"; */ ?>


Nov 16

No Ceremony

DI framework makes sense for OOP

In Java (or most OOP languages):

  • Objects need to be created
  • In most of the cases they are stateful
  • Dependencies (state) often need to be injected
  • Order of the creation needs to be determined/given for the injection to work

Hence an IoC framework such as Spring makes perfect sense (in Java):

for example creating a dataSource, a sessionFactory and a txManager in Spring

DI framework “hurts functionally”

In Clojure (or similar functional languages):

  • Explicit objects with state and behavior are discouraged
  • Code organized in namespaces and small functions
  • Functions are directly referenced across modules/namespaces

DI/IoC framework would hurt all of the above: “beans” with functionality can only be accessed via creating other framework managed “beans”: very much like a need to create an Object to access another Object’s stateful functionality.


Let’s say we need to find a user in a database.

we would need to connect to a database:

;; in reality would return a database connection instance
(defn connect-to-database [{:keys [connection-uri]}]
  {:connected-to connection-uri})

and find a user by passing a database connection instance and a username:

;; pretending to execute a query
(defn find-user [database username]
  (if (:connection database)
      (println "running query:"
               "SELECT * FROM users WHERE username = "
               username "on" database)
    (throw (RuntimeException. (str "can't execute the query => database is disconnected: " database)))))

examples are immediately REPL’able, hence we pretend to connect to a database, and pretend to execute the query, but the format and ideas remain.

Application Context

One way to use a stateful external resource(s) such as a database in the find-user function above, is to follow the Spring approach and to define an almost identical to Spring Lifecycle interface:

(defprotocol Lifecycle
  (start [this] "Start this component.")
  (stop [this] "Stop this component."))

Then define several records that would implement that interface.

By the way, Clojure records are usually used with methods (protocol implementations) that makes them “two fold”: they complect data with behavior, very much like Objects do. (Here is an interesting discussion about it)

(defrecord Config [path]
  (start [component]
    (let [conf path] ;; would fetch/edn-read config from the "path", here just taking it as conf for the sake of an example
      (assoc component :config conf)))
  (stop [component]
    (assoc component :config nil)))
(defrecord Database [config]
  (start [component]
    (let [conn (connect-to-database config)]
      (assoc component :connection conn)))
  (stop [component]
    (assoc component :connection nil)))
(defrecord YetAnotherComponent [database]
  (start [this]
    (assoc this :admin (find-user database "admin")))
  (stop [this]

Now as the classes (records above) are defined, we can create an “application context”:

(def config (-> (Config. {:connection-uri "postgresql://localhost:5432/clojure-spring"})
(def db (-> (Database. config) start))
(def yet-another-bean (-> (YetAnotherComponent. db) start))
;; >> running query: SELECT * FROM users WHERE username =  admin on #boot.user.Database{:config {:connection-uri postgresql://localhost:5432/clojure-spring}, :connection {:connected-to postgresql://localhost:5432/clojure-spring}}

and finally we get to the good stuff (the reason we did all this):

(:admin yet-another-bean)
;; >> :jimi

a couple of things to notice:

* Well defined order *

Start/stop order needs to be defined for all “beans”, because if it isn’t:

(def db (-> (Database. config)))
(def yet-another-bean (-> (YetAnotherComponent. db) start))
;; >> java.lang.RuntimeException: 
;;      can't execute the query => database is disconnected: boot.user.Database@399337a0
* Reality is not that simple *

All the “components” above can’t be just created as defs in reality, since they are unmanaged, hence something is needed where all these components:

  • are defined
  • created
  • injected into each other in the right order
  • and then destroyed properly and orderly

Library vs. Framework

This can be done as a library that plugs in each component into the application on demand / incrementally. Which would retain the way the code is navigated, organized and understood, and would allow the code to be retrofitted when new components are added and removed, etc. + all the usual “library benefits”.


It can be done as a framework where all the components live and managed. This framework approach is what Spring does in Java / Groovy, which in fact works great in Java / Groovy.

.. but not in Clojure.

Here is why: you can’t really do (:admin yet-another-bean) from any function, since this function needs:

: access to yet-another-bean
: that needs access to the Database
: that needs access to the Config
: etc..

Which means that only “something” that has access to yet-another-bean needs to pass it to that function. That “something” is.. well a “bean” that is a part of the framework. Oh.. and that function becomes a method.

Which means the echo system is now complected: this framework changes the way you navigate, :require and reason about the code.

It changes the way functions are created in one namespace, :required and simply used in another, since now you need to let the framework know about every function that takes in / has to work with a “component”.

This is exactly what frameworks mean
When they talk about requiring a “full app buy in”
And while it works great for Java and Spring
In Clojure you don’t create a bean after bean
You create a function and you’re “keeping it clean”

“Just doing” it

In the library approach (in this case mount) you can just do it with no ceremony and / or changing or losing the benefits of the Clojure echo system: namespaces and vars are beautiful things:

(require '[mount.core :as mount :refer [defstate]])
(defstate config :start {:connection-uri "postgresql://localhost:5432/clojure-spring"})
(defstate db :start {:connection (connect-to-database config)})
;; #'boot.user/db
(mount/start #'boot.user/db)
;; {:started ["#'boot.user/db"]}
(find-user db "admin")
;; running query: SELECT * FROM users WHERE username =  admin on
;; {:connection {:connected-to postgresql://localhost:5432/clojure-spring}}
;; :jimi


no ceremony.

in fact the db state would most likely look like:

(defstate db :start (connect-to-database config)
             :stop (disconnect db))

Managing Objects

While most of the time it is unnecessary, we can use records from the above example with this library approach as well:

boot.user=> (defstate db :start (-> (Database. config) start)
                         :stop (stop db))
boot.user=> (defstate config :start (-> (Config. {:connection-uri "postgresql://localhost:5432/clojure-spring"}) start)
                             :stop (stop config))

and they become intelligently startable:

boot.user=> (mount/start)
{:started ["#'boot.user/config" "#'boot.user/db"]}
boot.user=> (find-user db "admin")
;; running query: SELECT * FROM users WHERE username =  admin on
;; #boot.user.Database{:config #boot.user.Config{:path {:connection-uri postgresql://localhost:5432/clojure-spring},
;; :config {:connection-uri postgresql://localhost:5432/clojure-spring}},
;; :connection {:connected-to nil}}
;; :jimi

and intelligently stoppable:

boot.user=> (mount/stop)
{:stopped ["#'boot.user/db" "#'boot.user/config"]}
boot.user=> (find-user db "admin")
;; java.lang.RuntimeException: can't execute the query => database is disconnected:
;;   '#'boot.user/db' is not started (to start all the states call mount/start)

Easy vs. Simple

While usually a great argument, this is not it.

In this case this is pragmatic vs. dogma

Apr 16

The Way Nature Creates Things

Dogma From The Top

The best thing about the Clojure community is a large number of independent thinkers. The second best thing that allows this community to exist is the Clojure language itself, that have the hammock philosophy and great tools to enable thinkers to think and create. Often I notice myself following a “90 / 10” rule: where I spend 90% of the time thinking (brain, REPL, google, papers, people, sleep, etc..) and 10% of the time creating once thoughts are solidified.

As any vibrant community Clojure has “the powers that be” that provide guidance to people who need it. This guidance could be either: very helpful or lacking context + real world examples => let’s call it “dogma from the top”.

While I am not a biologist, I like to draw a parallel between software programs created by developers and things “naturally” created by nature. When I am in the beautiful garden or diving in the ocean, I find a lot less dogma, and a lot more fruits of mutation and centuries of trial and error development. It’s amazing. So much bigger and more honest than… “singletons are bad”.

Yes, this short story is about Mount, or rather it is about perception and understanding.

Singletons are Bad

If the context is not provided, “singletons”, or “global singletons” when referring to Clojure vars, have no quality of being bad or good. It’s just the word. Is “paper” bad? How about “inheritance”, is it bad? Well, what if it’s in millions of dollars?

The claim from the top is that keeping state in Clojure vars is bad, because global singletons are bad. Ok more context, but still no context to measure the goodness. When I ask for a concrete example, or two, that would “teach me” why Clojure var is such a horrible place to keep state, the only concrete example I get is:

“Because you can develop and run tests within the same REPL / runtime”

It turns out that some people develop and run tests within the same REPL.

I like my tests run in a separate REPL, moreover I like to run a watcher in that REPL which will rerun tests on any change I make and will notify me with a pass/fail sound / color in that terminal. This can be easily done with Boot: boot watch speak test. Can also be done with Leiningen via lein-test-refresh: lein test-refresh.

Another benefit of running tests in a separate REPL / runtime: it ensures a clean, isolated environment where tests are run. And since most people still use Leiningen this saves a lot of time.

But I do respect that people could have different workflows and they like running tests within the same REPL they develop. And there is Yurt that allows to create as many different yurts as needed in the same runtime.

False Dmitry I

… “the real Dmitriy died in Uglich” [source]

All other “examples” of why keeping state in vars is bad are simply.. not cutting it. Here are some.

“Because there are instances I will have more than one database”

– “Why would I ever have more than one database?”
– “Why would I not!?”

And the example usually goes like this:

 :user-db (database "sql://user-db")
 :product-db (database "sql://product-db")

“I can’t do that with namespaces and vars”. Yes you can. Namespaces and vars are beautiful, very capable things. Here is a simple example:

(defn database [uri]
  (connect-to uri))
(defstate user-db :start (database "sql://user-db"))
(defstate product-db :start (database "sql://product-db"))
;; + :stop functions

notice how “database” is just a function, can be reused as many times as needed. user and product DBs can live within different namespaces if needed.

Yes, if “defstate” was a simple “def”, “def user-db” would be limiting, since how would you restart it for example. But “defstate” plugs “user-db” into a lifecycle aware Mount where “user-db” becomes a managed component (in a good sense) of an application.

Context is King

And here we arrived at the context: (defstate state :start (f1) :stop (f2)) is not just some global singleton which is claimed to be bad, but it is a living component that can be started, restarted, referenced and healed when it’s abused. Is it bad? Not from my experience, it’s actually quite excellent :)

“Because if you recompile a namespace you end up with a stale state”

Now with the context provided, it is simply not true with Mount.

When a namespace is recompiled Mount will stop/restart/clean all the states in this namespace. It will also clean the deleted states. Here is a lot more details about it: Recompiling Namespaces with Running States and Cleaning up Deleted States

Just Ask

Again, the Clojure community is full of people who think and reason independently. The next time you hear something like:

” Whenever you’re about to make a singleton, ask yourself “When will I need 2 of these?” ”


” More than one instance of a resource in an app? Inconceivable! ”

Just ask for more context, you’ll see that most of the time this is just “dogma from the top” and has nothing to do with the way nature creates things.

Jan 16

Yurt: Mount’s Local Real (e)State

Components or states in mount are kept in Clojure vars, which means that they are accessible from any other namespaces that :require them. They can be made private of course but nothing stops a developer from accessing a private var in Clojure by its full name: i.e. #’any.namespace/any-var.

The flip side is that they “lie near”: they are really easy to use. Can usage of these vars be abused? Of course. Can any “other state management solutions” be abused? Of course. I like my fine balance between “easy” things for development and “simple” things for the architecture.

Several, Local and Simultaneous

In several reddit discussions, people pointed out that since mount keeps components in Clojure vars, those are singletons, hence you can’t have more than one.

While I honestly don’t know how often I would want to have more than one database connection to

* the same database
* with the same host / port / sid
* the same credentials and
* the same schema

(because if any of the above is not the same it would be a different resource / component all together) The example people gave would always come down to this one use case where you want to run a development “system” and a test “system” (potentially more than one) in the same REPL.

The way I do it today is simply running:

boot watch speak test

in a different REPL.

The need to run multiple systems in the same REPL might have something to do with the limitation of the framework (i.e. component), since you can’t just start and stop parts of a system, which means you can’t iterate quickly with sub systems in the REPL. So instead you end up predefining and running several sub systems simultaneously. But this is just an assumption.

I personally never needed to run multiple “systems” within the same REPL. But I truly believe there are multiple great ways to do things, an I simply can’t just dismiss the fact that people have different development flows.

So I sat down and started to think.

Yurts are Comfy

Mount relies on the Clojure compiler to tell it what states are defined and what order they should be started / stopped in. So, I thought, I can just use this intel, create a “system” and simply detach it from vars. In other words, using the intel mount has and only using the vars for the bootstrap I can spawn as many local systems as needed.

I don’t think this should be mount’s core functionality, plus I really like the way things work with vars and namespaces, but it would be really cool to have these local systems that people can use for testing, whether it is from REPL or for running tests in parallel.

That’s how Yurt was born. The docs go over Yurt’s API (i.e. blueprint, build, destroy) and have a couple of examples of running multiple Yurts simultaneously in the same REPL.

This of course comes down to a choice of either using vars directly or encapsulating components in Yurts, or maybe even both (?) in the same application. But what’s cool about either choice, there is still “nothing to buy”: no full app buy in.

Jan 16

Swapping Alternate Implementations with Mount

Getting ready for a talk at Clojure Remote gave me an excuse to work on several mount example apps that, I feel, should help others to start with mount, as well as just present certain ways to structure applications.

Sending SMS over Web

Using a great twilio library I wrote a small web app that sends texts over web (i.e. via HTTP POST). It serves as a good example of how to test mount states by swapping them with stubs/mocks. This application has 3 states:

  • config which is loaded from an external file

  • web-server a Jetty web server

  • send-sms which, once started, becomes a function with Twilio creds that sends texts

The app receives an HTTP POST request and sends an SMS message:

(POST "/sms/:from/:to/:msg" [from to msg]
    @(send-sms {:from from
                :to to
                :body msg}))))


send-sms is a mount state that is setup with Twilio credentials that come from the config:

(defn create-sms-sender [{:keys [sid auth-token]}]
  (fn [{:keys [from to body]}]
    (twilio/with-auth sid auth-token
        (twilio/sms from to body)))))
(defstate send-sms :start (create-sms-sender 
                            (:sms config)))

When an HTTP request is sent:

$ curl -X POST "http://localhost:4242/sms/+15104266868/+17180000000/mount%20is%20fun%20:)"

it gets wrapped to an SMS payload and is passed to Twilio which successfully delivers it:

I am sure you noticed, but the Twilio phone number this SMS is sent from is:

+1 (510) 42 MOUNT :)

Testing the App

Notice that while send-sms is a state, when started, it becomes just a function that takes args and passes them to Twilio as an SMS payload. Which means that if it is needed to be replaced, or swapped, during testing, it can be replaced with a test function that, for example, receives an SMS and puts in on a core.async channel:

(fn [sms] 
  (go (>! sms-ch sms)))

One thing to note, the real twilio/send-sms returns a future, which means it might be dereferenced somewhere in the codebase, and it is just safer to stay true to the “real thing”, so we’ll return a future as well:

(fn [sms] 
  (go (>! sms-ch sms))

Now all that needs to be done is to create a test function and let mount know to use it instead if the real one.

;; ...
(let [sms-ch (chan)
      send-sms (fn [sms]
                 (go (>! sms-ch sms))
                 (future))]                        ;; twilio API returns a future
  (mount/start-with {#'app.sms/send-sms send-sms})
;; ...

This way mount will start an application with swapping the real app.sms/send-sms for a send-sms stub function. Which means that any reference to app.sms/send-sms at runtime will be using this locally scoped send-sms stub.

Check out the working test to get a visual on how all the above pieces come together.
And here is more details on swapping alternate implementations from mount docs.

Dec 15

The Story of Booting Mount

Feeling The Code

I don’t agree with the opinion that “cool kids now use boot“. People who say that are just missing out on the power of “feeling the code” rather than being abstracted from the code by a “better XML”. Same deal with people 10 years ago who said “cool kids are using functional languages”.

Don’t get me wrong I like lein a lot. It is simple to start with, it is well documented, it is very googlable, it is sharing platform (i.e. templates), mature, etc.. But boot is very different, it does not aim to do what lein does, it aims to do “what you want”. There is a difference.

Mounting a Bootable Partition

Since the late 90s when I got in to Linux, I found bootable partitions most exciting, they actually bootstrap everything, they were these wizards waving their magic wands and systems appeared. Granted the wave could take minutes, but we are humans, we always wait for the magic, even if it takes the whole life.

First thing that needs to be done for the magic to happen, this bootable partition needs to be mounted.

I wanted to do it for some time now, when, I could not figure out why ClojureScript brought in as a dependency with :classifier “aot” caused compilation problems with lein/cljsbuild, David Nolen suggested that this is rather due to the lein environment issues. So 2 and 2 together: it was the right time to “boot” myself up.

And since the partition was already mounted it was ready to boot.

Grokking the New Simple

Rather than tell you how great boot is, I’ll share non obvious (to me) things that I stumbled upon converting mount from lein to boot. Let’s rock & roll:

REPL is just REPL

Since I needed a support for both Clojure and ClojureScript, I looked at many examples and noticed a pattern: usually in a dev mode one task groups several, where most of the examples have a (watch) task in that group.

I just wanted to start out, so I decided that at a minimum I need a REPL and (I guess) this watcher to be able to mimic the lein repl behavior, so I did:

(deftask dev [] 

And it worked! I ran boot dev and I got a REPL which would see all the updates from vim (via the updated vim-fireplace).

But then I decided to stop the REPL, and it just froze.. I ran jstack on the PID and saw lots of watcher threads locking and derefing futures. Ok, so that’s not a good combination.

The answer is simpler than I expected: it’s just boot repl. Nothing else is needed to get to the lein repl functionality.

“Bring on Your Own Data Readers” Party

The Clojure mount example app uses in memory Datomic, so when I tried to start the app, boot told me:

no reader to handle the #db/id tag

This was easily googlable, and revealed that boot has a (load-data-readers!) function that “refreshes *data-readers* with readers from newly acquired dependencies”.

An interesting bit here is that (load-data-readers!) can’t be a part of a “top level” task that is executed with boot since:

java.lang.IllegalStateException: Can't set!: *data-readers* from non-binding thread

So calling boot dev, in case “load-data-readers!” is there, is not an option. But getting into a REPL “boot repl“, and then calling (dev) works beautifully.

REPL Logging

At this point I could get into the boot REPL and start the mount example app. A slight problem was that I did not see any logging from the app within the REPL.

That’s when I found boot-logservice that brought the logging back to the REPL:

(def log4b
   [:appender {:name "STDOUT" :class "ch.qos.logback.core.ConsoleAppender"}
    [:encoder [:pattern "%-5level %logger{36} - %msg%n"]]]
   [:root {:level "TRACE"}
    [:appender-ref {:ref "STDOUT"}]]])
;; ...
(deftask dev []
  ;; ...
  (alter-var-root #'log/*logger-factory* 
                  (constantly (log-service/make-factory log4b)))
  ;; ... 
Shaking up tools.namespace

While it is not a requirement, and most of the time unnecessary, the example app uses tools.namespace to make it easier for people who rely on it heavily to get into mount.

By default “tools.namespace” won’t find anything to refresh, since boot uses its own “secret” temp directories for sources, and “tools.namespace” simply does not know about them.

This was an easy one, since it is well documented by boot. Hence having (apply set-refresh-dirs (get-env :directories)) in the “dev” task pointed “tools.namespace” to the right directories to refresh.

The Joy of Deploy: Build and Publish

At this point having the Clojure part figured out, before moving to the ClojureScript support, I decided to deploy mount to Clojars, to understand how it’s done with boot.

I found bootlaces, and just plugged it in, it was very straightforward:

(def +version+ "0.1.7-SNAPSHOT")
(bootlaces! +version+)
;; other things.. and
  pom {:project     'mount
       :version     +version+
       :description "managing Clojure and ClojureScript app state since (reset)"
       :url         "https://github.com/tolitius/mount"
       :scm         {:url "https://github.com/tolitius/mount"}
       :license     {"Eclipse Public License"

Then I did:

boot build-jar push-snapshot

and everything was going smoothly, it asked for my Clojars username, then password.. but then:

clojure.lang.ExceptionInfo: java.lang.AssertionError: 
Assert failed: current git branch is 0.1.7 but must be master
               (or (not ensure-branch) (= b ensure-branch))

Boot told me that it prefers publishing snapshots from the “master”. I don’t disagree, but for some projects I like snapshots from version branches. I don’t really like “git flow”, I like “git freedom”.

Looking at the bootlaces code it seems that “master” is hardcoded. By this time I already started to feel the concept of a “boot task” and noticed that it is hardcoded under the “push” internal task, which means that this task’s options can potentially be overridden:

;; ...
  push {:ensure-branch nil}       ;; <<<<<<<<<<
  pom {:project     'mount
       :version     +version+
       ;; ... 

And what d’you know, it worked! This was most likely the first “aha moment” which wired some of my neurons in boot ways.

Shall Not Pass!

Mount’s “test” root has both cljc tests and clj/cljs test apps that these tests use. The structure looks similar to:

| |~clj/...
| | `+tapp/
| |~cljs/...
| | `+tapp/
| |~mount/
| | |+test/...
| | `-test.cljc

In lein, I can give “test” + “test/clj” for Clojure tests, and “test” + “test/cljs” for ClojureScript tests as the sources paths.

In boot I can’t do that, boot says:

java.lang.AssertionError: Assert failed: 
The :source-paths, :resource-paths, and :asset-paths must not overlap.
    (empty? (set/intersection paths parents))

Since boot already read everything under “test”, it does not want to merge things from “test/clj”. Fair enough, so I had to change the structure a bit to make it work:

| |~clj/
| | `+tapp/
| |~cljs/
| | `+tapp/
| |~core/
| | `~mount/
| |   |+test/
| |   `-test.cljc

Now I can give “test/core” + “test/clj” and “test/core” + “test/cljs” respectively.

ClojureScript is Clojure, but.. not Always

ClojureScript took some time to get right. Many examples helped a lot especially these three: boot-cljs-example, tenzing and boot-cljs-multiple-builds.

The concept of dividing “cljs” options between “xyz.cljs.edn” and “task options” did not sink in immediately, and required some code digging to figure out where to put what and how to make sure it is being used.

It ends up to be quite simple. Options that are provided via “xyz.cljs.edn” can be referenced from task options via ids option:

(cljs :optimizations :advanced :ids #{"mount"})

would mean that it would look for mount.cljs.edn file within the classpath. That file should point to the entry point of the ClojureScript app. In case of the mount example app it would just be:

{:require  [app.example]}

where init-fns and compiler-options can be also added.

Testing ClojureScript

“mount does doo” for ClojureScript testing, and boot-cljs-test does it as well.

I would expect it to pick up “xyz.cljs.edn” files in the same way as “boot-cljs”, but it does it a bit differently. It is not all that obvious at first, but looking at the code I saw that it has a different name for ids, it calls it out-id. It also does not just take an “id”, it takes an “id” + “.js”, as I saw from the code.

So to get it to work is quite simple:

(tcs/test-cljs :out-file "mount.js"))

which would look for the same mount.cljs.edn file within the classpath.

Power it Up

There were other discoveries, like

* tasks are functions, but not really, they take arguments in the particular format and they better return a fileset

* tasks: “comp us please”. They like to be (comp ..)ed. Otherwise no go.

* there were others, but I liked Pods the most.

At this point I got all up and pumping, deployed to CircleCi using boot to build and run tests, published to Clojars as snapshot and release, etc.

One of the greatest things that I loved while debugging dependencies is boot show -p, it’s amazing!

Get up! Boot yourself up! Enjoy the runtime!