mount

riding side by side with tools.namespace to manage application state during development.

why?

Clojure is

• powerful
• simple
• and fun

Depending on how application state is managed during development, the above three superpowers can either stay, go somewhat or go completely.

If Clojure REPL (i.e. lein repl, boot repl) fired up instantly, the need to reload application state inside the REPL would go away. But at the moment, and for some time in the future, managing state by making it reloadable within the same REPL session is important to retain all the Clojure superpowers.

Here is a good break down on the Clojure REPL startup time, and it is not because of JVM.

mount is here to preserve all the Clojure superpowers while making application state enjoyably reloadable.

There is another Clojure superpower that mount is made to retain: Clojure community. Pull request away, let's solve this thing!

how

(require '[mount :refer [defstate]])

Creating State

Creating state is easy:

(defstate conn :start (create-conn))

where (create-conn) is defined elsewhere, can be right above it.

In case this state needs to be cleaned / destryed between reloads, there is also a :stop

(defstate conn :start (create-conn)
               :stop (disconnect conn))

That is pretty much it. But wait, there is more.. this state is a top level being, which means it can be simply required by other namespaces.

Using State

For example let's say an app needs a connection above. No problem:

(ns app
  (:require [above :refer [conn]]))

where above is an arbitrary namespace that have the above state / connection.

Dependencies

If the whole app is one big application context (or system), cross dependencies with a solid dependency graph is an integral part of the system.

But if a state is a simple top level being, these beings can coexist with each other and with other namespaces by being required instead.

If a managing state library requires a whole app buy-in, where everything is a bean or a component, it is a framework, and dependency graph is usually quite large and complex, since it has everything (every piece of the application) in it.

But if stateful things are kept lean and low level (i.e. I/O, queues, etc.), dependency graphs are simple and small, and everything else is just namespaces and functions: the way it shuold be.

Talking States

There are of course direct dependecies that mount respects:

(ns app.config
  (:require [mount :refer [defstate]]))

(defstate app-config 
  :start (load-config "test/resources/config.edn"))

this app-config, being top level, can be used in other namespaces, including the ones that create states:

(ns app.database
  (:require [mount :refer [defstate]]
            [app.config :refer [app-config]]))

(defstate conn :start (create-connection app-config))

here is an example of Datomic connection that "depends" on the app config.

The Importance of Being Reloadable

mount has start and stop functions that will walk all the states created with defstate and start / stop them accordingly: i.e. will call their :start and :stop defined functions.

This can be easily hooked up to tool.namespace, to make the whole application reloadable. Here is a dev.clj as an example.

mount and develop!

mount comes with an example app that has two states:

• config, loaded from the files and refreshed on each (reset)
• datamic connection that uses the config to create connection

Running New York Stock Exchange

To try it out, clone mount, get to REPL and switch to (dev):

$ lein repl

user=> (dev)
20:37:29.461 [nREPL-worker-0] INFO  mount.config - loading config from test/resources/config.edn
20:37:29.477 [nREPL-worker-0] INFO  mount.nyse - creating a connection to datomic: datomic:mem://mount
dev=>

everything is started and can be played with:

dev=> (create-nyse-schema)
dev=> (add-order "GOOG" 665.51M 665.59M 100)
dev=> (add-order "GOOG" 665.50M 665.58M 300)

dev=> (find-orders "GOOG")
({:db/id 17592186045418, :order/symbol "GOOG", :order/bid 665.51M, :order/qty 100, :order/offer 665.59M}
 {:db/id 17592186045420, :order/symbol "GOOG", :order/bid 665.50M, :order/qty 300, :order/offer 665.58M})

once something changed in the code, or you just need to reload everything (reset):

dev=> (reset)
20:38:43.244 [nREPL-worker-1] INFO  mount.nyse - disconnecting from  datomic:mem://mount
:reloading (mount mount.config mount.nyse mount.utils.datomic mount.app dev)

20:38:43.287 [nREPL-worker-1] INFO  mount.config - loading config from test/resources/config.edn
20:38:43.296 [nREPL-worker-1] INFO  mount.nyse - creating a connection to datomic: datomic:mem://mount
:ready

notice that it stopped and started again.

Since the schema was not there roght after the start, it is not there now as well:

dev=> (find-orders "GOOG")

IllegalArgumentExceptionInfo :db.error/not-an-entity Unable to resolve entity: :order/symbol  datomic.error/arg (error.clj:57)

hence the app is in its "clean" state, and ready to rock and roll as right after the REPL started:

dev=> (create-nyse-schema)
dev=> (find-orders "GOOG")
()

dev=> (add-order "AAPL" 111.712M 111.811M 250)

dev=> (find-orders "AAPL")
({:db/id 17592186045418, :order/symbol "AAPL", :order/bid 111.712M, :order/qty 250, :order/offer 111.811M})

license

Copyright © 2015 tolitius

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