That's nice. We could do something like:
(-> (swap! ...)
force
:access_token)
On Mon, Jun 15, 2020 at 7:57 PM Justin Smith <noisesm...@gmail.com> wrote:
> The usage of delay here is clever. I suggest as an addition, using
> `force` instead of `deref` to disambiguate delay vs. atom (of course
> if you take a few moments to think about it, swap! shouldn't return an
> atom etc., but I think it becomes clearer with force).
>
> On Mon, Jun 15, 2020 at 10:34 AM Ernesto Garcia <titogar...@gmail.com>
> wrote:
> >
> > Hi, it's a long time that this question was posted, but I have found it
> interesting in the implementation of token refreshes.
> >
> > First of all, for service invocation, given a `revise-oauth-token`
> method, I think this is good client code:
> >
> > (http/request
> > {:method :get
> > :url "https://example.com/";
> > :oauth-token (revise-oauth-token token-store)})
> >
> > If you find it too repetitive or fragile in your client code, you can
> make a local function, but I wouldn't abstract the service invocation at a
> higher layer.
> >
> > Regarding the implementation of the token store, we could initially
> think of a synchronized store, like an atom, and `revise-oauth-token` would
> swap its content when a refresh is required. This is inconvenient for
> multithreaded clients, because there could be several refresh invocations
> going on concurrently.
> >
> > In order to avoid concurrent refreshes, I propose to implement the token
> store as an atom of promises. Implementation of `revise-oauth-token` would
> be:
> >
> > (defn revise-oauth-token [token-store]
> > (:access_token
> > @(swap! token-store
> > (fn [token-promise]
> > (if (token-needs-refresh? @token-promise (Instant/now))
> > (delay (refresh-oauth-token (:refresh_token @token-promise)))
> > token-promise)))))
> >
> > Note that using a delay avoids running `refresh-oauth-token` within the
> `swap!` operation, as this operation may be run multiple times.
> > Also note that `token-needs-refresh` takes an argument with the present
> time. This keeps the function pure, which could help for unit testing, for
> example.
> >
> > There is an alternative implementation using `compare-and-set!` that
> avoids checking `token-needs-refresh?` several times, but it is more
> complicated. I have posted full sample code in a gist:
> https://gist.github.com/titogarcia/4f09bcc5fa38fbdc1076954b9a99a8fc
> >
> > Remark: None of this refers to "functional programming" per se. Dealing
> with state in a purely functional way involves using different constructs
> (like possibly monads, for which you can find Clojure libraries if you are
> interested), and best practices are still a topic of research. Clojure has
> taken the pragmatic approach of making purely functional code easy to
> write, but it doesn't reject the use of state, rather it provides
> well-behaved primitives like vars, atoms, agents, etc.
> >
> > Ernesto
> >
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