add SharedObjectContext example to cart_object.rs

[kpwebb]

SharedObjectContext is missing from the tutorial/example code, so added this CartObject/list handler. While this is covered in the SDK docs, nothing on the main Restate Docs site covers the shared/read-only functionality. Including it in the tour seems useful.

[gvdongen]

Hi @kpwebb,

Thank you for the addition. Shared handlers are part of the Virtual Objects explanation in the Rust docs:

I will add some extra docs there. I would prefer not adding it to the Tour since the Tour is already quite long.

I do understand though that this concept is not prominent enough... I need to think for a second on how this can be improved.

Thanks again.

[kpwebb]

Thanks! I think a mention or example of the read-only option early on would be helpful. I searched the docs site for read only but couldn’t find a reference so I didn’t check the SDK docs.

Also I just realized I that there are some places in your online tutorials (e.g. the Gentle Introduction video) where its mentioned that all calls are processed sequentially. I get the emphasis on preserving order / transactional integrity, but there are lots of applications that benefit from having a non-blocking read-only path. The way you all have implemented both options is fantastic and worthy of highlighting.