Code placement depends on what you're trying to do and when: if processing the event from javascript (ajax call) then your code must be separate from OnPreHTTP(); if processing on the server (after page submit) then your code should be in OnPreHTTP() or called from it (this last part depends on your coding style - jam everything in a single method or separate things in multiple smaller methods)

In simple terms (as Robert pointed out) , OnPreHTTP needs to handle any processing the server must perform prior to "displaying"/"printing" the page on the browser.

Correct. The alternative using TextReader was to "avoid" increasing memory but if you have that option then the answers provided by Timothy are the way to go.

Understand. I haven't tried this myself yet but in theory you should be able to initialize the Reader with an XML.Document object. This Document can be initialized (%OnNew) with a global of your choice in the same manner that TextReader can. 

If you want to check on the result of a LOCK command for example 

LOCK +<SOMERESOURCE>:<timeout>

IF $T {do something}

So I wouldn't call it bad practice just yet.

Agree with brace always. This really helps with new developers as the syntax is something they've seen before in other languages. 

I understand the accessing part but by creating a class mapping I'm enabling SQL access to the existing global. I guess that the question is more in line on whether sharding will be able to properly partition (shard) SQL tables that are the result of global mapping? Are there any constraints on how the %Persistent class (and the storage) is defined in order for it to work with sharding? Should they all use %CacheStorage or can they use %CacheSQLStorage (as with mappings)? 

Let's say I have an orders global with the following structure:

^ORD(<ID>)=customerId~locationId.....

And I create a mapping class for this global: MyPackage.Order

Can I use sharding over this table?

Alex, it's all based on your design. The decision of whether a method is an instance or class method is totally independent of whether that method can be private or not. Your design, and patterns or best practices you want to follow, will dictate which way to go. The benefits are in what you want to accomplish with the design: ease of testing, ease of change, etc. 

Can we infer form this that sharding can be applied to globals that have been mapped to classes (thus providing SQL access)?

You're totally right. I forgot about this syntax.

The only "caution" here is that referencing the property by name only works with properties where the SqlFieldName corresponds to a valid Cache property name:

1. Property myproperty As %String;
2. Property myproperty As %String [SqlFieldName = "mysqlname"];
3. Property myproperty As %String [SqlFieldName="my_property"];

You can access #1 & #2 by property name, however #3 you can only access by using %Get("my_property");

Can you provide more details on what exactly you're trying to accomplish? As you said, the last query works. What exactly is missing? Are you trying to get a prompt (outside of Cache I'm guessing) with the parameter name?

Yeah, that's what I was referring to :-(

Cache doesn't provide this "out of the box" but you can always create a class that works as a Collection (a collection of IDs if you will) and provides an Iterator interface/functionality for to you iterate over the values.  You can always expose and access data through Objects as you need to.

$ORDER would do. Not the cleanest (or preferred) but the point being you can do without SQL.

You can manage without. You have plenty of Object Oriented options to get to the items.  Or you could use $O over the global to get the IDs (similar to what Extent would do)

Can you elaborate on this: "For performance reasons we suggest using Foreign Keys instead of relationships."

My understanding is that Relationships will create the foreign keys (and the relationship table) for you similar as you would do in the relational world:

TableA

AID

TableB

BID

TableABRelationship

AID

BID

FK: AID

FK: BID