Building services with Duct and compojure-api

October 1, 2015

In this post we'll look at writing a RESTful service using Duct and compojure-api. Our service will use a SQLite database and illustrate how to do operations such as adding, remove, and authenticating users.


Creating the Project

Duct is a minimal web framework with emphasis on using component to manage stateful resources such as database connections. We can create a new Duct application by running the following command:

lein new duct swagger-service +example
This will generate a fresh application and add an example route component to it. Once the application is created we'll have to run the setup task to generate local assets in the root folder of the application:
cd swagger-service
lein setup

We can now test that our application works as follows:

lein run

If everything went well then we should be able to navigate to localhost:3000 and see Hello World displayed on the page. We're now ready to start working on creating our service.

Adding Dependencies

We'll start by adding some dependencies in project.clj that we'll need in order to create our service:

 [crypto-password "0.1.3"]
 [metosin/compojure-api "0.23.1"]
 [org.xerial/sqlite-jdbc ""]
 [yesql "0.5.0"]
 [migratus "0.8.4"]]

We'll use crypto-password to handle password hashing when we create user accounts and checking passwords during authentication. The compojure-api library will be used to generate the service endpoints. The sqlite-jdbc driver will be used as our data store, we'll access it using yesql, and we'll generate the database using migratus.

Configuring Migrations

Let's add the migratus plugin along with its configuration to our project:

:plugins [[lein-environ "1.0.1"]
          [lein-gen "0.2.2"]
          [migratus-lein "0.1.7"]]

:migratus {:store :database             
           :db {:classname "org.sqlite.JDBC"
                :connection-uri "jdbc:sqlite:service-store.db"}}          

We can now run the following commands to generate the migration files for the users table:

mkdir resources/migrations
lein migratus create users

This will produce files for up and down migrations such as:


The up migrations file will create the table:

 first_name VARCHAR(30),
 last_name VARCHAR(30),
 email VARCHAR(30),
 admin BOOLEAN,
 last_login TIME,
 is_active BOOLEAN,
 pass VARCHAR(100));

Conversely, the down migrations file will delete it:

We can now run the following command to create the database:
lein migratus migrate

Adding Database Queries

With the database created, we'll need to add some queries to access the database. We'll create a new file called resources/sql/queries.sql and put the following SQL queries in it:

-- name: create-user!
-- creates a new user record
(id, first_name, last_name, email, pass)
VALUES (:id, :first_name, :last_name, :email, :pass)

-- name: get-user
-- retrieve a user given the id.
WHERE id = :id

-- name: get-users
-- retrieve a user given the id.
SELECT id, first_name, last_name, email FROM users

-- name: delete-user!
-- delete a user given the id
WHERE id = :id

Creating the Database Component

Now, let's create a component that will be used to access it. We'll create a new namespace called swagger-service.component.db then put the following code there:

(ns swagger-service.component.db
  (:require [yesql.core :refer [defqueries]]
            [com.stuartsierra.component :as component]
            [crypto.password.bcrypt :as password]
            [environ.core :refer [env]]))

(defqueries "sql/queries.sql")

(defn create-user-account! [user db]
  (create-user! (update user :pass password/encrypt) db))

(defn authenticate [user db]
   (when-let [db-user (-> user (get-user db) first)]
     (password/check (:pass user) (:pass db-user)))))

(defrecord DbComponent [connection]
  (start [component]
         (assoc component :connection connection))
  (stop [component]
        (dissoc component :connection)))

(defn db-component [connection]
  (->DbComponent connection))

The namespace will define query functions by calling the defqueries macro and giving it the path to the queries.sql file we just created.

Then we'll add a couple of helper functions to create a user account with a hashed password and to check whether the user and the password match the stored credentials.

Next, we define the DbComponent record that will manage the lifecycle of the database. The start function in the component will associate the given connection settings with the :connection key in the component, and the stop function will remove the connection.

The connection is specified in the swagger-service.config namespace and points to the connection-uri key that is expected to be found in the environment.

(def environ
  {:http {:port (some-> env :port Integer.)}
   :db   {:connection-uri (:connection-uri env)}})

We'll add the actual connection information under the :env key in profiles.clj:

;; Local profile overrides

{:profiles/dev  {:env {:connection-uri "jdbc:sqlite:service-store.db"}}
 :profiles/test {}}

Finally, we have a helper function to instantiate the component called db-component.

Adding a New Component to the System

With the component created we can now add it to the system found in the swagger-service.system namespace:

(ns swagger-service.system
 (:require ...
           [swagger-service.component.db :refer [db-component]]))

(defn new-system [config]
 (let [config (meta-merge base-config config)]
   (-> (component/system-map
        :db      (db-component (:db config))
        :app     (handler-component (:app config))
        :http    (jetty-server (:http config))
        :example (endpoint-component example-endpoint))
        {:http [:app]
         :app  [:example]
         :example []          
         :db []}))))            

Creating an HTTP Endpoint Component

The final step is to add the service endpoint that will provide the RESTful interface to the database. We'll create a new namespace called swagger-service.endpoint.service. The namespace will use the compojure-api library to define the service operations. The library requires us to declare the types of request parameters and responses for each endpoint using the schema library.

Let's start by creating the namespace declaration with the following references:

(ns swagger-service.endpoint.service
 (:require [ :as io]
           [ring.util.http-response :refer :all]
           [compojure.api.sweet :refer :all]
           [schema.core :as s]
           [swagger-service.component.db :as db]))

Then we'll create the schema for the User type that matches the user table in our database:

(s/defschema User
 {:id String
  (s/optional-key :first_name) String
  (s/optional-key :last_name) String
  (s/optional-key :email) String
  (s/optional-key :pass) String})

Finally, let's create the service-endpoint component that will define the service routes. The component accepts the config as its parameter. The config will contain the :db key that we added to our system earlier with the database connection.

The routes are created by calling the api macro from compojure-api:

(defn service-endpoint [config]
    {:info {:title "User API"}})
   (context* "/api" []
             :tags ["users"]

             (GET* "/users" []
                   :return  [User]
                   :summary "returns the list of users"
                   (ok (db/get-users {} (:db config))))
             (GET* "/user/:id"  []
                   :return      User
                   :path-params [id :- String]
                   :summary     "returns the user with a given id"
                   (ok (db/get-users {:id id} (:db config))))

             (POST* "/authenticate" []
                    :return         Boolean
                    :body-params    [user :- User]
                    :summary        "authenticates the user using the id and pass."
                    (ok (db/authenticate user (:db config))))
             (POST* "/user"      []
                    :return      Long
                    :body-params [user :- User]
                    :summary     "creates a new user record."
                    (ok (db/create-user-account! user (:db config))))
             (DELETE* "/user"    []
                    :return      Long
                    :body-params [id :- String]
                    :summary     "deletes the user record with the given id."
                    (ok (db/delete-user! {:id id} (:db config)))))))

Notice that we call ring.swagger.ui/swagger-ui and swagger-docs at the beginning of the definition of api. This will automatically produce the API documentation for the service operations defined within it. Once our service is hooked up, we'll be able to navigate to localhost:3000/swagger-ui and see an interactive page for testing the API endpoints.

As you may have noticed, compojure-api mimics Compojure route definitions with the difference that the route method name has a * after it. The route definition also has some additional keys associated with it.

  • the :return key specifies the return type for the service operation
  • the :summary key provides the documentation about the purpose of the operation
  • the parameters are specified using different keys depending on the parameter type, such as :path-params and :body-params.

Finally, each route will return a response type with the result of calling the handler associated with it.

If we look at the "/users" route we see that it calls the get-users function from the database and passes it the value of the :db key from the config. This will be used to resolve the database connection at runtime.

Adding the Endpoint to the System

With the route added we can now navigate back to the swagger-service.system namespace and add the component there:

(ns swagger-service.system
 (:require ...
           [swagger-service.component.db :refer [db-component]]
           [swagger-service.endpoint.service :refer [service-endpoint]]))

(defn new-system [config]
 (let [config (meta-merge base-config config)]
   (-> (component/system-map
        :db      (db-component (:db config))
        :app     (handler-component (:app config))
        :http    (jetty-server (:http config))
        :example (endpoint-component example-endpoint)
        :service (endpoint-component service-endpoint))
        {:http [:app]
         :app  [:example :service]
         :service [:db]
         :example []          
         :db []}))))            

The service component is initialized using the endpoint-component Duct helper. Next, the component relationships have to be described explicitly. We can see that the :service component depends on the :db component, and the :app in turn depends on both the :example and the :service.

We can now restart our app and navigate to localhost:3000/swagger-ui to see the service test page. Using this page we can test all the service operations that we defined such as creating new users, authenticating, and listing users.

The full source for this tutorial is available on GitHub.


As you can see, compojure-api allows us to easily define RESTful services with type assertions, documentation, and a helpful test page. I've found this approach to be extremely effective when creating service APIs as it documents what each endpoint is doing and makes it easy to collaborate with consumers of the service. Meanwhile, Duct provides an excellent base for building services using the component pattern.

Copyright © Dmitri Sotnikov

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