synchronized counter in clojure

问题:

If I want to keep a global counter (e.g. to count number of incoming requests across multiple threads), then the best way to do in java would be to use a volatile int. Assuming, clojure is being used is there a better (better throughput) way to do?

回答1:

I would do this with an atom in Clojure:

(def counter (atom 0N))

;; increment the counter
(swap! counter inc)

;; read the counter
@counter
=> 1

This is totally thread-safe, and surprisingly high performance. Also, since it uses Clojure's abitrary-precision numeric handling, it isn't vulnerable to integer overflows in the way that a volatile int can be.....

回答2:

Define a global counter as an agent

(def counter (agent 0))

To increase the value contained in the agent you send a function (in this case inc) to the agent:

(send counter inc)

To read the current value you can use deref or the @ reader macro:

@counter ;; same as (deref counter)

Agents are only one of several available reference types. You can read more about these things on the Clojure website:

• High-level overview
• Software transactional memory with refs
• Asynchronous agents
• Atoms