closeable-map

Your application state is like your hens: it's safe when it is securely contained in a chicken coop with automated doors to prevent chicken run-away. Think about Zelda: when hens are free to propagate everywhere, they attack you and it becomes a mess.

This small library defines a new type of Clojure map that you may (.close m). See it in action above. It is a tiny alternative to more capable projects:

• Application state management: stuartsierra/component, weavejester/integrant, tolitius/mount, et al.

• Extension of with-open: jarohen/with-open

• Representing state in a map: robertluo/fun-map

In your project, require:

(require '[piotr-yuxuan.closeable-map :as closeable-map :refer [close-with with-tag]])

Define an application that can be started, and closed.

(defn start
  "Return a map describing a running application, and which values may
  be closed."
  [config]
  (closeable-map/closeable-map
    {;; Kafka producers/consumers are `java.io.Closeable`.
     :producer (kafka-producer config)
     :consumer (kafka-consumer config)}))

You can start/stop the app in the repl with:

(comment
  (def config (load-config))
  (def system (start config))

  ;; Stop/close all processes/resources with:
  (.close system)
)

It can be used in conjunction with with-open in test file to create well-contained, independent tests:

(with-open [{:keys [consumer] :as app} (start config)]
  (testing "unit test with isolated, repeatable context"
    (is (= :yay/🚀 (some-business/function consumer)))))

You could also use thi library while live-coding to stop and restart your application whenever a file is changed.

More details

(defn start
  "Return a map describing a running application, and which values may
  be closed."
  [config]
  (closeable-map/closeable-map
    {;; Kafka producers/consumers are `java.io.Closeable`.
     :producer (kafka-producer config)
     :consumer (kafka-consumer config)

     ;; File streams are `java.io.Closeable` too:
     :logfile (io/output-stream (io/file "/tmp/log.txt"))

     ;; Closeable maps can be nested. Nested maps will be closed before the outer map. 
     :backend/api {:response-executor (close-with (memfn ^ExecutorService .shutdown)
                                        (flow/utilization-executor (:executor config)))
                   :connection-pool (close-with (memfn ^IPool .shutdown)
                                      (http/connection-pool {:pool-opts config}))

                   ;; These functions receive their map as argument.
                   ::closeable-map/before-close (fn [m] (backend/give-up-leadership config m))
                   ::closeable-map/after-close (fn [m] (backend/close-connection config m))}

     ;; Any exception when closing this nested map will be swallowed
     ;; and not bubbled up.
     :db ^::closeable-map/swallow {;; Connection are `java.io.Closeable`, too:
                                   :db-conn (jdbc/get-connection (:db config))}

     ;; Some libs return a zero-argument function which when called
     ;; stops the server, like:
     :server (with-tag ::closeable-map/fn (http/start-server (api config) (:server config)))
     ;; Gotcha: Clojure meta data can only be attached on 'concrete'
     ;; objects; they are lost on literal forms (see above).
     :forensic ^::closeable-map/fn #(metrics/report-death!)

     ::closeable-map/ex-handler
     (fn [ex]
       ;; Will be called for all exceptions thrown when closing this
       ;; map and nested items.
       (println (ex-message ex)))}))

When (.close system) is executed, it will:

• Recursively close all instances of java.io.Closeable and java.lang.AutoCloseable;

• Recursively call all stop zero-argument functions tagged with ^::closeable-map/fn;

• Skip all nested Closeable under a ^::closeable-map/ignore;

• Silently swallow any exception with ^::closeable-map/swallow;

• Exceptions to optional ::closeable-map/ex-handler in key or metadata;

• If keys (or metadata) ::closeable-map/before-close or ::closeable-map/after-close are present, they will be assumed as a function which takes one argument (the map itself) and used run additional closing logic:

  (closeable-map
    {;; This function will be executed before the auto close.
     ::closeable-map/before-close (fn [this-map] (flush!))
  
     ;; Kafka producers/consumers are java.io.Closeable
     :producer (kafka-producer config)
     :consumer (kafka-consumer config)
  
     ;; This function will be executed after the auto close.
     ::closeable-map/after-close (fn [this-map] (garbage/collect!))
    }
  )
  

Some classes do not implement java.lang.AutoCloseable but present some similar method. For example instances of java.util.concurrent.ExecutorService can't be closed but they can be .shutdown:

{:response-executor (close-with (memfn ^ExecutorService .shutdown)
                      (flow/utilization-executor (:executor config)))
 :connection-pool (close-with (memfn ^IPool .shutdown)
                    (http/connection-pool {:pool-opts config}))}

You may also extend this library by giving new dispatch values to multimethod [[piotr-yuxuan.closeable-map/close!]]. Once evaluated, this will work accross all your code. The multimethod is dispatched on the concrete class of its argument:

(import '(java.util.concurrent ExecutorService))
(defmethod closeable-map/close! ExecutorService
  [x]
  (.shutdown ^ExecutorService x))

(import '(io.aleph.dirigiste IPool))
(defmethod closeable-map/close! IPool
  [x]
  (.shutdown ^IPool x))

All or nothing: opening a map atomically

In some circumstances you may need to handle exception on the creation of a closeable map. If an exception happens during the creation of the map, values already evaluated will be closed. No closeable objects will be left open with no references to them.

For instance, this form would throw an exception:

(closeable-map/closeable-map {:server (http/start-server (api config))
                              :kafka {:consumer (kafka-consumer config)
                                      :producer (throw (ex-info "Exception" {}))}})
;; => (ex-info "Exception" {})

The problem is: consumer and server stay open but with no references. Kafka messages keep being consumed and the port stays locked. Using with-closeable prevents that kind of broken, partially open maps:

(closeable-map/with-closeable [server (http/start-server (api config))
                               consumer (kafka-consumer config)
                               producer (throw (ex-info "Exception" {}))]
  (closeable-map/closeable-map {:server server
                                :kafka {:consumer consumer
                                        :producer producer}}))
;; `consumer` is closed, then `server` is closed, and finally the
;; exception is bubbled up.
;; => (ex-info "Exception" {})

Technicalities

Some Clojure datastructures implement IFn:

({:a 1} :a) ;; => 1
(remove #{:a} [:a :b :c]) ;; => '(:b :c)
([:a :b :c] 1) ;; => :b

Clojure maps (IPersistentMap) implement IFn, for invoke() of one argument (a key) with an optional second argument (a default value), i.e. maps are functions of their keys. nil keys and values are fine.

This library defines a new data strucure, CloseableMap. It is exposed as an instance of java.io.Closeable which is a subinterface of java.lang.AutoCloseable. When trying to close its values, it looks for instances of the latter. As such, it tries to be most general.

(require '[clojure.data])

(clojure.data/diff
  (ancestors (class {}))
  (ancestors CloseableMap))

;; =>
[;; Ancestors of Clojure map only but not CloseableMap.
 #{clojure.lang.AFn ; Concrete type, but see below for IFn.
   clojure.lang.APersistentMap
   clojure.lang.IEditableCollection
   clojure.lang.IKVReduce
   clojure.lang.IMapIterable
   java.io.Serializable}

 ;; Ancestors of CloseableMap only.
 #{clojure.lang.IType
   java.io.Closeable
   java.lang.AutoCloseable
   java.util.Iterator
   potemkin.collections.PotemkinMap
   potemkin.types.PotemkinType}

 ;; Ancestors common to both types.
 #{clojure.lang.Associative
   clojure.lang.Counted
   clojure.lang.IFn
   clojure.lang.IHashEq
   clojure.lang.ILookup
   clojure.lang.IMeta
   clojure.lang.IObj
   clojure.lang.IPersistentCollection
   clojure.lang.IPersistentMap
   clojure.lang.MapEquivalence
   clojure.lang.Seqable
   java.lang.Iterable
   java.lang.Object
   java.lang.Runnable
   java.util.Map
   java.util.concurrent.Callable}]