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https://github.com/piotr-yuxuan/closeable-map

Application state management made simple: a Clojure map that implements java.io.Closeable.
https://github.com/piotr-yuxuan/closeable-map

clojure clojure-maps closeable component integrant map mount state state-management system with-open

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Application state management made simple: a Clojure map that implements java.io.Closeable.

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# `closeable-map`



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``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.``



![](./doc/automatische-huehnerklappe.jpg)



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)`. ``See it in

action above``.



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](https://github.com/juxt/clip)

- Simple library to initialise an app stack: [sathyavijayan/upit](https://github.com/sathyavijayan/upit)

- Application state management:

  [stuartsierra/component](https://github.com/stuartsierra/component),

  [weavejester/integrant](weavejester/integrant),

  [tolitius/mount](https://github.com/tolitius/mount), _et al_.

- Extension of `with-open`:

  [jarohen/with-open](https://github.com/jarohen/with-open)

- Representing state in a map:

  [robertluo/fun-map](https://github.com/robertluo/fun-map)



## Usage



In your project, require:



``` clojure

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

```



Define an application that can be started, and closed.



``` clojure

(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:



``` clojure

(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:



``` clojure

(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



``` clojure

(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:



    ``` clojure

    (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`:



``` clojure

{: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:



``` clojure

(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.



``` clojure

(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*`:



``` clojure

(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:



``` clojure

(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:



``` clojure

(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.



``` clojure

(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.

  

``` clojure

(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`:



``` clojure

({: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.



``` clojure

(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}]

```