modeling-users — For users: questions, support, announcements, etc.

Re: [Modeling-users] How the transactions are done in the saveChanges?

[Marco B. <m.b...@ic...>]

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Sebastien Bigaret wrote:
...

|
| BTW: if you set the env. variable 'MDL_ENABLE_DATABASE_LOGGING' then
you'll get
| a copy of the framework db-related actions on stderr (or anywhere else
if you
| modify Modeling.logging) --see the appropriate annex in the User's Guide.
|

Ok, I will take a look

|>Also, I've noticed that the ec.fetch() is a transaction which
|>terminates with an abort. Is this correct?
|
|
| This is intentional, anyhow ;)  Such a transaction should be
terminated, and I
| cannot see any reason why it should be COMMITed, hence it is ABORTed!
  But if
| you think differently, please let us know!
|

One of the basic reason for this is in the debugging/maintenance of the
system.

When you need to look for the DB log, it is useful to spot the problems
by mean of ABORTed transactions. If the aborted transactions are the
standard, this can be a problem.

|
|>2) how are the db transaction executed/completed inside a Zope
|>transaction? In a non-zope application, I can see each fetch()
|>produces a transaction, as well as the ec.saveChanges(). Is this true
|>also in Zope applications?
|
|
| Yes it is.  The zope support basically consists in providing a default
| EditingContext per session, possibly also connecting this EC's
| saveChanges() to the transaction machinery (i.e. to each
request/response loop).
|  That's all, and everything else will be handled in your ZProducts'
code, just
| the same way you would make it in pure python.
|
| 	Hope this helps,
|
| -- Sébastien.

Yes and no... I would prefer to be able to tell ModelingFramework when
it should start/stop a transaction on the db... howevr, thanks of the
answer...
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Re: [Modeling-users] Aggregate values in objects

[Sebastien B. <sbi...@us...>]

Kertis, Dennis wrote:
> Is there a way I can get a field populated with an aggregate value. 
> i.e., I want a manager to have a field which would contain how many 
> employees work for him by doing a select count(*).  I parsed through 
> the fetching section of the doc and didn't find a way to do this.

For COUNT(*) there is a dedicated method: EC.fetchCount(), which 
has the very same syntax as EC.fetch() and returns the number of
objects matching the qualifier.

I recommend that you define a method returning this count, rather
than have an integer field populated with such a value, or else
you should be prepared to deal with every possible situations
where this count can change if you do not want that this field's
value get out of sync.

  Other aggregates are not yet supported; I believe we formerly
discussed this here, probably with Ernesto, among others.
I do not have access to the archives here, Erny could you maybe
confirm?

-- Sébastien.

Re: [Modeling-users] inconsistencies with PyModels

[Sebastien B. <sbi...@us...>]

Lorenzo Gil Sanchez wrote:
> Hi all,
> 
> I'm finding some problems when creating a PyModel:
> 
> model.addEntity(my_entity) does nothing but
> model.entities.append(my_entitye) works
> 
> This is not really important and I guess it's easy to fix.

Probably easy to fix, yes, I'll check that, thanks for reporting.

> The next problem is more important:
> 
> model.build
> defaultModelSet().addModel(model)
> 
> fails with this exception:
[...]
> defaultModelSet().addModel(model.component)
> works without a warning but I'm not sure if this is the preferred way 
> of adding a model to the modelset.

Yes, this is a known bug, ModelSet.addModel() (and other methods in 
ModelSet as well) only accepts Modeling.Model objects, not PyModels.
In the meantime, adding the PyModel.component is the correct
workaround.

-- Sébastien.

Re: [Modeling-users] Many-to-many relationship example doesn't work?

[Sebastien B. <sbi...@us...>]

Oh well, sorry, I'm sure it's not the first time it's been 
reported but left uncorrected, isn't it?

co...@sa... wrote:
> I've looked into CustomObject.py, and i could'nt find any assigment of
> REAL editing, context, only initial definition 'lambda self: None'
> Did i misunderstood something, or this example code in User's guide
> is
> just out-dated?

With Erny's correction this should work.

And about CustomObject.editingContext(): it's assigned whenever the object is
inserted or fetched within an EC.

-- Sébastien.


Ernesto Revilla wrote:
> 
> Hi,
> 
> You have to add the instance to both sides of the relation, the
> a.setPerson(p)
> 
> is missing.
> 
> To add an instance to both sides of a relation, you can do also:
> 
> p.addObjectToBothSidesOfRelationshipWithKey(a, ’addresses’)
> 
> Erny
> 
> 
> co...@sa... escribió:
> 
> >Hello,
> >
> >First of all thanks to Sabastien for his efforts creating this
> framework.
> >Projects I'm involved now requires lots of Many-To-Many relations in
> E-R
> >model, so I've tried adding methods (for tranparent access to
> correlation
> >entity) as described in the example in UsersGuide 2.6.3. But those
> methods
> >doesn't work. So I've copy-pasted example from user's guide, but it
> did'nt
> >work as well.:
> >i.e I add methods getAddresses(), addToAddresses()
> >to the class "Person", Then I try to run a code, that uses those
> classes:
> >-------------
> >#test.py
> >from Modeling.EditingContext import EditingContext
> >from MM1 import Person, Address
> >ec=EditingContext()
> >p = Person.Person()
> >a = Address.Address()
> >p.addToAddresses(a)
> >ec.insert(p)
> >ec.insert(a)
> >ec.saveChanges()
> >-----------------
> >I get an error:
> >Traceback (most recent call last):
> >  File "./test.py", line 8, in ?
> >    p.addToAddresses(a)
> >  File "/tmp/test2/MM1/Person.py", line 76, in addToAddresses
> >    self.editingContext().insert(_pa)
> >AttributeError: 'NoneType' object has no attribute 'insert'
> >-----------------
> >I've looked into CustomObject.py, and i could'nt find any assigment
> of
> >REAL editing, context, only initial definition 'lambda self: None' 
> >Did i misunderstood something, or this example code in User's guide
> is
> >just out-dated?
> >
> >Can someone please give me some guidlines?
> >
> >I'm using: Python 2.3, Modeling: 0.9-pre-17.1

Re: [Modeling-users] one model, multiple database instances

[Sebastien B. <sbi...@us...>]

Mario Ruggier wrote:
[...]
> Another situation in which this could apply is for migrating from one
> db+model to another... a little modeling mapping code could read from
> one EC and populate another that uses a modified model. This may not 
> be the most efficient to way to do this, but possibly the most 
> flexible and one that would guarantee data integrity and consistency 
> the most. Any opinions on whether this would be a good idea or not?

I'm sure we've played with that idea some day in the past, if I remember well it
was probably with Yannick Gingras (I can't check the archives right now).

  The problem is that, while this sounds interesting, this could
  probably only be done with small databases --because, unless we
  want to automatically analyse a model in detail, handling
  relationships will end up in loading a whole database in memory.

Moreover, I'm convinced that it will be difficult to cover every
transformations. But I can be proved wrong!

-- Sébastien.

Re: [Modeling-users] How the transactions are done in the saveChanges?

[Sebastien B. <sbi...@us...>]

Marco Bizzarri wrote:
> Hi all.
> 
> I'm experimenting with ModelingFramework, and I've some questions.
> 
> I've tried the sample model in from the website, and I've analyzed the 
> database logs. I've noticed that the ec.saveChanges() in the model is 
> composed of 5 transactions: the first 4 transactions get the key 
> numbers from the sequence, the last one is the transaction actually 
> doing the work.
> 
> Is this a correct behaviour? What are the reasons behind it?

The first 4 transactions get the PK value from the sequence; in fact, you'll
notice that there are as many transactions for obtaining PK values as there are
newly inserted objects that will be saved.

  Is this correct? It is, even if it's inefficient --this is a TODO item, PKs
values could be obtained in a single roundtrip to the database!

By the way, the PK values are obtained before any change is saved, just because
saving changes requires that those PKs values are known (because of
relationships).  Now if ec.saveChanges() fails, those PKs values won't be used,
ever.

  So, in theory, there are two transactions, one for obtaining the
  new primary keys values (in practice you get N transactions there),
  the other one for actually saving the changes.

BTW: if you set the env. variable 'MDL_ENABLE_DATABASE_LOGGING' then you'll get
a copy of the framework db-related actions on stderr (or anywhere else if you
modify Modeling.logging) --see the appropriate annex in the User's Guide.

> Also, I've noticed that the ec.fetch() is a transaction which 
> terminates with an abort. Is this correct?

This is intentional, anyhow ;)  Such a transaction should be terminated, and I
cannot see any reason why it should be COMMITed, hence it is ABORTed!  But if
you think differently, please let us know!

> I've not yet experimented with Zope. I see I can connect the
> ec.saveChanges() to the transactions machinery in Zope. Some
> questions:
> 
> 1) how many db transaction are done for each saveChanges (i.e. same as
> above?)

Yes.

> 2) how are the db transaction executed/completed inside a Zope 
> transaction? In a non-zope application, I can see each fetch() 
> produces a transaction, as well as the ec.saveChanges(). Is this true 
> also in Zope applications?

Yes it is.  The zope support basically consists in providing a default
EditingContext per session, possibly also connecting this EC's
saveChanges() to the transaction machinery (i.e. to each request/response loop).
 That's all, and everything else will be handled in your ZProducts' code, just
the same way you would make it in pure python.

	Hope this helps,

-- Sébastien.

Re: [Modeling-users] Query bug when not using ec.saveChanges

[Sebastien B. <sbi...@us...>]

Ernesto Revilla wrote:
> Dear all,
> 
> just a question: can't I fetch objects by attribute access, without 
> having used ec.saveChanges first? See the last 4 lines of the 
> following source code. What restrictions exist using fetch, in respect 
> with ec.saveChanges?
> 
> Thanx,
> Erny
[example snipped]

To be perfectly clear: there should not exist such a restriction.

I'm currently away from any development environment, with an access to my emails
but that's all, so I'm not able to check and diagnose this.  You have found a
bug for sure --even if, for now, I cannot be completely positive about the fact
that it can be easily solved.

  Anyhow, thanks for reporting! may I ask you to fill a bug report? I'll get
back to you next week (I can't say exactly when, I'll be extremely busy for the
10 coming days).

  In case you're in a hurry, you may want to check this by yourself by checking
EditingContext.fetch(): at the end of it, it applies the qualifier (here,
'reservations.startDay like "2004-08-09*"') to its inserted objects of the
correct type/entity.  I suspect that you get no objects because the 'like'
operator tries to compare a string to a Date object, which obviously never
selects any object.

  If this is the case, then the solution is probably to check the type of the
destination attribute (startDay), and, if it is a date, then adjust the
qualifier's value accordingly (may be changing the operator from 'like' to
'between'); or an other (more efficient?) solution could be to compare the
string with the string representation of the attribute's value... but this
would add the constraint that such a string representation should be the same
than the database's representation for dates...

Oh well, anyhow, you get the idea, don't you?-)  I'm pretty confident that the
bug is only triggered with attributes whose type is Date.

-- Sébastien.

Re: [Modeling-users] one model, multiple database instances

[Sebastien B. <sbi...@us...>]

Delio Brignoli in...@dr... wrote:
> Sebastien Bigaret <sbi...@us...> wrote:
> > Without more details, I guess you intend to do some load-balancing 
> > between db-instances in aread-only app., right?
> 
> not at this stage, load balancing if further down the track, now we 
> need to access several DBs with the same schema, it is a requirement 
> that each customer has is own separate database instance; fixing one 
> fixes the other as well.

Oh oh, so this is something completely different than what I initially had in
mind.

  If I understand you right, you have a reference model, something like
  a template model, that you want to use for every customer account.
  Each one will get its own model, say a copy of the reference model,
  that it should use to access its data in a separate database.

But since in the framework, entities' names share a single namespace, you won't
be able to load two different models with entities having the same name in the
same process.

  What seems to be a serious problem at first sight also suggests that
  there exists a possible and easy solution.

Say your reference model contains one entity 'Document', then for each account
'login', you can create a model named e.g. 'model_login', containing the entity
'Document_login'.  This way you'll be able to dynamically create the models,
create or destroy the dedicated databases from them (see the script
mdl_generate_db_schema.py for details on how this can be done), and access each
account's data in a dedicated database (since every 'model_login' will get its
own connection dictionary, as expected).

 [...]
> My knowledge of the framework is limited, I literaly unpacked the 
> source less than 24 hours ago, ported some of our biz object, logic 
> and have one test from the test suit succesfully complete (by the way,
> being able to express the model in python made my life easier) (in
> case you are interested I tried the dynamic metaclass based approach
> first but I'd get an exception when populating the class as soon as I
> defined an __init__ method: check_old_init method tried to len() args
> but python chocked ;) ) ... 

I'm interested, if you could supply a simple example and fill a bug report
@sf.net that would help a lot ;)

> but back to the issue,
> I'll look into the framework more to understand exactly what it's 
> involved in adding this feature and since I can already hear my boss 
> asking it, I'll ask you straight away how much it would cost to 
> contract you to add this feature (and how long it would take).

Hopefully the solution exposed above can make it; if not, then do not hesitate
to get back to me!


Oh, but wait... sorry, I didn't realize you sent an other email with more
informations:

> What I want to achieve:
> 
> When I create an EC, I'd like to pass the ID string as a parameter and 
> this should give me an EC that works 'like' a plain old connection to 
> the mapped database instance.
> 
> How to get there:
> 
> - change the mapping into: ID -> ( connection params, model )
> ?- subclass EC to support the ID string be passed on __init__
> ?- subclass ObjectStoreCoordinator to use the ID string information in
> EC to create/select a proper 		DatabaseContext.
> ?- at which level caches work in modeling: I obviously don't want rows 
> from different database instances to mixup :-P

Okay, so good news, we're definitely talking about the same thing ;)

So what you need to do is: use the 'ID' (login e.g.) to dynamically build (or
load or...) the corresponding model 'model_ID', use the correct entity in this
model e.g. 'Document_ID' when fetching objects for example, and use the correct
class (that you'd probably want to
dynamic.build...() from the model) to build new objects --this way they get
automatically inserted into the right table in the right database 
(look in class ClassDescription or EntityClassDescription for a method
automatically building the right object of the right class given its entity).

  I think this makes everything work the way you want; and obviously you won't
run into any cache trouble or so, since you're just using different models,
that all!

  What do you think?

-- Sébastien.

[Modeling-users] Aggregate values in objects

[Kertis, D. <dk...@vo...>]

Is there a way I can get a field populated with an aggregate value.  i.e., I
want a manager to have a field which would contain how many employees work
for him by doing a select count(*).  I parsed through the fetching section
of the doc and didn't find a way to do this.

 

 

 

 

[Modeling-users] inconsistencies with PyModels

[Lorenzo G. S. <lg...@si...>]

Hi all,

I'm finding some problems when creating a PyModel:

model.addEntity(my_entity) does nothing but
model.entities.append(my_entitye) works

This is not really important and I guess it's easy to fix. The next
problem is more important:

model.build
defaultModelSet().addModel(model)

fails with this exception:

  File
"/home/lgs/proyectos/gruppy/middletier/DOF/m/DomainObjectFramework.py",
line 40, in __init__
    defaultModelSet().addModel(model)
  File "/opt//lib/python2.3/site-packages/Modeling/ModelSet.py", line
138, in addModel
    if aModel.name() in self.modelsNames():
TypeError: 'str' object is not callable

defaultModelSet().addModel(model.component)

works without a warning but I'm not sure if this is the preferred way of
adding a model to the modelset.

Cheers

Lorenzo Gil Sanchez

[Modeling-users] inconsistencies with PyModels

[Lorenzo G. S. <lg...@si...>]

Hi all,

I'm finding some problems when creating a PyModel:

model.addEntity(my_entity) does nothing but
model.entities.append(my_entitye) works

This is not really important and I guess it's easy to fix. The next
problem is more important:

model.build
defaultModelSet().addModel(model)

fails with this exception:

  File
"/home/lgs/proyectos/gruppy/middletier/DOF/m/DomainObjectFramework.py",
line 40, in __init__
    defaultModelSet().addModel(model)
  File "/opt//lib/python2.3/site-packages/Modeling/ModelSet.py", line
138, in addModel
    if aModel.name() in self.modelsNames():
TypeError: 'str' object is not callable

defaultModelSet().addModel(model.component)

works without a warning but I'm not sure if this is the preferred way of
adding a model to the modelset.

Cheers

Lorenzo Gil Sanchez

[Modeling-users] How the transactions are done in the saveChanges?

[Marco B. <m.b...@ic...>]

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Hi all.

I'm experimenting with ModelingFramework, and I've some questions.

I've tried the sample model in from the website, and I've analyzed the
database logs. I've noticed that the ec.saveChanges() in the model is
composed of 5 transactions: the first 4 transactions get the key numbers
from the sequence, the last one is the transaction actually doing the work.

Is this a correct behaviour? What are the reasons behind it?

Also, I've noticed that the ec.fetch() is a transaction which terminates
with an abort. Is this correct?

I've not yet experimented with Zope. I see I can connect the
ec.saveChanges() to the transactions machinery in Zope. Some questions:

1) how many db transaction are done for each saveChanges (i.e. same as
above?)

2) how are the db transaction executed/completed inside a Zope
transaction? In a non-zope application, I can see each fetch() produces
a transaction, as well as the ec.saveChanges(). Is this true also in
Zope applications?

Thanks for the support.

Regards
Marco
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[Modeling-users] Query bug when not using ec.saveChanges

[Ernesto R. <er...@si...>]

Dear all,

just a question: can't I fetch objects by attribute access, without 
having used ec.saveChanges first? See the last 4 lines of the following 
source code. What restrictions exist using fetch, in respect with 
ec.saveChanges?

Thanx,
Erny


"""
A simple model
+-------+    1          0,*   +--------+   1         0,*          +-------------+
| Hotel |<-hotel------rooms->>| Room   |<-room-----reservations->>| Reservation |
+-------+  nullify    cascade +--------+  nullify   cascade       +-------------+
| name  |                     | number |                          | startDay    |
+-------+                     +--------+                          | endDay      |
                                                                  +-------------+
"""

from Modeling.PyModel import *

# valores por defecto para el modelo
Attribute.defaults['usedForLocking']=1
AString.defaults['externalType']='TEXT'
AString.defaults['width'] = 0
AString.defaults['usedForLocking']=1

Association.defaults['delete']=['nullify', 'nullify']

Entity.defaults['properties'] = [
  APrimaryKey('id', isClassProperty=0, isRequired=1, doc='PK')
]

_connDict = {'database': 'reservations.sqlite'}
model = Model('Reservations',adaptorName='SQLite', connDict=_connDict, version='0.1')
model.doc=__doc__

model.entities = [
  Entity('Hotel',
         properties=[ AString('name', isRequired=1),
                    ],
         ),

  Entity('Room',
         properties=[ AInteger('number', isRequired=1),
                      ]
         ),

  Entity('Reservation',
         properties=[ 
                      ADateTime('startDay', isRequired=1),
                      ADateTime('endDay', isRequired=1),
                      ]
         ),
]

# Associations will allways be defined towards the 'one' part.
model.associations = [
    Association('Reservation', 'Room', relations=['room','reservations'],
                multiplicity=[ [1, 1] , [0, None] ],
                delete=['nullify','cascade']),
    Association('Room', 'Hotel', relations=['hotel','rooms'],
                multiplicity=[ [1, 1] , [0, None] ],
                delete=['nullify','deny']),
]

model.build()


def test():
    from Modeling.EditingContext import EditingContext
    from mx.DateTime import Date, Time
    from Reservations.Hotel import Hotel
    from Reservations.Room import Room
    from Reservations.Reservation import Reservation
    ec=EditingContext()
    h=ec.fetch('Hotel','name == "Ritz"') #!!! tiene q ser "Ritz" no 'Ritz'
    if not h:
        h=Hotel(name="Ritz"); ec.insert(h)
        ec.saveChanges()
    else:
        h=h[0]
    r=ec.fetch('Room','number == 101')
    if not r:
        r=Room(number=101); ec.insert(r)
        r.setHotel(h)
        ec.saveChanges()
    else:
        r=r[0]
    
    res=Reservation()
    ec.insert(res)
    res.setRoom(r); r.addToReservations(res)
    res.setStartDay(Date(2004,8,9)); res.setEndDay(Date(2004,8,14))

    # ec.saveChanges()	

    ## This does return an instance in each case
    res=ec.fetch("Reservation",'startDay like "2004-08-09*"')
    print "res normal fetch:",res
    

    ## !!! without ec.saveChanges this does return None !!!
    res=ec.fetch("Room",'reservations.startDay like "2004-08-09*"')
    print "res fetch with attribute traversal:",res




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Re: [Modeling-users] Many-to-many relationship example doesn't work?

[Ernesto R. <er...@si...>]

Hi,

You have to add the instance to both sides of the relation, the
a.setPerson(p)

is missing.

To add an instance to both sides of a relation, you can do also:

p.addObjectToBothSidesOfRelationshipWithKey(a, ’addresses’)

Erny


co...@sa... escribió:

>Hello,
>
>First of all thanks to Sabastien for his efforts creating this framework.
>Projects I'm involved now requires lots of Many-To-Many relations in E-R
>model, so I've tried adding methods (for tranparent access to correlation
>entity) as described in the example in UsersGuide 2.6.3. But those methods
>doesn't work. So I've copy-pasted example from user's guide, but it did'nt
>work as well.:
>i.e I add methods getAddresses(), addToAddresses()
>to the class "Person", Then I try to run a code, that uses those classes:
>-------------
>#test.py
>from Modeling.EditingContext import EditingContext
>from MM1 import Person, Address
>ec=EditingContext()
>p = Person.Person()
>a = Address.Address()
>p.addToAddresses(a)
>ec.insert(p)
>ec.insert(a)
>ec.saveChanges()
>-----------------
>I get an error:
>Traceback (most recent call last):
>  File "./test.py", line 8, in ?
>    p.addToAddresses(a)
>  File "/tmp/test2/MM1/Person.py", line 76, in addToAddresses
>    self.editingContext().insert(_pa)
>AttributeError: 'NoneType' object has no attribute 'insert'
>-----------------
>I've looked into CustomObject.py, and i could'nt find any assigment of
>REAL editing, context, only initial definition 'lambda self: None'
>Did i misunderstood something, or this example code in User's guide is
>just out-dated?
>
>Can someone please give me some guidlines?
>
>I'm using: Python 2.3, Modeling: 0.9-pre-17.1
>
>
>
>
>
>-------------------------------------------------------
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[Modeling-users] Many-to-many relationship example doesn't work?

[<co...@sa...>]

Hello,

First of all thanks to Sabastien for his efforts creating this framework.
Projects I'm involved now requires lots of Many-To-Many relations in E-R
model, so I've tried adding methods (for tranparent access to correlation
entity) as described in the example in UsersGuide 2.6.3. But those methods
doesn't work. So I've copy-pasted example from user's guide, but it did'nt
work as well.:
i.e I add methods getAddresses(), addToAddresses()
to the class "Person", Then I try to run a code, that uses those classes:
-------------
#test.py
from Modeling.EditingContext import EditingContext
from MM1 import Person, Address
ec=EditingContext()
p = Person.Person()
a = Address.Address()
p.addToAddresses(a)
ec.insert(p)
ec.insert(a)
ec.saveChanges()
-----------------
I get an error:
Traceback (most recent call last):
  File "./test.py", line 8, in ?
    p.addToAddresses(a)
  File "/tmp/test2/MM1/Person.py", line 76, in addToAddresses
    self.editingContext().insert(_pa)
AttributeError: 'NoneType' object has no attribute 'insert'
-----------------
I've looked into CustomObject.py, and i could'nt find any assigment of
REAL editing, context, only initial definition 'lambda self: None'
Did i misunderstood something, or this example code in User's guide is
just out-dated?

Can someone please give me some guidlines?

I'm using: Python 2.3, Modeling: 0.9-pre-17.1

Re: [Modeling-users] one model, multiple database instances

[Mario R. <ma...@ru...>]

On Aug 10, 2004, at 7:47 AM, Sebastien Bigaret wrote:
> Delio Brignoli <in...@dr...> wrote:
>> Hello,
>>
>> I'm evaluating the use of modeling for our new project(s) and was
>> wondering if some of you could give me pointers on how to access
>> multiple database instances of the same model in the same
>> application. (I understand that each database instance will be
>> isolated and don't expect to insert object from an objectStore into
>> another) After creating my first model definition and 30 minutes dive
>> in the docs, it seems I'll have to create an EC passing a 'properly
>> crafted' :-) ObjectStore instance. Am I on the right track?
>
> Without more details, I guess you intend to do some load-balancing 
> between db-instances in aread-only app., right?

Another situation in which this could apply is for migrating from one 
db+model to another... a little modeling mapping code could read from 
one EC and populate another that uses a modified model. This may not be 
the most efficient to way to do this, but possibly the most flexible 
and one that would guarantee data integrity and consistency the most. 
Any opinions on whether this would be a good idea or not?

mario

Re: [Modeling-users] one model, multiple database instances

[Delio B. <in...@dr...>]

On 10 Aug 2004 07:47:43 +0200
Sebastien Bigaret <sbi...@us...> wrote:

Hi,
I realized I wrote my first reply in a hurry so I'm writing this up to give you more info on what I want to achieve so you can have a better idea of what's involved.

The application runs in a multithreaded application server, a client to database instance mapping
is done by the application already ( in its first incarnation it was accessing the DB (almost-)directly )
let's say that we have a mapping between an ID string and a database connection string.
These are my ideas and the may not play well with the way modeling is designed, but I hope will be a starting point.

What I want to achieve:

When I create an EC, I'd like to pass the ID string as a parameter and this should give me an EC
that works 'like' a plain old connection to the mapped database instance.

How to get there:

- change the mapping into: ID -> ( connection params, model )
?- subclass EC to support the ID string be passed on __init__
?- subclass ObjectStoreCoordinator to use the ID string information in EC to create/select a proper 		DatabaseContext.
?- at which level caches work in modeling: I obviously don't want rows from different database instances to mixup :-P

regards
--
Delio

Re: [Modeling-users] one model, multiple database instances

[Sebastien B. <sbi...@us...>]

        Hi,

Delio Brignoli <in...@dr...> wrote:
> Hello,
>=20
> I'm evaluating the use of modeling for our new project(s) and was
> wondering if some of you could give me pointers on how to access
> multiple database instances of the same model in the same
> application. (I understand that each database instance will be
> isolated and don't expect to insert object from an objectStore into
> another) After creating my first model definition and 30 minutes dive
> in the docs, it seems I'll have to create an EC passing a 'properly
> crafted' :-) ObjectStore instance. Am I on the right track?

Without more details, I guess you intend to do some load-balancing between =
db-instances in aread-only app., right?

  First, if you want to do that in different processes, simply change
  the model's connection dictionary before using it; each process will
  then use its own db instance.

Regarding EC(parentObjectStore): it's usually used to make child ECs;
passing it a dedicated ObjectStore is completely untested. But you're on
the right track: you can either give it a dedicated DBContext, or use
the default behaviour (which assigns a ObjectStoreCoordinator) and
design your own ObjStoreCoord. so that e.g. it returns different
DBContexts for different ECs.
  In both cases, you'll have to take care of the DBContext's Database
(see e.g. DBContext.handleNotification): either each one gets its own,
or they all share the same.  If every DBContext gets its own Database
object, then each EC should *always* get the same DBContext, or you'll
defeat the snapshotting ref. counting.

-- S=E9bastien.

Re: [Modeling-users] Database Connections

[<so...@la...>]

On Friday 06 August 2004 21:44, Sebastien Bigaret wrote:

[snip]

=46irst .. i'm very busy w/ computer off, right now so ..

Anyways

> I'm not sure this is exact --I've read your post there, and I do not=20
> agree w/ your analysis: I can be wrong of course since my experience w/
> mod_python is really thin, but to my understanding there is a global
> namespace available w/ mod-python, the global namespace that exists
> within the subinterpreter dedicated to an application.


=46irst, i won't talk about apache1 but take care that apache2 isn't really=
=20
stable. I use it in production right now but i disabled the thread support.
I get to much trouble w/ it.  They is 3 kind of process handling in apache2
fork / thread / mixed (i don't recall it but i hope it's true ..:)=20

I won't go deeper in explain but cut/paste the answer i got about MPServlet
(which use mod_python + apache2) from Daniel Popowich:


"""=20
Your questions are related to each other and are frequently asked on
the mod_python discussion list.  The problem with maintaining global
data with mod_python (and servlets doesn't solve this issue) is
tightly related to how apache maintains processes and threads.  If you
are on linux or other unix-like OS, then apache uses the prefork MPM
by default.  This is a multi-process MPM where each request is handled
by a separate process, so you end up with N python interpreters, where
N is the number of processes apache has forked.  In such an
environment, the only way to have global data is through some external
storage: sessions, db, shared memory, etc.

=46or unix, there is the worker MPM which is a hybrid
multi-process/multi-threaded server.  With worker you can control
apache such that you have ONE process with multiple threads.  I know
one person who is using servlets in such a manner.  (This is the
default on windows with the winnt MPM.)  In such an environment you
can have global data shared across requests, but of course, you'll
have to wrap your data in mutexes of some sort to avoid threads
stepping on each other.

If the data you want shared across requests is tied to a particular
user, then sessions are an excellent solution.  Light-weight and
pretty fast.  Servlets has builtin support for sessions; see the api
doc or the tutorial.
"""

In short, using if you're using the fork behaviour, you can't share
data. So in modeling =3D> time consuming import .. and multiple
(with no way to reduce without restart ).. interpreter .. not talking
about the trouble you can get if you have some multiple virtual host=20
etc etc ..

If you want to share data, you need to use the multi-thread, but=20
you have to play w/ python thread/mutexes. And everybody around=20
know I don't want to play w/ thread. In modeling, i don't know if=20
you can do something like this. I mean, share a 'global' editing=20
context, and garantues that it's thread safe. (without of course
ending w/ a lock/unlock around each http request). I use SQObject=20
for a couple of things right now, and I get exactly this kind of=20
troubles. For example SQLite or MySQL DB Api aren't thread safe.=20
(I get weird issues about lost connection in MySQL) ..=20


So that's the dilemma (the post in my blog, is about all kind=20
of web frameworks, but here is example)  If you don't want to=20
share objects, ok but you will loose time because you can use=20
some cache behaviours ..=20
I you want to share, it's ok, but you need to play w/ generator
(=E0la Twisted) or to play w/ the mutexes. And i didn't find a way to
do it the nice way, because most of python modules doesn't
scale very well in threaded env.=20

In my point of view, quixote offer a good way to handle this.=20
Because it use the same prefork / reuse process that apache
does. So you don't have to deal w/ threadObuggy modules, and=20
you can really handle multiples connections without any other
artifact, and without loosing time in big lock/unlock.=20

(I know Zope can handle multiple cnx without too much pain
 by threading. But Zope's threads aren't really used to do=20
 multiple task at the same time. It's just a way to accept http
 request without locking ..)=20

> > Look at http://www.larsen-b.com/Article/130.html for little deeper
> > explain of the problem.
> >
> > I haven't do this kind of stuff for a while.. but i think Modeling need
> > something like Durus do .. a server process.. :)
>
> Interesting indeed, I'll have a look at that. Do you have experience
> with this?  How do they serialize objects that are transmitted between
> the server and its client?

I played w/ it in a recent project, it's more simple than ZODB + ZEO ..=20
but i don't know how it works.=20


While finishing this mail, i remember that's a modeling mailing list, and=20
need to apologize to other that don't want to hear about http handling=20


Hope this helps. / I know i need to go to english course sooooonnnn :)

[Modeling-users] one model, multiple database instances

[Delio B. <in...@dr...>]

Hello,

I'm evaluating the use of modeling for our new project(s) and was wondering if some of you could give me pointers on how to access multiple database instances of the same model in the same application. (I understand
that each database instance will be isolated and don't expect to insert object from an objectStore into another)
After creating my first model definition and 30 minutes dive in the docs, it seems I'll have to create an EC
passing a 'properly crafted' :-) ObjectStore instance. Am I on the right track?

regards
--
Delio

Re: [Modeling-users] Tutorial or example programs

[Mario R. <ma...@ru...>]

On Aug 9, 2004, at 6:12 PM, Sebastien Bigaret wrote:
> Dennis Kertis <djk...@ya...> wrote:
>> Hello,
>>
>> I am trying to get started with Modeling and have
>> reviewed the users guide, but I do not see a complete
>> example application.
>>
>> Rather than try to piece different sections of the
>> users guide together, It would be easier to start with
>> an application that works "out of the box" I can
>> modify to suit my needs.
>
>
> Sorry Dennis, I'm afraid I've no such example at hand.  I know we need=20=

> a
> little demo app., probably along with a real tutorial, but it's not=20
> been
> made yet.
>
>   Maybe someone on the list already have a sample little app. just for
>   demonstration?
>
> -- S=E9bastien.

Ya, a nice complete example would be... quite nice ;) As also would be=20=

a tutorial...
Dennis, the quick example I assume you have seen? It is the core of any=20=

modeling based app, and a more evolved app will necessarily introduce=20
all sorts of other issues that are in principle independent of=20
modeling, such as web/appservers (and which one to use for such  an=20
example?), gui or command line interface issues, and so on. This is in=20=

principle... as clearly each of these imposes some definite=20
constraints, such as the EC and sessions ongoing discussion.

mario

Re: [Modeling-users] Tutorial or example programs

[Sebastien B. <sbi...@us...>]

Dennis Kertis <djk...@ya...> wrote:
> Hello,
>=20
> I am trying to get started with Modeling and have
> reviewed the users guide, but I do not see a complete
> example application.
>=20
> Rather than try to piece different sections of the
> users guide together, It would be easier to start with
> an application that works "out of the box" I can
> modify to suit my needs.


Sorry Dennis, I'm afraid I've no such example at hand.  I know we need a
little demo app., probably along with a real tutorial, but it's not been
made yet.

  Maybe someone on the list already have a sample little app. just for
  demonstration?

-- S=E9bastien.

[Modeling-users] Tutorial or example programs

[Dennis K. <djk...@ya...>]

Hello,

I am trying to get started with Modeling and have
reviewed the users guide, but I do not see a complete
example application.

Rather than try to piece different sections of the
users guide together, It would be easier to start with
an application that works "out of the box" I can
modify to suit my needs.

thanks,
Dennis




		
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Re: [Modeling-users] Database Connections

[Sebastien B. <sbi...@us...>]

Duncan McGreggor <py...@ad...> wrote:
> Since the basic requirements were to have minimal connects to the database
> server, the default behaviour of keeping the connection open is just fine.
>=20
> However, what happens when I use EditingContextSessioning? It seems to cr=
eate
> an EC keyed off a session id. Will that keep one complete editing context=
 per
> user session (and thus one opened database connection per session)? or am=
 I
> missing something?

By default ECSessioning creates an EC per session id, right --but then,
each EC has the same parent ObjectStore, an ObjectStoreCoordinator by
default, which in turn forwards the request (e.g. a fetch) to the
appropriate elements underneath it.

In a word: you get as much ECs as you want, but only one DB-connection
is opened (and left opened).

> >>> If this is the case, then we'll need an other ObjectStoreCoordinator,
> >>> assigning a dedicated DBContext to each EC... Hmmm, just thinking lou=
dly
> >>> here, okay, I'll wait til you confirm the need for such a feature.
> >>
> >> How much work is entailed in this? What would the other OSC do?
> >
> > It would assign specific elements of the Database (DBContext, DBChannel)
> > and Adaptor (AdaptorContext, AdaprotChannel) layers per EC.  At least,
> > that's what I initially thought. There may be another way to ensure
> > different fetchs could run concurrently --but if you can accept that any
> > fecth waits for the running one to finish, it's probably not worth the
> > effort, or am I still missing something?
>=20
> There is no business need or user demand for this app that will require
> super-speeds and concurrent queries per session, so I think it's fine wit=
hout
> going through this trouble... yet ;-)

Fine :)  but that's an idea we should keep in mind.  That will probably
be requested in the future.

  Oh and BTW, Soif hasn't made the remark yet ;) but there is a
  potential problem with caching, since there is no way for now to
  "forget" an object and its corresponding snapshot --except by deleting
  all ECs that reference it.  Keep that in mind, I'll get back on the
  subject before the next release I guess.

-- S=E9bastien.

Re: [Modeling-users] dynamic, metaclasses and Virtual properties

[Ernesto R. <er...@si...>]

Sebastien Bigaret escribió:

>        Hi Erny and all,
>
>On Wed, 30 Jun 2004, Ernesto Revilla <er...@si...> wrote:
>[original post can be found at:
> https://sf.net/mailarchive/forum.php?thread_id=5035017&forum_id=10674]
>
>  
>
>>I think that Python has a design error with properties, but it is easy to
>>correct this. 
>>    
>>
>[demonstration that properties aint behaving very good when it comes to
> inheritance]
>  
>
>>In Modeling, when inheriting from class created with properties (dynamic), it
>>does NOT the inherited methods. Is this reasonable?
>>    
>>
Sometimes I just forget the VERB. I mean 'apply' behind 'NOT' in the 
previous sentence. Properties bind directly to methods/functions (read 
method, write method, delete method). This is the result of descriptors 
implemented in python 2.2. There is still no syntactic sugar to use 
'virtualized' properties.

>You're right, this needs some attention, really.  Could you submit this
>as a bug report or as a patch proposal @sf.net please?
>  
>
I've sent a patch proposal.

>>Here I have attached a
>>little patch for dynamic.py. I made a local function mkProp, so that variable
>>binding is correct, otherwise  it does not work the way it should.
>>    
>>
>
>Can you be more explicit on this point?
>
>  
>
As you can see in the mkProp I create the methods like this:

getter=lambda s: getattr(s,'get'+name)()

Now, as lambda is evaluated, when it is called, it depends of what is 
the binding of the 'name' variable at this time. If I leave this outside 
of  a function and I have this is called several times, name changes, 
and at the end will have the last assigned value, so all getters created 
with the statement will refere effectively to the same attribute, as all 
getters will evaluate 'name' to the same value. Say you have 3 
attributes: name, surname and age, and call the above statement, 3 times 
in the same order for each attribute, you'll get 3 functions, but all 
the three functions will call getAge, because 'name' will bind to 'age'. 
To force the binding, I built the mkProp function and put the 
statement   ( getter=lambda s: getattr(s,'get'+name)() ) into the 
function body. This forces to copy the binding to a local variable, the 
function parameter. So each time I call mkProp, name will be bound to 
another value, and will be retained as the lambda expression refers to it.

I think the same thing can be accomplished in another way, e.g. as you 
do by creating methods from source code. I still haven't tried using 
this approach.

>>Just a
>>remark about the setter. As in the original code, a virtual setter is created
>>only if the attribute is settable in the base class. In this case, we have two
>>alternatives: 1. do not create a setter, as the original code, 2. create
>>always a virtual setter and the setter in the base class raises a
>>ReadOnlyAttributeException. (This is was I have done in a experimental
>>framework.)
>>    
>>
>
>For the moment being we're not handling any 'readOnly' properties on
>attributes --and the one in entity is still ignored for now, so I can't
>really see whazt your point is, I suspect I'm misunderstanding something
>here.
>
>  
>
I used that for a mini testing framework I wrote, where i defined 
read-only attributes. So, please ignore it at this time.

>  Oh, and BTW sorry for taking such a long time to answer :/
>  
>
No problem at all. I just wanted to make the point clear of using 
properties.

Regards, Erny