Re: Prevayler - update data

I knew from the start what you meant and I will indeed write text something like that. There are a few books out there I've found extremely pedagogic and I'm going to browse them a bit before I get down and dirty with text.  "Lucene in action" is one of those, they first write a bit about a subject and then demonstrate it using snippets of unit test code and then elaborate further using more text, not too different from what you suggest.

I do however find that it make sense to also demonstrate caveats they way I've done. My hypothesis is that if they are all uniform (i.e. all the test cases I've committed so far are more or less the same with just a minor change to demonstrate a specific problem) it's easy to under them all once you figured out the first one. They are however not to be the base for the booklet I'm about to write, if anything they're to be appendices and something to point users to when a frequently asked question show up in this forum, etc.

Just downloaded OpenOffice in order to get started with the writing... I have about two weeks to spend on this before my next paying gig starts.

		karl

On Nov 3, 2012, at 1:11 AM, Naveen Chawla wrote:

> Just to clarify, everything I wrote in between < > was incomplete stuff.
> 
> On 2 November 2012 12:45, Naveen Chawla <[email protected]> wrote:
> Dear Karl,
> 
> Sorry I didn't mean to suggest I think it should necessarily be exemplified, only that I use a simple "stick to prevalentSystem (when making changes)" motto, which covers all cases.
> 
> I've made an example skeleton of how I roughly pictured the manual to begin, although you can ignore this 100% if you wish:
> 
> <...Short intro why Prevayler is so phenomenally fantastic etc.>
> 
> The following guide shows how to make a Java object prevalent:
> 
> Let's say you have a class:
> 
> -------
> class Root{
> 	<...>
> }
> -------
> 
> To make it prevalent via Prevayler, type:
> 
> -------
> Prevayler<Root> prevayler = PrevaylerFactory.createPrevayler(new Root());
> -------
> 
> To be recoverable, any changes to your prevalent object need to be encapsulated in Transactions:
> 
> ------
> class MyTransaction implements Transaction<Root>{
> 	public void executeOn(Root prevalentSystem, Date date){
> 		//...
> 	}
> }
> ------
> 
> ...and performed via the given <b>prevalentSystem</b> reference.
> 
> <You could but not necessarily put an example overlooking this requirement (i.e. a simple change made outside a Transaction) here>
> 
> When you want to execute the transaction from your program, call Prevayler#execute:
> 
> ------
> prevayler.execute(new MyTransaction());
> ------
> 
> You can pass objects in to your transaction as follows:
> 
> ------
> prevayler.execute(new MyTransaction("Somebody");
> ------
> 
> ...with a class such as:
> 
> ------
> class MyTransaction implements Transaction<Root>{
> 	
> 	String name;
> 
> 	public MyTransaction(String name){
> 		this.name = name;
> 	}
> 
> 	public void executeOn(Root prevalentSystem, Date date){
> 		prevalentSystem.entities.put(this.name, new Entity(this.name));
> 	}
> }
> ------
> 
> However, you must bear one thing in mind: <b>your Transaction object is serialized to disk when executed live, so anything it references will be deserialized from scratch during recovery execution</b>. This basically means only the <b><code>prevalentSystem</code></b> reference can be relied upon to reference anything in your prevalent object. Here's an example overlooking this (also known as the "baptism problem"):
> 
> <...>
> 
> etc.
> 
> On 2 November 2012 09:05, Karl Wettin <[email protected]> wrote:
> 
> On Nov 1, 2012, at 9:28 PM, Naveen Chawla wrote:
> 
>> Although it's not part of the "baptism problem", any change to the prevalent object outside of any transaction is also not recoverable. So I like to simplify it by just thinking "stick to prevalentSystem (when making changes)" (while picturing the "prevalentSystem" parameter in a Transaction's "executeOn" method). This may or may not help.
>> 
> 
> <https://github.com/karlwettin/prevayler-examples/blob/master/src/test/java/org/prevayler/examples/e105/E105Test.java>
> 
> How's that?
> 
> 
> 			karl
> 
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> 
> 
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Stay in control, update software, and manage PCs from one command center
Diagnose problems and improve visibility into emerging IT issues
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