The context, cancelations, and timeouts
In Chapter 2, we showed that closing a channel shared between multiple goroutines is a good way to signal cancelation. Cancelations may happen in different ways: a failure in a part of the computation may invalidate the entire result, the computation may last so long that it times out, or the requester notifies the server application that it is no longer interested in the result by closing the network connection. So, it makes sense to pass a channel to the functions that are called to handle a request. But you have to be careful: you can close a channel only once. Closing a closed channel will panic. Here, the term “request” should be taken in an abstract sense: it can be an API request submitted to a server, or it can simply be a function call to handle a particular piece of a larger computation.