Re: Transparent Object Persistence

Hakan, changes to all non-transient fields would be encapsulated in
transactions, obviously. And yes, of course all reachable classes should be
enhanced. Anything less would be pointless.

As for finding objects, as I said they can be done in any way: field
access, method calls, whatever.

The point is that the bytecode instrumentation would be able to track all
of it, regardless, so you wouldn't even have to think about how you are
accessing and/or modifying the members of the object.

On 26 November 2013 10:12, hakan eryargi <[email protected]> wrote:

> I guess such a system may be possible without annotations but will require
> very sophisticated code analysis. I see two main difficulties where
> annotations may really help:
>
> * what should be encapsuated in a transaction? naming convention may help,
> such as methods named as setXX can be assumed transactions. but what about
> boundaries? starting from root class, should we enhance all reachable
> classes? including third party and system classes?
>
> * how to find an object starting from root? to execute a transaction on an
> object we should first find it, but how?
>
>
> On Mon, Nov 25, 2013 at 11:59 PM, Naveen Chawla <[email protected]>wrote:
>
>> Hi Hakan, are you sure that annotations would be necessary? I wrote a
>> compact suggestion that doesn't require annotations a while back ("Future
>> Prevayler").
>>
>> If bytecode manipulation is used, then surely field accesses can be
>> intercepted, and perhaps even object locks can be acquired via bytecode
>> automatically during accesses, allowing you to do something like:
>>
>> final MyObject prevalentObject = Prevayler.<MyObject>connect("name");
>>
>> /*or ("name", new MyObject(x, y, z)) if there's no default constructor or
>> you wanted to set a special default state.*/
>>
>> on start up, upon which "prevalentObject" would become the object
>> recovered from the snapshot and/or journal if it exists, and then all
>> subsequent access to "prevalentObject" in your code would be synchronized
>> over (and journalled in the case of changes) automatically.
>>
>> i.e. nothing else necessary.
>>
>> Am I wrong that this is possible? I haven't done bytecode manipulation
>> before but it definitely says in AspectJ
>> http://theholyjava.wordpress.com/tag/aspectj/ that it can intercept
>> field access, and acquiring locks over objects is a simple bytecode
>> instruction.
>>
>>
>> On 24 November 2013 21:32, hakan eryargi <[email protected]> wrote:
>>
>>> come on, AFAIK object databases does not store POJO's. I'm talking about
>>> persisting POJO's transparently here.
>>>
>>> for tx boundaries, it's determined by methods with @Writer (@Transaction
>>> is better I suppose) annotations which are in classes that can be accessed
>>> starting from Root.
>>> On Nov 24, 2013 10:31 PM, "Justin T. Sampson" <[email protected]>
>>> wrote:
>>>
>>>> That's not revolutionary, that's an object-oriented database. :) Not a
>>>> bad idea, but a different idea than Prevayler.
>>>>
>>>> The essential issue is how to demarcate transaction boundaries. It's
>>>> not sufficient for individual setter calls to be their own transactions, so
>>>> you've gotta have some higher-level coordination.
>>>>
>>>> Prevayler itself does this using a variant of the Command Object design
>>>> pattern, where each command object encapsulates a single transaction. The
>>>> Facade design pattern is the most obvious alternative, where every method
>>>> declared on the facade demarcates a transaction.
>>>>
>>>> Both of those options are legitimate object-oriented design patterns. :)
>>>>
>>>> Domain-Driven Design offers another way to think of transactional
>>>> boundaries: The Aggregate design pattern is intended precisely to demarcate
>>>> transactional boundaries in a domain model. So that might be a fruitful
>>>> area of study if you're concerned about the design side of things.
>>>> On Nov 24, 2013 10:10 AM, "hakan eryargi" <[email protected]>
>>>> wrote:
>>>>
>>>>> Justin, thanks for the pointer:) The Facade idea is definetely not
>>>>> bad, but I think regarding the possibilities it's a bit unsufficient. The
>>>>> problem IMHO is, Prevayler itself honors object oriented design but the
>>>>> Facade idea -in contrast- utilizes service oriented design and introduces
>>>>> an uncessary layer for persistence.
>>>>>
>>>>> Wouldnt it be great if objects are transparently persisted?
>>>>>
>>>>> Root root = Persister.create(Root.class)
>>>>>
>>>>> class Root implements Serializable {
>>>>>    @Writer
>>>>>    void setSomeProperty(X x) {}
>>>>>
>>>>>    @Writer
>>>>>    void setSomeOtherProperty(Y y) {}
>>>>>
>>>>>    @Reader
>>>>>    X getSomeProperty() {}
>>>>> }
>>>>>
>>>>>
>>>>> For top level class (the root of object graph) I suppose that's a
>>>>> piece of cake to implement with aspects or bytecode enhancement.
>>>>>
>>>>> For child classes, there is a difficulty of accesing target object
>>>>> while executing the transaction. But I believe this can be solved to some
>>>>> degree by replacing collections with wrapper collections.
>>>>>
>>>>> For example a typical pattern is:
>>>>>
>>>>> class Root {
>>>>>    Map<Integer, Child> children;
>>>>>
>>>>>    @Writer
>>>>>    void saveChild(Child c) {
>>>>>        children.put(c.id, c);
>>>>>    }
>>>>>
>>>>>     @Reader
>>>>>    Child getChild(int id) {
>>>>>       return children.get(id);
>>>>>    }
>>>>> }
>>>>>
>>>>>  class Child {
>>>>>    @Key
>>>>>    int id;
>>>>>
>>>>>    @Writer
>>>>>    void setSomeProperty(X x) {}
>>>>> }
>>>>>
>>>>> Now, If we replace children map with a special wrapper map, at the
>>>>> point of insertion, we can store the access path to the child in a special
>>>>> bytecode enhanced field of Child. Something like: parentClass: Root,
>>>>> id=idValue. So the transaction will be like (or the reflective equivalent) :
>>>>>
>>>>> root.children.get(id).setSomeProperty(x);
>>>>>
>>>>> If the Child has no filled in path field than it means it's not
>>>>> attached to Root and write action should not be encapsulated in a
>>>>> transaction. If it's later inserted into object graph, then it will be
>>>>> written as it's (its properties are set etc) in a transaction.
>>>>>
>>>>> For deeper classes in hierarchy it's more complicated but I guess same
>>>>> principle can be applied. Just store the parent's key value in
>>>>> path-to-object field, and when write method is executing, traverse up in
>>>>> the hierarchy and check if Root is accesible. If so, object can be accessed
>>>>> in the same way:
>>>>>
>>>>> root.children.get(id).someOtherChilren.get(otherId)...
>>>>> setSomeProperty(x);
>>>>>
>>>>> Some annotations may also help this process.
>>>>>
>>>>> @Path("Root/chilren/{id}/someOtherChildren/{otherId}")
>>>>> class SomeChild {
>>>>> }
>>>>>
>>>>> Of course this is much more complicated then I wrote here and several
>>>>> unforseen problems may occur (for example arrays can not be handled this
>>>>> way) but I belive it may worth the effort.
>>>>>
>>>>>  Combine this with Spring's dependency injection:
>>>>>
>>>>> <prevayler:component-scan base-package="some.package" />
>>>>>
>>>>> package some.package;
>>>>> @Root // a persisted object graph root
>>>>> class Root {
>>>>> }
>>>>>
>>>>> Root can be injected:
>>>>>
>>>>> class SomeBean {
>>>>>    @Autowired
>>>>>    Root root;
>>>>> }
>>>>>
>>>>> And maybe even child classes can be injected:
>>>>>
>>>>> class SomeOtherBean {
>>>>>    @Autowired
>>>>>    @PathToChild("/Root/42") // 42 is id of child
>>>>>    Child child;
>>>>> }
>>>>>
>>>>> And other beans can be injected into persisted objects too
>>>>>
>>>>> class Root  {
>>>>>     @Autowired
>>>>>     transient EmailService emailService;
>>>>> }
>>>>>
>>>>> Wouldn't that be revolutionary? What do you think?
>>>>>
>>>>> *r a f t*
>>>>>
>>>>>
>>>>> On Fri, Nov 22, 2013 at 8:40 AM, Justin T. Sampson <[email protected]
>>>>> > wrote:
>>>>>
>>>>>> Yes, Prevayler uses synchronized(prevalentSystem){...} internally for
>>>>>> both queries and transactions (plus additional synchronization for
>>>>>> transactions to get the journal sequencing right).
>>>>>>
>>>>>> A few people have played with the latter idea. There's even a
>>>>>> proof-of-concept included with Prevayler, in the extras/facade module.
>>>>>>
>>>>>>
>>>>>>
>>>>>>  On Thu, Nov 21, 2013 at 11:20 AM, hakan eryargi <
>>>>>> [email protected]> wrote:
>>>>>>
>>>>>>>  is it enough to sync on root object for reads? writing a query
>>>>>>> class for each read is really verbose.
>>>>>>>
>>>>>>> indeed even writing tx classes for writes is also verbose. a future
>>>>>>> improvement may be using magic @annotations very much like spring's.
>>>>>>>
>>>>>>> r a f t
>>>>>>>
>>>>>>>
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