Use-case diagram: When to decompose use-case and when not to?

For an example, say, I want to draw a use-case diagram for an artificial vending machine like so:

• The vending machine have 3 options: carbonated drinks, tea, and pure water.

• The user can choose only 1 of the above for an order:

  • If the user chooses carbonated drinks, the user needs to choose which brand: say Coke or Pepsi.
  • Else if the user chooses tea, then he/she have to choose the amount of sugar.
  • Else if the user chooses pure water, then that's it.

• After choosing the drinks, the user then has to choose the number of drinks (a little bit contrived). The machine then sends out a confirmation message for the user to confirm.

I tried this but not really sure of it, don't know what to keep, what to add or what to remove..

The questions are:

• Should the 2 Choose the number of drinks and Confirm here be kept independently as use-cases, or should they be merge with the grand use-case: Buy a kind of drink?

• If at least 1 of them is to be kept, should I draw them as including use-case of Buy a kind of drink or should they be drawn as independent use-cases?

• The 3 use-cases Buy carbonated, Buy tea, Buy pure water I considered as the discretization of Buy a kind of drink. For Buy carbonated and Buy tea, should I keep the including use-cases: Choose brand and Choose sugar amount or should I remove them also?

• More general question: When should a step of doing some big use-case should be kept as a use-case, and when shouldn't? There are a lot of questions on here that are the same, but the answers are non-consistent.

Use cases are about the what and not about the how. The latter goes into Activity and is split to Actions. Use case only show the added value a system under consideration brings to its actors.

So the only added value is Sell drinkand that's it.

I recommend reading Bittner/Spence about use cases. They call it synthesis and that's the right word. Not analysis and even less decomposition.

Strange, that techies always want to decompose things. Well, I know it's hard to learn since I had been scratching my head too. It's probably so complicated because its about simplicity.