Thanks, Dan!

That definitely makes sense.

What I like about object callbacks is the code readability. Compare:

Method %OnBeforeSave(insert as %Boolean) As %Status [ Private, ServerOnly = 1 ]

{

if insert s ..CreationDate=$H

q $$$OK }

And 

Property CreationDate As %TimeStamp [ SqlComputeCode = { set {*}=$zdt($h,3) }, SqlComputed, SqlComputeOnChange = %%INSERT ];

What if I would need to refer to other properties in

{*)=expression() 

Or the expression would be complex and if I want to debug it?

Is there a way to keep callbacks readability and have the callback methods  be fired for object and SQL access both?

Something like:

Property CreationDate As %TimeStamp [ SqlComputeCode = { set {*}=..%OnBeforeSave(1) }, SqlComputed, SqlComputeOnChange = %%INSERT ];

Thank you, Rubens, Fabian, Sean.

Agreed, that Initial expression is the best option here. And thanks for the explanations on the callbacks!

Thanks, Sean!

JSON-RPC library

Would be great to look into this if you want to expose it some day.

Thanks, Fabian!

Yes, you are right and I agree. George, thank you very much for thorough explanations.

So, it's never safe to go without storage schema (even with default storage schema) if you want to either to delete or add new class properties.

That's quite clear. Question closed.

So, in the end, if I want to store in VCS only class body (without storage schema)  it is safe to do it if:

I use only default storage schema in this project;

I never delete properties and use "deprecated" clause instead.

Right?

Right. But I would never delete and change the name of the properties if you have the data behind them already. I prefer "deprecated" approach of "deleting" unused properties.

I think it is a good candidate for another question. 

Yes, sure. If you customized the default storage (which is not very recommended, see @Kyle.Baxter's answer) on purpose it becomes the part of the solution and should be stored in VCS along with class def. 

There is a qualifier in export utility:

$System.OBJ.Export()

d $System.OBJ.ShowQualifiers()
/skipstorage

If you have customised the storage schema

But how can I customize it? Do you mean manually?

When I export the class def to the file I have the regenerated schema every time. So the only way to customize it is to change the schema manually in exported file. And this would be the risk to experience problems after importing that class.

To be clear, I do keep schema with the class def in Git and I never ever changed (hope so) this part of the class definition. It is useful for me only for information purposes to know where  the data is stored.

 So here is my question: why should I export and keep this part of class def if I don't want to know where is the data and don't want to have risk to manually change the storage data?

This can have serious consequences as data may get stored in a different place from data stored by previous versions of the class.

I believe if you change properties and indexes of the persistent class it would definitely cause  serious consequences on the data of previous versions of the class (but happily here we have the recipe of data conversion and etc).

Why not: if you import the class without schema it would be generated automatically according to the class description.  So why to export and keep it in VCS?