Integrant elides Midje metaconstants from config

Integrant

integrant /ˈɪntɪɡr(ə)nt/

(of parts) making up or contributing to a whole; constituent.

Integrant is a Clojure (and ClojureScript) micro-framework for building applications with data-driven architecture. It can be thought of as an alternative to Component or Mount, and was inspired by Arachne and through work on Duct.

Rationale

Integrant was built as a reaction to fix some perceived weaknesses with Component.

In Component, systems are created programmatically. Constructor functions are used to build records, which are then assembled into systems.

In Integrant, systems are created from a configuration data structure, typically loaded from an edn resource. The architecture of the application is defined through data, rather than code.

In Component, only records or maps may have dependencies. Anything else you might want to have dependencies, like a function, needs to be wrapped in a record.

In Integrant, anything can be dependent on anything else. The dependencies are resolved from the configuration before it's initialized into a system.

Installation

Add the following dependency to your deps.edn file:

integrant/integrant {:mvn/version "0.8.1"}

Or this to your Leiningen dependencies:

[integrant "0.8.1"]

Presentations

Enter Integrant

Usage

Configurations

Integrant starts with a configuration map. Each top-level key in the map represents a configuration that can be "initialized" into a concrete implementation. Configurations can reference other keys via the ref (or refset) function.

For example:

(require '[integrant.core :as ig])

(def config
  {:adapter/jetty {:port 8080, :handler (ig/ref :handler/greet)}
   :handler/greet {:name "Alice"}})

Alternatively, you can specify your configuration as pure edn:

{:adapter/jetty {:port 8080, :handler #ig/ref :handler/greet}
 :handler/greet {:name "Alice"}}

And load it with Integrant's version of read-string:

(def config
  (ig/read-string (slurp "config.edn")))

Initializing and halting

Once you have a configuration, Integrant needs to be told how to implement it. The init-key multimethod takes two arguments, a key and its corresponding value, and tells Integrant how to initialize it:

(require '[ring.adapter.jetty :as jetty]
         '[ring.util.response :as resp])

(defmethod ig/init-key :adapter/jetty [_ {:keys [handler] :as opts}]
  (jetty/run-jetty handler (-> opts (dissoc :handler) (assoc :join? false))))

(defmethod ig/init-key :handler/greet [_ {:keys [name]}]
  (fn [_] (resp/response (str "Hello " name))))

Keys are initialized recursively, with the values in the map being replaced by the return value from init-key.

In the configuration we defined before, :handler/greet will be initialized first, and its value replaced with a handler function. When :adapter/jetty references :handler/greet, it will receive the initialized handler function, rather than the raw configuration.

The halt-key! multimethod tells Integrant how to stop and clean up after a key. Like init-key, it takes two arguments, a key and its corresponding initialized value.

(defmethod ig/halt-key! :adapter/jetty [_ server]
  (.stop server))

Note that we don't need to define a halt-key! for :handler/greet.

Once the multimethods have been defined, we can use the init and halt! functions to handle entire configurations. The init function will start keys in dependency order, and resolve references as it goes:

(def system
  (ig/init config))

When a system needs to be shut down, halt! is used:

(ig/halt! system)

Like Component, halt! shuts down the system in reverse dependency order. Unlike Component, halt! is entirely side-effectful. The return value should be ignored, and the system structure discarded.

It's also important that halt-key! is idempotent. We should be able to run it multiple times on the same key without issue.

Integrant marks functions that are entirely side-effectful with an ending !. You should ignore the return value of any function ending in a !.

Both init and halt! can take a second argument of a collection of keys. If this is supplied, the functions will only initiate or halt the supplied keys (and any referenced keys). For example:

(def system
  (ig/init config [:adapter/jetty]))

Suspending and resuming

During development, we often want to rebuild a system, but not to close open connections or terminate running threads. For this purpose Integrant has the suspend! and resume functions.

The suspend! function acts like halt!:

(ig/suspend! system)

By default this functions the same as halt!, but we can customize the behavior with the suspend-key! multimethod to keep open connections and resources that halt-key! would close.

Like halt-key!, suspend-key! should be both side-effectful and idempotent.

The resume function acts like init but takes an additional argument specifying a suspended system:

(def new-system
  (ig/resume config system))

By default the system argument is ignored and resume functions the same as init, but as with suspend! we can customize the behavior with the resume-key multimethod. If we implement this method, we can reuse open resources from the suspended system.

To illustrate this, let's reimplement the Jetty adapter with the capability to suspend and resume:

(defmethod ig/init-key :adapter/jetty [_ opts]
  (let [handler (atom (delay (:handler opts)))
        options (-> opts (dissoc :handler) (assoc :join? false))]
    {:handler handler
     :server  (jetty/run-jetty (fn [req] (@@handler req)) options)}))

(defmethod ig/halt-key! :adapter/jetty [_ {:keys [server]}]
  (.stop server))

(defmethod ig/suspend-key! :adapter/jetty [_ {:keys [handler]}]
  (reset! handler (promise)))

(defmethod ig/resume-key :adapter/jetty [key opts old-opts old-impl]
  (if (= (dissoc opts :handler) (dissoc old-opts :handler))
    (do (deliver @(:handler old-impl) (:handler opts))
        old-impl)
    (do (ig/halt-key! key old-impl)
        (ig/init-key key opts))))

This example may require some explanation. Instead of passing the handler directly to the web server, we put it in an atom, so that we can change the handler without restarting the server.

We further encase the handler in a delay. This allows us to replace it with a promise when we suspend the server. Because a promise will block until a value is delivered, once suspended the server will accept requests but wait around until it's resumed.

Once we decide to resume the server, we first check to see if the options have changed. If they have, we don't take any chances; better to halt and re-init from scratch. If the server options haven't changed, then deliver the new handler to the promise which unblocks the server.

Note that we only need to go to this additional effort if retaining open resources is useful during development, otherwise we can rely on the default init and halt! behavior. In production, it's always better to terminate and restart.

Like init and halt!, resume and suspend! can be supplied with a collection of keys to narrow down the parts of the configuration that are suspended or resumed.

Resolving

It's sometimes useful to hide information when resolving a reference. In our previous example, we changed the initiation from:

(defmethod ig/init-key :adapter/jetty [_ {:keys [handler] :as opts}]
  (jetty/run-jetty handler (-> opts (dissoc :handler) (assoc :join? false))))

To:

(defmethod ig/init-key :adapter/jetty [_ opts]
  (let [handler (atom (delay (:handler opts)))
        options (-> opts (dissoc :handler) (assoc :join? false))]
    {:handler handler
     :server  (jetty/run-jetty (fn [req] (@@handler req)) options)}))

This changed the return value from a Jetty server object to a map, so that suspend! and resume would be able to temporarily block the handler. However, this also changes the return type! Ideally, we'd want to pass the handler atom to suspend-key! and resume-key, without affecting how references are resolved in the configuration.

To solve this, we can use resolve-key:

(defmethod ig/resolve-key :adapter/jetty [_ {:keys [server]}]
  server)

Before a reference is resolved, resolve-key is applied. This allows us to cut out information that is only relevant behind the scenes. In this case, we replace the map with the container Jetty server object.

Prepping

Sometimes keys also require some preparation. Perhaps you have a particularly complex set of default values, or perhaps you want to add in default references to other keys. In these cases, the prep-key method can help.

(defmethod ig/prep-key :adapter/jetty [_ config]
  (merge {:port 8080} config))

The prep-key method will change the value of a key before the configuration is initialized. In the previous example, the :port would default to 8080 if not set.

All keys in a configuration can be prepped using the prep function before the init function:

(-> config ig/prep ig/init)

If prep-key is not defined, it defaults to the identity function. Prepping keys is particularly useful when adding default references to derived keywords.

Derived keywords

Keywords have an inherited hierarchy. Integrant takes advantage of this by allowing keywords to refer to their descendants. For example:

(derive :adapter/jetty :adapter/ring)

This sets up a hierarchical relationship, where the specific :adapter/jetty keyword is derived from the more generic :adapter/ring.

We can now use :adapter/ring in place of :adapter/jetty:

(ig/init config [:adapter/ring])

We can also use it as a reference, but only if the reference is unambiguous, and only refers to one key in the configuration.

Composite keys

Sometimes it's useful to have two keys of the same type in your configuration. For example, you may want to run two Ring adapters on different ports.

One way would be to create two new keywords, derived from a common parent:

(derive :example/web-1 :adapter/jetty)
(derive :example/web-2 :adapter/jetty)

You could then write a configuration like:

{:example/web-1 {:port 8080, :handler #ig/ref :handler/greet}
 :example/web-2 {:port 8081, :handler #ig/ref :handler/greet}
 :handler/greet {:name "Alice"}}

However, you could also make use of composite keys. If your configuration contains a key that is a vector of keywords, Integrant treats it as being derived from all the keywords inside it.

So you could also write:

{[:adapter/jetty :example/web-1] {:port 8080, :handler #ig/ref :handler/greet}
 [:adapter/jetty :example/web-2] {:port 8081, :handler #ig/ref :handler/greet}
 :handler/greet {:name "Alice"}}

This syntax sugar allows you to avoid adding extra derive instructions to your source code.

Composite references

Composite references complement composite keys. A normal reference matches any key derived from the value of the reference. A composite reference matches any key derived from every value in a vector.

For example:

{[:group/a :adapter/jetty] {:port 8080, :handler #ig/ref [:group/a :handler/greet]}
 [:group/a :handler/greet] {:name "Alice"}
 [:group/b :adapter/jetty] {:port 8081, :handler #ig/ref [:group/b :handler/greet]}
 [:group/b :handler/greet] {:name "Bob"}}

One use of composite references is to provide a way of grouping keys in a configuration.

Refs vs refsets

An Integrant ref is used to reference another key in the configuration. The ref will be replaced with the initialized value of the key. The ref does not need to refer to an exact key - the parent of a derived key may be specified, so long as the ref is unambiguous.

For example suppose we have a configuration:

{:handler/greet    {:name #ig/ref :const/name}
 :const.name/alice {:name "Alice"}
 :const.name/bob   {:name "Bob"}}

And some definitions:

(defmethod ig/init-key :const/name [_ {:keys [name]}]
  name)

(derive :const.name/alice :const/name)
(derive :const.name/bob   :const/name)

In this case #ig/ref :const/name is ambiguous - it could refer to either :const.name/alice or :const.name/bob. To fix this we could make the reference more specific:

{:handler/greet    {:name #ig/ref :const.name/alice}
 :const.name/alice {:name "Alice"}
 :const.name/bob   {:name "Bob"}}

But suppose we want to greet not just one person, but several. In this case we can use a refset:

{:handler/greet-all {:names #ig/refset :const/name}
 :const.name/alice  {:name "Alice"}
 :const.name/bob    {:name "Bob"}}

When initialized, a refset will produce a set of all matching values.

(defmethod ig/init-key :handler/greet-all [_ {:keys [names]}]
  (fn [_] (resp/response (str "Hello " (clojure.string/join ", " names))))

Specs

It would be incorrect to write specs directly against the keys used by Integrant, as the same key will be used in the configuration, during initiation, and in the resulting system. All will likely have different values.

To resolve this, Integrant has an pre-init-spec multimethod that can be extended to provide Integrant with a spec to test the value after the references are resolved, but before they are initiated. The resulting spec is checked directly before init-key, and an exception is raised if it fails.

Here's how our two example keys would be specced out:

(require '[clojure.spec.alpha :as s])

(s/def ::port pos-int?)
(s/def ::handler fn?)

(defmethod ig/pre-init-spec :adapter/jetty [_]
  (s/keys :req-un [::port ::handler]))

(s/def ::name string?)

(defmethod ig/pre-init-spec :handler/greet [_]
  (s/keys :req-un [::name]))

If we try to init an invalid configuration:

(ig/init {:adapter/jetty {:port 3000} :handler/greet {:name "foo"}})

Then an ExceptionInfo is thrown explaining the error:

ExceptionInfo Spec failed on key :adapter/jetty when building system
val: {:port 3000} fails predicate: (contains? % :handler)

Loading namespaces

It can be hard to remember to load all the namespaces that contain the relevant multimethods. If you name your keys carefully, Integrant can help via the load-namespaces function.

If a key has a namespace, load-namespaces will attempt to load it. It will also try concatenating the name of the key onto the end of its namespace, and loading that as well.

For example:

(load-namespaces {:foo.component/bar {:message "hello"}})

This will attempt to load the namespace foo.component and also foo.component.bar. A list of all successfully loaded namespaces will be returned from the function. Missing namespaces are ignored.

Reloaded workflow

See Integrant-REPL to use Integrant systems at the REPL, in line with Stuart Sierra's reloaded workflow.

Further Documentation

API docs

License

Released under the MIT license.

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Integrant

Rationale

Installation

Presentations

Usage

Configurations

Initializing and halting

Suspending and resuming

Resolving

Prepping

Derived keywords

Composite keys

Composite references

Refs vs refsets

Specs

Loading namespaces

Reloaded workflow

Further Documentation

License

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