Hi Tianxiang,
In my experience a good way to decouple a non-trivial fn with metrics is to
make the fn provide hooks for various events when metrics may be gathered.
(defn nop [& args])
(defn foo->bar [foo {:keys [on-foo on-bar] :or {on-foo nop on-bar nop} :as
options}]
(on-foo)
(let [bar (produce-bar foo)]
(on-bar)
bar))
Now, when unit testing for functionality of foo->bar you can pass {} as
options, which wouldn't do any metrics collection. In actual usage, or for
unit testing metrics collection, you would pass {:on-foo #(counter/inc!
foo-counter) :on-bar #(counter/inc! bar-counter)} as options.
This is a small fn, so it may not be apparent how can providing hooks be
useful, but for large fns with multiple metrics-collection I've found
providing hooks to be working quite well.
HTH
Shantanu
On Saturday, 12 November 2016 08:24:40 UTC+5:30, Tianxiang Xiong wrote:
>
> What are some good ways of handling metric gathering (or, more generally,
> AOP) in Clojure?
>
> Suppose we have an application in which there is a transformation
> function, foo->bar. Every time a foo is input to foo->bar, we'd like to
> increment the foo-counter. Every time a bar is produced, we'd like to
> increment the bar-counter.
>
> To keep foo->bar pure, we can wrap foo->bar in another function like:
>
> (fn [& args]
> (counter/inc! foo-counter)
> (let [result (apply f args)]
> (counter/inc! bar-counter)
> result))
>
> But the side effect of incrementing counters makes it difficult to write a
> unit test.
>
> I have a vague idea of what I'd *like* to see, which is the function
> returning:
>
> - The result
> - Some operation to be carried out, OR some data allowing the receiver
> to carry out some operation?
>
> I'm sure this is not a new problem for Clojure applications; what are some
> strategies for handling it?
>
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