README.md

closeable-map

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

This small Clojure library tries to get as close as possible to the bare essential complexity of state management in code. It defines a new type of Clojure map that you may (.close m). <scherz>See it in action above</scherz>.

This library defines a new type of map that you may use like any other map. This map may contain stateful Java objects like a server, a Kafka producer, a file output stream. When you want to clean your state, you just .close the map and all nested stateful objects will be closed recursively.

It is a tiny alternative to more capable projects:

• Application state management in a map: juxt/clip
• Simple library to initialise an app stack: sathyavijayan/upit
• 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

Usage

In your project, require:

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

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}))}

Java objects

A Java object does not implement interface clojure.lang.IObj so it is unable to carry Clojure metadata. As such, you can't give a it a tag like ::closeable-map/fn. You may also extend this library by giving new dispatch values to multimethod 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 close! ExecutorService
  [x]
  (.shutdown ^ExecutorService x))

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

(import '(clojure.lang Atom))
(defmethod close! Atom
  [x]
  (reset! x nil))

(import '(clojure.core.async.impl.protocols Channel))
(defmethod close! Channel
  [x]
  (async/close! x))

(import '(xtdb.node XtdbNode))
(defmethod c/close! XtdbNode
  [^Closeable x]
  (doto x
    xt/sync
    (.close)))

A Java object may be wrapped as a closeable*.

All or nothing

No half-broken closeable map

You may also avoid partially open state when an exception is thrown when creating a CloseableMap. This is where closeable-map* comes handy. It outcome in one of the following:

• Either everything went right, and all inner forms wrapped by closeable correctly return a value; you get an open instance of CloseableMap.

• Either some inner form wrapped by closeable didn't return a closeable object but threw an exception instead. Then all closeable forms are closed, and finally the exception is bubbled up.

(closeable-map*
  {:server (closeable* (http/start-server (api config)))
   :kafka {:consumer (closeable* (kafka-consumer config))
           :producer (closeable* (kafka-producer config))
           :schema.registry.url "https://localhost"}})

Closeable objects not directly within the map may still be closed if wrapped in closeable*:

(closeable-map*
  (closeable* (kafka-consumer config))
  (closeable* (kafka-producer config))
  {:server (closeable* (http/start-server (api config)))
   :schema.registry.url "https://localhost"})

No half-broken state in general code

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" {})

with-closeable* prevents that kind of broken, partially open states for its bindings:

(with-closeable* [server (http/start-server (api config))
                  consumer (kafka-consumer config)
                  producer (throw (ex-info "Exception" {}))]
  ;; Your code goes here.
)
;; Close consumer,
;; close server,
;; finally throw `(ex-info "Exception" {})`.

You now have the guarantee that your code will only be executed if all these closeable are open. In the latter example an exception is thrown when producer is evaluated, so consumer is closed, then server is closed, and finally the exception is bubbled up. Your code is not evaluated. In the next example the body is evaluated, but throws an exception: all bindings are closed.

(with-closeable* [server (http/start-server (api config))
                  consumer (kafka-consumer config)
                  producer (kafka-producer config)]
  ;; Your code goes here.
  (throw (ex-info "Exception" {})))
;; Close producer,
;; close consumer,
;; close server,
;; finally throw `(ex-info "Exception" {})`.

When no exception is thrown, leave bindings open and return like a normal let form. If you prefer to close bindings, use with-open as usual.

(with-closeable* [server (http/start-server (api config))
                  consumer (kafka-consumer config)
                  producer (kafka-producer config)]
  ;; Your code goes here.
  )
;; All closeable in bindings stay open.
;; => result

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}]