^self shouldBeInASpecialCluster ifTrue: [anFLAnalyser clusterWithId: MySpecialCluster id] ifFalse: [super fuelClusterIn: anFLAnalyser] This makes it easier to find a specific unique cluster to handle a group of objects specially.
I can't imagine a concrete example but I see that it is more flexible... the cluster obtained via double dispatch can be anything polymorphic with MySpecialCluster... that's the point?
To be honest I'm not sure. But passing in the analyser in things like fuelCluster or objectToSerialize is I think a good idea as it provides a convenient communication path which in turn provides considerable flexibility.
Lastly, the class-side cluster ids are a bit of a pain. It would be
nice
to know a) are these byte values or general integer values, i.e. can there be more than 256 types of cluster?, and b) is there any meaning to the ids? For example, are clusters ordered by id, or is this just an integer tag? Also, some class-side code to assign an unused id would be nice. You might think of virtualizing the id scheme. For example, if FLCluster maintained a weak array of all its subclasses then the id of a cluster could be the index in the array, and the array could be cleaned up occasionally. Then each fuel serialization could start with the list of cluster class names and ids, so that specific values of ids are specific to a particular serialization.
I do agree, these ids are an heritage from the first prototypes of fuel, they should be revised. a) yes, it is encoded in only one byte; b) just an integer tag, the only purpose of the id was for decoding fast: read a byte and then look in a dictionary for the corresponding cluster instance. We could even store the cluster class name but that's inefficient.
Yes, but how inefficient? What's the size of all the cluster names? FLCluster allSubclasses inject: 0 into: [:t :c| t + c name size + 1] 670
So you'd add less than a kilobyte to the size of each serialization and get complete freedom from ids. Something to think about.
Virtualizing the id scheme is a good idea. Much more elegant and extensible. The current mechanism not only limits the number of possible clusters, but also "user defined" extensions can collide, for example if your Glue cluster id is the same of the Moose cluster id.
I added an issue in our tracker.
If it makes sense, maybe the weak array you suggest can be also used to avoid instantiating lots of FLObjectCluster like we are doing in Object:
fuelCluster ^ self class isVariable ifTrue: [ FLVariableObjectCluster for: self class ] ifFalse: [ FLFixedObjectCluster for: self class ]
the second time you send fuelCluster to an object, it can reuse the cluster instance.
Right. I think that's important, and is one reason why I think passing in the analyser is important, because it allows certain objects to discover existing clusters in the analyzer and join them if they want to, instead of having to invent and maintain their own cluster uniquing solution .
again thanks for a great framework.
Thanks for your words and the feedback. Is Glue published somewhere?
No, and its extremely proprietary :) Newspeak however is available and we may end up maintaining a port of Fuel for Newspeak. best regards, Eliot
regards Martin
best, Eliot
On Mon, Jun 13, 2011 at 10:16 AM, Mariano Martinez Peck <marianopeck@gmail.com> wrote:
On Thu, Jun 9, 2011 at 3:35 AM, Eliot Miranda <
eliot.miranda@gmail.com>
wrote:
Hi Martin and Mariano, a couple of questions. What's the right way to exclude certain objects from the serialization? Is there a way of excluding certain
inst
vars from certain objects?
Eliot and the rest....Martin implemented this feature in Fuel-MartinDias.258. For the moment, we decided to put #fuelIgnoredInstanceVariableNames at class side.
Behavior >> fuelIgnoredInstanceVariableNames "Indicates which variables have to be ignored during serialization."
^#()
MyClass class >> fuelIgnoredInstanceVariableNames ^ #('instVar1')
The impact in speed is nothing, so this is good. Now....we were thinking if it is common to need that 2 different instances of the same class need different instVars to ignore. Is this common ? do you usually need this ? We checked in SIXX and it is at instance side. Java uses the prefix 'transient' so it is at class side...
thanks
-- Mariano http://marianopeck.wordpress.com
-- best, Eliot