`watch` Carries State; `broadcast` Carries Events

Tokio has two channels for sending updates to multiple receivers. The choice starts with the receiver’s job: does it need the current value, or does it need to handle messages in send order? Tokio’s channel overview describes both patterns.

Use watch for a current value

A watch channel retains only the latest value. Each receiver tracks whether it has seen that value. If several updates arrive while a receiver is busy, it can observe the newest value without observing every intermediate one. That fits a configuration snapshot or a shutdown state. The receiver can catch up by reading what is true now.

A watch channel starts with an initial value. That value is already marked as seen for changed(), including on a newly subscribed receiver. Read it directly with borrow() or borrow_and_update() when starting work; waiting for changed() alone waits for a later send. In a loop that awaits changed(), Tokio recommends borrow_and_update() to avoid processing the same value twice if a send races with the read.

Use broadcast for a stream of messages

A broadcast channel lets each active receiver read sent messages in order. A new subscriber receives messages sent after it subscribes. This suits an event stream where each receiver has a separate reaction to each delivered message.

Capacity matters. Tokio bounds the retained messages. If a receiver falls behind and an old message is overwritten, its next receive reports RecvError::Lagged. The receiver then resumes at the oldest retained message. It must decide whether that gap is acceptable. A larger capacity gives a slow receiver more room, but it does not turn the channel into a durable event history.

What to do

1. Write down what a receiver needs after a pause: the latest state or every event.
2. Choose watch for state, and read its initial value before waiting for changes.
3. Choose broadcast for messages that each active receiver should handle. Set a capacity and handle Lagged explicitly.
4. If losing an event would break the task, make recovery from a gap part of the design before relying on broadcast.