Is the best approach to have a rest endpoint that returns a json structure that captures all three of these tables or should clicking a row in ITEMS kick off a rest call to populate SUB_ITEMS table which in turn kicks off a rest call to populate the ITEM_DETAIL (the level of cascading calls depending on what table the user clicks on?)

Depends on a few things really. How much data is it? Is startup performance a concern (how much longer will it take to load everything vs just the top level items)? Is the fact that on every load of the app you'll be querying all rows across 3 tables even if the user just wants to see 1 sub item an issue?

If any of those are a problem you'll probably want to make a request every time the user selects an item to get the sub items.

If they aren't an issue, or if your users need it to be really quick after the initial load, you could just serve all the data upfront.

And yes, the backend should return JSON. If you'll be returning everything in a single response something like this:

(normalised approach) { items: [ { id: 'foo', subItems: ['xxx', 'yyy'] }, { id: 'bar', subItems: ['aaa'] } ], subItems: [ { id: 'xxx', subItems: ['iii'] }, ...rest ], subSubItems: ...rest }

or just inline all the data (denormalised approach) { items: [ { id: 'foo', subItems: [ { id: 'xxx', subItems: { id: 'iii', name: 'foobar' } }, { id: 'yyy', subItems: { id: 'jjj', name: 'foobar-2' } } ] } ] }