100-exercises-to-learn-rust/book/src/07_threads/10_patch.md

Update operations

So far we've implemented only insertion and retrieval operations.
Let's see how we can expand the system to provide an update operation.

Legacy updates

In the non-threaded version of the system, updates were fairly straightforward: TicketStore exposed a get_mut method that allowed the caller to obtain a mutable reference to a ticket, and then modify it.

Multithreaded updates

The same strategy won't work in the current multi-threaded version, because the mutable reference would have to be sent over a channel. The borrow checker would stop us, because &mut Ticket doesn't satisfy the 'static lifetime requirement of SyncSender::send.

There are a few ways to work around this limitation. We'll explore a few of them in the following exercises.

Patching

We can't send a &mut Ticket over a channel, therefore we can't mutate on the client-side.
Can we mutate on the server-side?

We can, if we tell the server what needs to be changed. In other words, if we send a patch to the server:

struct TicketPatch {
    id: TicketId,
    title: Option<TicketTitle>,
    description: Option<TicketDescription>,
    status: Option<TicketStatus>,
}

The id field is mandatory, since it's required to identify the ticket that needs to be updated.
All other fields are optional:

• If a field is None, it means that the field should not be changed.
• If a field is Some(value), it means that the field should be changed to value.