New link.

Consider renaming bookid into Book to have consistent properties names.

I have an Atelier question.

How's that Atelier-related though?

There are some utilities for work with images. For example QR code generation via %SYS.QRCode.  There is also this Captcha generation project.

That said, imagemagick is doubtlessly the best solution.

SQL way (query docs, 1 means BS):

SELECT *
FROM Ens_Config.Production_EnumerateConfigItems('Your.Production', 1)

Object way:

set rs = ##class(Ens.Config.Production).EnumerateConfigItemsFunc("Your.Production", 1)
do rs.%Display()

Constants for Business Host Type (defined in EnsConstants.inc):

#define eHostTypeUnknown   0
#define eHostTypeService   1
#define eHostTypeProcess   2
#define eHostTypeOperation 3
#define eHostTypeActor     4

Postconditionals are to be used only for the most simple checks, for example:

set:a="" a=10

instead of

if a="" {
  set a=10
}

But several conditions (or several lines inside) should be as if, I agree.

For one-line if I prefer postconditionals:

set:a b=a

As for For - the option with braces. It's more readable. And one line inside For loop quickly and often becomes several lines, so braces become mandatory anyway.

Why do you need to check it at runtime? If source and target are from different but related  classes you can copy by their common ancestor (or by specified ancestor class). Or do you copy properties between two unrelated classes?

Nice example of using method generators.

Why do you need ExistsProperty? Here's a check I usually employ for class properties in generators:

 if prop.Internal || prop.Calculated || prop.ReadOnly || prop.Private || prop.Identity || prop.MultiDimensional  CONTINUE

It filters out all systems properties like Concurrency, %%OID and so on.

When talking about speed, one of the most important questions is what exactly do you need to speed up? You need to determine:

• what queries are the most popular
• what queries take the most time

to fix it. Some ways to fix performance are (easy to hard):

• Add indices
• Change classes
• Change application architecture

Because in your case I see at least two request types which require different actions to speed them up:

• Get last X changes
• Get last X modified subjects

Versioning for example. Temporal queries and so on.

Header itself is another layer because it's meta information for the BPL state machine, which you're inside of. So I don't see how you can get header Id from inside the state machine without violating abstractions in place.

Still, %Context, %Process and %LastError are documented and acceptable to use.

You can access %Ensemble("%Process") as ..%Process so:

..%Process.%PrimaryRequestHeader.%Id()

Reference WebDAV implementation for Caché.

Check out Caché FileServer project for a sample of file upload/download.

Use any of the existing Git hooks for Studio or Atelier.  They would work with GitHub and GitLab both.

Check EvaluateRules method of Ens.Rule.Definition class. It evaluates rule by name and context. You can call it from Studio or Terminal.