Becoming friends with Clojure protocols

I’ve been programming Clojure for several years, and yet I’ve managed to avoid protocols during all that time (I’ve also avoided macros, but that is another story). I found myself always having a colleague do the “dirty work” or some sad excuse as of why it wasn’t necessary right now. No more… this week I got my hands dirty.

For me, Clojure protocols solves the same problem, that I previously used interfaces in Java and PHP for: Dependency Injection (DI) and Iversion of Control (IoC). This kind of abstraction probably have several purposes, but I use it for being able to reason about a “service” without the knowledge of its implementation.

Having your services “hidden” behind a protocol will make it very pleasant to test functions that would normally require external access causing side effects (like API endpoints, database and queues). But it also ties well in with applications state management libraries like Mount and Component, when needing a “standin” for one of these external resources, e.g. for some manual testing in the REPL.

As soon as I dived into the example about protocols found on the Clojure website, I found that it was too superficial for someone like me. I’ve never approached programming very academically. For some unknown reason, most things with fancy words (polymorphism included) just refuse to stick to the inside of my skull until I see and feel it in action. My pleading for help was heard by Clojurian Slack, and after I understood (a bit more), I decided to create a more elaborate example, that maybe others would find useful.

The protocol (interface)

For a more realistic example than the one on the Clojure website, imagine some entity in a database with CRUD operations (Create, Read, Update & Delete):

(defprotocol EntityStore
  (create [this id] [this id initial-data])
  (fetch [this id])
  (save [this id data])
  (delete [this id]))

For the Read operation I choose to use a function named fetch (over get and read) and for the Update operation I use save (over update and replace). I think both fetch and save clearly describes the intention of the operation without conflicting with existing function names in clojure.core. The otherwise overlap of naming could confuse for developers, and at the same time the choice avoids linting warnings like … already refers to ….

Adding doc-strings prior implementation, will force you to evaluate the exact needs of your protocol in order to articulate them. I found myself finding errors in my design on several occasions during this:

(defprotocol EntityStore
  "All operations to the store are atomic (e.g. a DB implementation
   would use transactions or something similar)."
  (create [this id] [this id initial-data]
    "Creates a new entity in the store, and returns a map representing
     the new entity.")
  (fetch [this id]
    "Fetches (reads) an entity from the store or returns nil if it
     doesn't exist.")
  (save [this id data]
    "Saves (updates) an entity with the id `id` overwriting its data,
     returns a map representing the updated entity.")
  (delete [this id]
    "Deletes an entity with the id `id` from the store and returns
     nil."))

I decided to put the protocol definition in the namespace my-app.service.entity-store, because it would allow me to use it in the code like so:

(ns my-app.core
  (:require [my-app.service.entity-store :as entity-store-service]
            ...))

...
(let [entity-a (entity-store-service/fetch entity-store "id-for-A")
  ...

The service part of the NS, emphasizes that implementation details are “hidden away” on purpose, and I think entity-store-service/fetch read very well in the code.

Not having the protocol definition in the same namespace as where it is used, tricked me at first and caused the error: Unable to resolve symbol: <symbol name> in this context. It took me a while to figure out that methods defined using defprotocol “live” in the same namespace as the namespace where they are defined.

The (mock) implementation

I’m going to start a bit backwards with a mock of the entity store, because it will be simpler in the sense that it does not require any third party libraries and such to implement.

(ns my-app.service.impl.in-memory-entity-store
  (:require [my-app.service.entity-store :as entity-store-service]))

(defn create
  ([store-atom id]
   (create store-atom id {}))
  ([store-atom id data]
   (swap! store-atom assoc id data)))

(defn fetch
  [store-atom id]
  (get @store-atom id))

(defn save
  [store-atom id data]
  (swap! store-atom assoc id data))

(defn delete
  [store-atom id]
  (swap! store-atom dissoc id))

(deftype InMemoryEntityStore [store-atom]
  entity-store-service/EntityStore
  (create [_this id] (create store-atom id))
  (create [_this id data] (create store-atom id data))
  (fetch [_this id] (fetch store-atom id))
  (save [_this id data] (save store-atom id data))
  (delete [_this id] (delete store-atom id)))

A classic mistake to make at this point is to remove either create or save on the protocol, since the implementation is identical. But they are only identical (for now), because this mock is a very naive implementation. Also remember, the protocol should never know about the implementation details of the exposed functionality.

For convenience

Consider adding an extra convenience function in the “implementation” namespace (in above example: my-app.service.impl.in-memory-entity-store). Such a function allows you to avoid importing the class that deftype creates, which would otherwise require your code to look something like:

(ns my-app.core
  (:require [my-app.service.impl.in-memory-entity-store])
  (:import [my-app.service.impl.in-memory-entity-store InMemoryEntityStore]))

...

(InMemoryEntityStore. (atom {}))

Instead, add a function like new-store:

(ns my-app.service.impl.in-memory-entity-store
  ...

(defn new-store
  "Convenience function for creating an in memory entity store."
  [store-atom]
  (InMemoryEntityStore. store-atom))

Which would allow something like:

(ns my-app.core
  (:require [my-app.service.impl.in-memory-entity-store :as in-memory-entity-store]))

...

(in-memory-entity-store/new-store (atom {}))

Real implementation

The following NoSQL implementation using Monger, a Clojure client for MongoDB is also very naive: 😅

(ns my-app.service.impl.mongo-entity-store
  (:require [monger.collection :as mongo-document]
            [monger.core :as mongo]
            [my-app.service.entity-store :as entity-store-service]))

(def coll
  "Collection in which entities are stored in MongoDB."
  "entities")

(defn create
  ([db oid]
   (create db oid {}))
  ([db oid data]
   (mongo-document/insert-and-return db coll (assoc data :_id oid))))

(defn fetch
  [db oid]
  (mongo-document/find-map-by-id db coll oid))

(defn save
  [db oid data]
  (mongo-document/update-by-id db coll oid data))

(defn delete
  [db oid]
  (mongo-document/remove-by-id db coll oid))

(deftype MongoEntityStore [db]
  entity-store-service/EntityStore
  (create [_this id] (create db id))
  (create [_this id data] (create db id data))
  (fetch [_this id] (fetch db id))
  (save [_this id data] (save db id data))
  (delete [_this id] (delete db id)))

(defn new-store
  "Convenience function for creating a NoSQL entity store."
  [uri]
  (let [{:keys [db]} (mongo/connect-via-uri uri)]
    (MongoEntityStore. db)))

On the surface, the above solution looks fine and dandy, but it has (at least) one flaw. It requires that the id given through the protocol is a BSON ObjectId (MongoDB specific Java object). Though in-memory implementation using an atom would not complain about using ObjectId as lookup keys, it is often preferable to avoid bleeding DB specifics outside the protocol. The following three functions (hexify, pad & s->oid) is a somewhat hacky attempt to work around it and use strings instead (here be dragons 🔥🐉):

(ns my-app.service.impl.mongo-entity-store
  (:require [monger.collection :as mongo-document]
            [monger.core :as mongo]
            [my-app.service.entity-store :as entity-store-service])
  (:import [org.bson.types ObjectId]))

; Shamelessly copied from https://stackoverflow.com/questions/10062967/clojures-equivalent-to-pythons-encodehex-and-decodehex
(defn hexify
  "Convert byte sequence to hex string"
  [coll]
  (let [hex [\0 \1 \2 \3 \4 \5 \6 \7 \8 \9 \a \b \c \d \e \f]]
    (letfn [(hexify-byte [b]
              (let [v (bit-and b 0xFF)]
                [(hex (bit-shift-right v 4)) (hex (bit-and v 0x0F))]))]
      (apply str (mapcat hexify-byte coll)))))

;; Strongly inspired by https://stackoverflow.com/questions/27262268/idiom-for-padding-sequences
(defn pad
  [n val coll]
  (take n (concat coll (repeat val))))

(defn s->oid
  [^String s]
  (->> (.getBytes s)
       (pad 12 0xFF)
       (hexify)
       (ObjectId.)))

(def coll
  "Collection in MongoDB in which entities are stored."
  "entities")

(defn create
  ([db id]
   (create db id {}))
  ([db id data]
   (mongo-document/insert-and-return db coll (assoc data :_id (s->oid id)))))

(defn fetch
  [db id]
  (mongo-document/find-map-by-id db coll (s->oid id)))

(defn save
  [db id data]
  (mongo-document/update-by-id db coll (s->oid id) data))

(defn delete
  [db id]
  (mongo-document/remove-by-id db coll (s->oid id)))

(deftype MongoEntityStore [db]
  entity-store-service/EntityStore
  (create [_this id] (create db id))
  (create [_this id data] (create db id data))
  (fetch [_this id] (fetch db id))
  (save [_this id data] (save db id data))
  (delete [_this id] (delete db id)))

(defn new-store
  "Convenience function for creating a NoSQL entity store."
  [uri]
  (let [{:keys [db]} (mongo/connect-via-uri uri)]
    (MongoEntityStore. db)))

The above solution have the following advantages:

There are still a bit of Mongo hiding in the shadows because the id must be a string and only the first 12 bytes are considered for magically generating the ObjectId behind the scenes. Also, not being able to easily correlate the id "my-juicy-idA" with ObjectId("6d792d6a756963792d696441") is a bit of a bummer.

It might be possible to use UUID’s encapsulated in Mongo BSON Binary though, but that is outside the scope of this post.

The business logic

Leaving all the exiting challenges with Mongo behind and moving on…

An “Entity store service” is now available, which business logic can leverage oblivious to its implementation.

Consider the following code describing some super important business logic:

(ns my-app.core
  (:require [my-app.service.entity-store :as entity-store-service]
            [my-app.service.impl.mongo-entity-store :as mongo-entity-store]))

(def entity-store
  (mongo-entity-store/new-store "mongodb://admin:secret@172.21.0.2/customer1"))

(defn apply-business-logic
  [{:keys [entity-id id] :as _event}]
  (when-let [entity (entity-store-service/fetch entity-store entity-id)]
    (if-not (= (:name entity) "Donald Duck")
      entity
      (do ; Someone have been testing (again) - cleanup
        (entity-store-service/delete entity-store entity-id)
        nil))))

The code in apply-business-logic, doesn’t care if entity-store is of the type MongoEntityStore or InMemoryEntityStore. This is very useful for testing, among other things.

Tests (using the mock)

Notice how the following test allows testing of apply-business-logic without having a database available during testing, or preparing test data in the database (and cleaning data in the database afterwards).

(ns my-app.core-test
  (:require [clojure.test :refer [deftest is testing]]
            [my-app.core :as sut] ; System Under Testing
            [my-app.service.impl.in-memory-entity-store :as in-memory-entity-store]))

(deftest apply-business-logic
  (testing "Normal entity"
    (with-redefs [my-app.core/entity-store
                  (in-memory-entity-store/new-store
                    (atom {"123" {:name "John Doe"}}))]
      (is (= {:name "John Doe"} (sut/apply-business-logic {:entity-id "123"})))))
  (testing "Bad entity"
    (let [store-atom (atom {"123" {:name "Donald Duck"}})]
      (with-redefs [my-app.core/entity-store
                    (in-memory-entity-store/new-store store-atom)]
        (is (contains? @store-atom "123"))
        (is (nil? (sut/apply-business-logic {:entity-id "123"})))
        (is (not (contains? @store-atom "123"))))))
  (testing "Unknown entity"
    (with-redefs [my-app.core/entity-store
                  (in-memory-entity-store/new-store (atom {}))]
      (is (nil? (sut/apply-business-logic {:entity-id "non-existing"}))))))

The above code can be found on GitHub.

This post is getting long… so before I even make the stubborn and enduring people tired, I will stop with:

Protocols are your friend (that maybe you just need to get to know). 💜

Creative Commons License
Becoming friends with Clojure protocols by Jacob Emcken is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.