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leav·en /ˈlevən/

noun - a pervasive influence that modifies something or transforms it for the better.

verb - permeate and modify or transform (something) for the better.

A lightweight component model for clojure and clojurescript.


Add [com.palletops/leaven "0.3.1"] to your :dependencies.


For users of components, com.palletops.leaven provides the start and stop functions, and the defsystem macro. There is also a status function that can be used with components that support it.

Both start and stop take a single component as an argument and return an updated component - components are normally immutable, so you should always use the updated component in the return value.

The defsystem macro is used to define a composite component, made up of a map of components, each identified by a symbol. The sub-components are specified as a vector of symbols, and are started in the order specified and stop in the reverse order. The macro defines a record, and you instantiate the record with the sub-component instances.

You can specify a body in defsystem, just as you would to defrecord, in order to implement other protocols on your system.

You can pass a map of options to defsystem, after the component names and before any body.

The :depends option takes a map from system component to the components it depends on, and ensures components see the started versions of their dependencies. If the dependent components are named the same as the component, then a sequence of keywords is used as the value for the map entry. If the dependent components have different names, then a map is used from the component keyword to the matching dependent component keyword. Note that this does not (currently) influence the component ordering.

The :on-start and :on-stop options are used to specify functions that will be called when a component is started or stopped. The value is a map from the component keyword to a function that takes the system and the keyword of the started component, and returns a possibly updated system.

For component providers, com.palletops.leaven.protocols provides the Startable and Stoppable protocols, that require the implementation of the start and stop methods respectively. The Queryable protocol provides for a status method.


We define a component that will provide an increasing sequence of numbers via a core.async channel. We implement the Startable and Stoppable protocols for the component.

(require '[clojure.core.async :as async]
         '[com.palletops.leaven :as leaven]
         '[com.palletops.leaven.protocols :refer [Startable Stoppable])

(defrecord Counter [init-val channel loop-chan]
  (start [component]
    (assoc component :loop-chan
           (async/go-loop [n init-val]
             (async/>! channel n)
             (recur (inc n)))))
  (stop [component]
    (async/close! channel)
    (assoc component :loop-chan nil)))

Note that the record contains fields for both the configuration and the runtime state of the component.

We instantiate the component with one of the record's constructor functions. In this example we use a var to hold the component, but this is in no way required.

(def c (async/chan))
(def counter (map->Counter {:init-val 1 :channel c}))

We can start the component:

(alter-var-root #'counter leaven/start)

Now we can get values from the channel:

(async/<!! c) ; => 1
(async/<!! c) ; => 2
(async/<!! c) ; => 3

If we had tried to read the channel before starting the component, the call to async/<!! would have blocked.

Stopping the channel:

(alter-var-root #'counter leaven/stop)

And now the channel is closed, so will return nil.

(async/<!! c) ; => 4
(async/<!! c) ; => nil

We're going to define another component now that, take a channel and will double what is put into it.

(defrecord Doubler [in-chan out-chan ctrl-chan loop-chan]
  (start [component]
    (let [ctrl-chan (async/chan)]
      (assoc component
        :loop-chan (async/go
                     (loop []
                       (let [[v _] (async/alts! [in-chan ctrl-chan])]
                         (if (not= ::stop v)
                           (let [[v _] (async/alts!
                                        [[out-chan (* 2 v)] ctrl-chan])]
                             (if (not= ::stop v)
                     (async/close! out-chan))
        :ctrl-chan ctrl-chan)))
  (stop [component]
    (async/>!! ctrl-chan ::stop)
    (assoc component :loop-chan nil :ctrl-chan nil)))

We'll use these components to define a system

(leaven/defsystem Evens [counter doubler])
(defn evens [out-chan]
  (let [c1 (async/chan)]
      (map->Counter {:init-val 1 :channel c1})
      (map->Doubler {:in-chan c :out-chan out-chan}))))

(def c (async/chan))
(def sys (evens c))
(alter-var-root #'sys leaven/start)
(async/<!! c) ; => 2
(async/<!! c) ; => 4
(alter-var-root #'sys leaven/stop)
(async/<!! c) ; => nil

Comparison with Component Example

This is the example from the Component readme, translated for leaven.

(ns com.example.your-application
  (:require [com.palletops.leaven :as leaven]))

(defrecord Database [host port connection]
  (start [component]
    (let [conn (connect-to-database host port)]
      (assoc component :connection conn)))

  (stop [component]
    (.close connection)
    (assoc component :connection nil)))

(defn database [host port]
  (map->Database {:host host :port port}))

(defrecord ExampleComponent [options cache database]
  (start [this]
    (assoc this :admin (get-user database "admin")))

  (stop [this]

(defn example-component [{:keys [config-options db]}]
  (map->ExampleComponent {:db db
                          :options config-options
                          :cache (atom {})}))

(defsystem ExampleSystem [db app])

(defn example-system [config-options]
  (let [{:keys [host port]} config-options
        db (database host port)]
    (map->ExampleSystem                 ; normal record constructor
      :db db
      :app (example-component
             {:config-options config-options
              :db db}))))

Library authors

Library authors are encouraged to provide a leaven component in a separate namespace. By making the dependency on leaven have a provided scope, you do not force the dependency on your users.

In leiningen, you can make a dependency have provided scop by adding it under the :provided profile.

Why another component library?

The Component framework pioneered a component model for clojure, and provides an excellent rationale for components.

We wanted something that:

  • did explicit dependency order, with no need for a using function,
  • would work in clojurescript


Copyright © 2014 Hugo Duncan

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

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