ReadStrategy details

[Keith Hodges]

Dear Florian,

I had similar problems.

If I understand correctly with the current scheme a negative value on a 
variable will not prevent that variable actually being read. It may 
lessen the propagation of the depth of that strategy down that branch. 

I finally gave up on the existing numeric/array type implementation of 
the read strategy scheme and reimplemented it with a proper object 
oriented model that I could fully understand and am able to dissect in 
order to test alternative approaches.

The existing approach works best with a cautious read strategy, one 
which reads 1 level as normal with high levels reserved for those 
objects which are entirely safe to read en-mass

If you do specify higher levels you run the risk of an infinite 
recursion if your model has circular references. I.e. get me an A which 
contains B which points to A which I haven't got yet so, get me A.

I wanted to use read strategies which say, read lots, except this 
branch. Or if not reading lots, at least ensure you read frequently 
indexable attributes such as id, name, type.

This work is part of the "Magma seaside" package, and you can simply use 
WAMagmaReadStrategyDefault instead of the standard read strategy class. 
You may also find the WAMagmaReadStrategyWithQ2Logging helpful. It uses 
the Q2Log from Gjallar project, which I think is also available on 
SqueakMap under another name.
(SLog or SLLog ?)

My basic approach is the same, except that I fully support negative 
depths. 

In my scheme: for any class you can specify a default depth for all 
vars, or all vars that are mine, (i.e. not the inherited inst vars) or 
for a specific variable name. These specifications are inherited by 
subclasses, including any as yet unspecified vars will inherit the 
defaults from their superclasses.

best regards

Keith
> Hi again Chris,
>
> Chris Muller wrote:
>> Hi Florian, recall from http://wiki.squeak.org/squeak/2638:
>>
>> "The minimumDepth is how far it reads for any object, all objects,
>> every time, all the time."
>
> My understanding of this is that if I ask Magma for a Player object 
> with a minDepth of 3, it will follow references and reify objects on a 
> depth of 3.
>
>> "As a programmer, you can change this default 0, 1, 2 or 3 (or maybe
>> even 4, but I wouldn't go beyond that) by supplying a ReadStrategy."
>
> My problem is that my application is a game. And every 12h it need to 
> go through the whole hierarchy of the whole players to update them. If 
> I let the minDepth to 1, the read, update and commit of one player 
> takes something like 15min due to the heavy number of requests to 
> magma (lot of very small objects). So I want to reify the whole object 
> hierarchy of my Player before updating it. Therefore I use a minDepth 
> of 10 (the max depth is : 
> Player->(1)Dungeon->(*)MonsterRoom->(*)Adventurer->(1)Treasure->(*)SmallInteger). 
>
>
> And it works great for Players without any Letters, it only takes 
> something like 30s instead of several minutes. But if the Player has 
> some Letters, Magma will reify the recipients and therefore other 
> Players. The read-update-commit then takes ~5-7min (quite annoying to 
> get a real value).
>
>> So here are a couple of options for you.
>>
>> Option 1:
>>
>> The minimumDepth is the global minimum that cannot be overridden
>> except by using a negative depth on a variable-specific specification;
>> but I tend to prefer staying positive just for clarity.  So you could
>> just specify a very low minimumDepth and then larger depths for each
>> variable you want to go deeper on:
>>
>> myReadStrategy := MaReadStrategy minimumDepth: 0.
>> (Player allInstVarNames copyWithout: 'letters') do:
>>  [ : each |
>>  myReadStrategy
>>    forVariableNamed: each
>>    onAny: Player
>>    readToDepth: 10 ]
>>
>> Something like that.  Since a minimumDepth of 0 can really thrash the
>> server with a lot of small requests, you should specify 1 for
>> everything else.  Not too much trouble though, do you have a main
>> superclass for your hierarchy?
>>
>> myReadStrategy onAny: GameObject readToDepth: 1
>>
>> Player, even though it inherits from FlorianGame will still employ a
>> minimum-depth of 0.  This is a convenience feature for just this
>> situation.
>
> I didn't have the need to a single superclass for the whole hierarchy 
> (~30 class) but it could be bothersome, but if needed I could try.
>
> I didn't test this option because my need is the opposite : full depth 
> everywhere and no depth for Desk. And the object hierarchy of a Player 
> is quite complex. I could use reflection to specify a depth of 1 to 
> every variable except myDesk but it's a little bit heavy.
>
>> Option 2:
>>
>> Even still, option 1 you don't get a minimumDepth of 1 for everything
>> else outside your GameObject, like Associations or other
>> out-of-the-box Smalltalk classes (you do with Collections, they always
>> get 1).
>>
>> So, another option would be to stick with a minimumDepth of 1
>> everywhere but then "back up" one level just for Players letters:
>>
>> myReadStrategy := (MaReadStrategy minimumDepth: 1)
>>  forVariableNamed: 'letters' onAny: Player readToDepth: -1;
>>  yourself.
>>
>> Even though it uses a negative, this might be easier..
>
> I tried this method :
>
>     session readStrategy: (
>         (MaReadStrategy minimumDepth: 10)
>         forVariableNamed: 'myDesk' onAny: Dungeon readToDepth: -1;
>         yourself
>     ).
>
> But the Desk is still reified. I tried with -11 but with the same 
> result :/. Did I miss something ?
>
> Florian
>
>
>> - Chris
>>
>>
>>
>> On 7/5/07, Florian Minjat <florian.minjat at gmail.com> wrote:
>>> Hi Chris,
>>>    I did some tests and have to ask for this again :
>>>    As you saw I have a Desk with letters which refers to other
>>> Players. But Players also have Rooms with Adventurers and Treasure.
>>> The structure can be simplified like that (varName:varType):
>>>
>>> Player
>>> ->(1)Desk->(*)Letter
>>> ->(*)Room->(*)Adventurer->(1)Treasure->(1)SmallInteger
>>>
>>> I don't want theses letters to be reified but I want object deeper
>>> like the value of the treasure of an Adventurer to be reified. So I
>>> tried something like that :
>>>
>>> session readStrategy: (
>>>     (MaReadStrategy minimumDepth: 10)
>>>        forVariableNamed: 'letters' onAny: Desk readToDepth: 0
>>> )
>>>
>>>    But it didn't seem to work :
>>> ((player instVarNamed: 'desk') instVarNamed: 'letters') class =>
>>> OrderedCollection
>>> (((player instVarNamed: 'desk') instVarNamed: 'letters') instVarAt: 1)
>>> class => Array
>>> ((((player instVarNamed: 'desk') instVarNamed: 'letters') instVarAt:
>>> 1) instVarAt: 1) class => Letter
>>>
>>>    So how can I make the rule on Desk precedes the more global rule ?
>>> Currently, retrieving a Player without Letters takes a few seconds,
>>> with Letters a few minutes :/.
>>>
>>> Florian
>>>
>>
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