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

(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)}))

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

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

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

(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)))}))

{: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}))}

(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:

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

(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" {})

(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" {})

In your project, require:

``` clojure
(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:

``` 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)))))
```

## 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}))}
```

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:

``` clojure
(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:

``` clojure
(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:

``` clojure
(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" {})
```

Use `with-closeable` to remediate this situation. It prevents broken,
partially open maps:

``` clojure
(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" {})
```