In the previous part we have briefly covered the Duct, how to create a project based on Duct, the project structure and routing. In this part, we will discuss how to implement the API, how Duct helps with Dependency injection and we will introduce a concept of boundaries.

## The API

In the previous chapter, we described routes and their configuration. Let’s do some real work and add a resource for creating messages. Our resource handler will accept a message with the following keys:

• receiver: a phone number as a String
• text: a message text as a String

Let’s start with renaming the namespace sms.handler.example to sms.handler.api. We also need to rename the :sms.handler/example component in config.edn and remove the test namespace sms.handler.example-test. You can directly remove the namespace file because we will cover it later.

The handler file should be like:

(ns sms.handler.api
(:require [compojure.core :refer compojure]
[integrant.core :as ig]))

(defmethod ig/init-key :sms.handler/api [_ options]
(compojure/context "/messages" []
(compojure/GET "/" []
{:body {:example "data"}})))
﻿

The configuration file should be like:

{:duct.profile/base
{:duct.core/project-ns sms
[#ig/ref [:sms.handler/api]]
:sms.handler/api
{:db #ig/ref :duct.database/sql}}
:duct.profile/dev #duct/include "dev"
:duct.profile/local #duct/include "local"
:duct.profile/prod {}
:duct.module/logging {}
:duct.module.web/api
{}
:duct.module/sql
{}}

We can test the changes by calling curl http://localhost:3000/messages. If you have already started the REPL, you need to refresh the code in the JVM by calling (reset) from the REPL. Otherwise, it should work when you start the server.

The resource handler would accept a message with keys (receiver, text) and return the same message with the new key id (as String). Sending a message is not an idempotent process so we also need to change the HTTP method to post.

Let’s use TDD and start with defining the test. Create a file sms.domain.message.impl_test.clj with these contents:

(ns sms.domain.message.impl-test
(:require [clojure.test :as t]
[sms.domain.message.impl :refer
[map->MessageServiceImpl]]
[sms.domain.message.sender :refer [Sender]]
[sms.domain.message.service :as service])
(:import [java.util UUID]))

(t/deftest send!-test
(t/testing "should successfully send a message"
(let [id (UUID/randomUUID)
expected-message {:id id
:text "Hej Clojure!"}
conf {:sender
(reify Sender
(send! [_ message]
(t/is (= expected-message message))))}
result (service/send! (map->MessageServiceImpl conf)
request)]
(t/is (= expected-message result)))))

To be able to compile a test we need to also define the protocols and the implementation file. Let’s define sms.domain.message.sender namespace like:

(ns sms.domain.message.sender)

(defprotocol Sender
(send! [this message]))

And the service itself sms.domain.message.service:

(ns sms.domain.message.service)

(defprotocol MessageService
(send! [this message]))

And the implementation namespace sms.domain.message.impl:

(ns sms.domain.message.impl
(:require [sms.domain.message.service :refer [MessageService]]))

(defrecord MessageServiceImpl []
MessageService
(send! [_ message]))

We did several things here. Let’s describe them to make them clear. First, we created a domain folder with a message subfolder. By that step, we made an explicit sign that the domain folder only contains our domain logic. A message is a domain object so everything related to it should be placed in sms.domain.message namespace. This structure increases the code cohesion because the functions that operate on the same data structures are together [BobCC]. When we would like to add another domain model, we just add another namespace or subfolder to the domain folder. Everybody who looks into the domain folder will see what the domain objects are, and where the functions that operate on them are.

We created a MessageSender protocol to separate behavior from implementation. The protocol provides several benefits:

• The domain behavior is not tightly coupled with the sender’s implementation.
• The protocol provides an explicit boundary between the domain logic and the rest of the system.
• It’s easy to test domain objects and their behavior.
• It’s possible to replace a sender at runtime.

Now we should be able to run tests via REPL by calling (test) or from your editor or IDE. Don’t forget to (reset) the code after every change when you run the tests from the REPL. In one of our previous articles, we described how to work with REPL.

No matter how you call the tests, they should fail and the output should look like this:

And that’s good! Because we see that our code works and it fails in the expected state. As you can see the results says that a message map was expected but the function returned nil.

Now we can implement the resource handler of course we will not be implementing real message sending. Rather we would delegate sending messages to the Sender protocol, but before that we need to talk about boundaries.

## Boundaries

When a system is being designed it’s a good practice to put the domain logic into the core of the system (ideally as pure functions) and move all communication with the outside world to the edges or boundaries of that system [BobCC]. This design approach has many advantages:

• It makes explicit what is a part of the core and what is not.
• It allows us to test the system components independently.
• These boundaries can be replaced at runtime.
• It allows us to develop the system even if we don’t know the boundary’s details.
• This separation is done on the architecture and structure layer (project’s layout, files, …) of the system.

This may sound too abstract or theoretical, so let’s show it in an example. The Sender protocol defines a boundary. This boundary is shaped at the core of the system. But its implementation is shifted outside of the core of the system. E.g. an implementation could be placed in sms.boundaries.gateway namespace.

Another example of a boundary is the repository pattern [FowlerPEAA]. The repository provides a collection-like interface for accessing domain objects. Its implementation typically connects to a database.

The Sender with one method send! takes a configuration and message for sending. Now we can continue with implementing the system even if we don’t know the real API for sending messages.

Now we can define the expected Sender behavior in the test. The Only thing we know is that the API returns an ID of a sent message and we want to add this ID to the message and return it to the caller. In case of an error, it would return an error result. The updated test could look like this:

(ns sms.domain.message.impl-test
(:require [clojure.test :as t]
[sms.domain.message.impl :refer
[map->MessageServiceImpl]]
[sms.domain.message.sender :as sender]
[sms.domain.message.service :as service])
(:import [java.util UUID]))

(def ^:private id (UUID/randomUUID))

(def ^:private expected-message
{:id id
:text "Hej Clojure!"})

(t/deftest send!-test
(t/testing "should successfully send a message"
(let [conf {:sender
(reify sender/Sender
(send! [_ message]
(t/is (= (dissoc expected-message :id)
message))
(assoc message :id id)))}
result (service/send! (map->MessageServiceImpl conf)
request)]
(t/is (= expected-message result))))

(t/testing "should return an error"
(let [conf {:sender
(reify sender/Sender
(send! [_ message]
{:error :unexpected-error}))}
result (service/send! (map->MessageServiceImpl conf)
request)]
(t/is (= {:error :unexpected-error} result)))))

We have defined the configuration for the API component with a boundary Sender. Actually we have reified the protocol (an anonymous implementation) in place just to simulate a response from the service and also a given message is asserted to the expected one (line 18).

If we run the tests again, nothing would change! Because we haven’t changed the handler’s implementation. So let’s update the implementation to use the Sender’s function send!.

(ns sms.domain.message.impl
(:require [sms.domain.message.sender :as sender]
[sms.domain.message.service :refer [MessageService]]))

(defrecord MessageServiceImpl [sender]
MessageService
(send! [_ request]
(sender/send! sender request)))

There are a few new things. First, the function uses the Sender’s send! function. Second, a sender field was added to  MessageServiceImpl record.

If we run the tests now they should work.

Now we have finished the domain logic. The architecture may look too complicated for that simple task. But real systems are more complex and too complicated for the presentation.

## Handling the HTTP requests

At this point, we have implemented the domain logic, but we don’t have any entry points to access the code. We are going to create an HTTP handler (or you can call it a controller). As in the previous section, we start with tests.

The test may look like this:

(ns sms.services.messages-test
(:require [clojure.test :as t]
[integrant.core :as ig]
[ring.mock.request :as mock]
[sms.domain.message.service :refer [MessageService]]
[sms.handler.api])
(:import [java.util UUID]))

(defn- send-message-api
[conf params]
(let [handler (ig/init-key :sms.handler/api conf)]
(-> :post
(mock/request "/messages")
(assoc :body-params params)
handler)))

(def ^:private expected-message
{:id (UUID/randomUUID)
:text "Hej Clojure!"})

(t/deftest send-message-test
(t/testing "should successfully send a message"
(let [conf {:message-service
(reify MessageService
(send! [_ message]
(t/is (= (select-keys expected-message
message))
expected-message))}
{:keys [body status]} (send-message-api conf params)]
(t/is (= expected-message body))
(t/is (= 201 status))))

(t/testing "should return 503, Sender failed"
(let [conf {:message-service
(reify MessageService
(send! [_ message]
{:error :unexpected-error}))}
{:keys [status]} (send-message-api conf params)]
(t/is (= 503 status)))))

Let’s also add the handler function, then we can compile the code:

(ns sms.handler.api.message)

(defn send! [message-service req])

When we run the tests, they should fail.

We should probably describe the test file and some of its interesting parts. We have defined the send-message-api function that wraps the API call. It calls ig/init-key on the API components with some configuration (the same thing happens when the application’s server starts), the application calls a post request with the given params.

The test itself is pretty simple, we have defined the component’s configuration as conf with one key called message-service. Under that key, we reified (mocked) the service protocol to expected behavior. We have made actual calls of the API and assert the result (body and status in this case).

The second test is almost the same as the first one, except the service returns an error result.

The very simple implementation of the handler may look like:

(ns sms.handler.api.message
(:require [ring.util.response :as response]
[sms.domain.message.service :as service]))

(defn send! [message-service req]
(let [result (service/send! message-service
(select-keys req
(if (= {:error :unexpected-error} result)
(response/status {} 503)
(response/created (format "/messages/%s" (:id result))
result))))

Now tests should be green.

## Production implementation

Well, we have implemented the HTTP handler and also added tests. But if we call the handler e.g. via curl

curl localhost:3000/messages -X POST -H 'Content-Type: application/json' -d '{"receiver": "+4207000000000", "text": "Hej Clojure!"}'

We would get something like this:

java.lang.IllegalArgumentException: No implementation of method: :send! of protocol: #'sms.domain.message.service/MessageService found for class: nil

The Message Service needs production implementation. Don’t be afraid we are not going to implement a real SMS sender. Instead, we will forward the message to another service via HTTP. First, let’s add a new dependency to the project HTTP-Kit, the latest stable version is 2.3.0. Update project.clj and restart the REPL, yes this is really needed. Start the REPL and create a file src/sms/boundaries/sms_gateway.clj with the following content:

(ns sms.boundaries.sms-gateway
(:require [duct.logger :as logger]
[integrant.core :as ig]
[jsonista.core :as jsonista]
[org.httpkit.client :as http]
[sms.domain.message.sender :refer [Sender]]))

(def mime-type "application/json")

{"Accept" mime-type
"Content-Type" mime-type})

(defrecord SmsGateway [logger url]
Sender
(send! [_ message]
(let [{:keys [body status] :as response}
(http/post url
{:as :text
:body (jsonista/write-value-as-string message)
(case status
(do
(logger/log logger :error response)
{:error :unexpected-error})))))

(defmethod ig/init-key :sms.boundaries/sms-gateway
[_ opts]
(map->SmsGateway opts))

On line 14 the Sender protocol is implemented. The record takes a logger instance and a url of the remote service. An implementation of send! The method is pretty straightforward, the API is called and if a response is successful (HTTP code 200 or 201) the response is parsed, otherwise, the response is logged and :error is returned. On line 27 the Integrant component is defined. The component converts a given configuration to a new record Sms Gateway.

Now we have a component for the Sender, but we also need a component for MessageService to be able to pass the Sender’s implementation. Let’s create a file sms.services.message with the following content:

(ns sms.services.message
(:require [integrant.core :as ig]
[sms.domain.message.impl :refer
[map->MessageServiceImpl]]))

(defmethod ig/init-key ::service
[_ opts]
(map->MessageServiceImpl opts))

The code is very simple. The Integrant component just creates an implementation of MessageService.

Let’s initialize the component in the project’s configuration. Let’s update resources/sms/config.edn like this:

{:duct.profile/base
{:duct.core/project-ns sms
[#ig/ref [:sms.handler/api]]
:sms.boundaries/sms-gateway
{:logger #ig/ref :duct/logger
:url #duct/env ["SMS_GATEWAY_URL" Str]}
:sms.services.message/service
{:sender #ig/ref :sms.boundaries/sms-gateway}
:sms.handler/api
{:message-service #ig/ref :sms.services.message/service}}
:duct.profile/dev #duct/include "dev"
:duct.profile/local #duct/include "local"
:duct.profile/prod {}
:duct.module/logging {}
:duct.module.web/api
{}
:duct.module/sql
{}}

On line 7 the SmsGateway component is initialized with the Duct’s logger and the SMS Gateway URL that is taken from an environment variable called SMS_GATEWAY_URL (as String). On line 11 the MessageService component is initialized with a sender.

This usage of Inversion of Control allows us to easily change the Sender’s implementation in tests, when the system starts, and even at runtime.

Unfortunately in local development we probably aren’t able to connect to the real SMS Gateway. But fortunately, we can fix this problem by creating a mock of the gateway. Let’s create a silly mock of the gateway dev/src/sms_dev/boundaries/sms_gateway_mock.clj

(ns sms-dev.boundaries.sms-gateway-mock
(:require [integrant.core :as ig]
[sms.domain.message.sender :refer [Sender]])
(:import [java.util UUID]))

(defrecord SmsGatewayMock []
Sender
(send! [_ message]
(assoc message :id (UUID/randomUUID))))

(defmethod ig/init-key :sms-dev.boundaries/sms-gateway-mock
[_ opts]
(map->SmsGatewayMock opts))

As you can see the silly implementation of Sender protocol just put an id on a given message to simulate the real behavior.

Now we have to tell the Duct to use SmsGatewayMock just for local development. Update dev/resources/local.edn file:

{:sms-dev.boundaries/sms-gateway-mock {}
:sms.services.message/service
{:sender #ig/ref :sms-dev.boundaries/sms-gateway-mock}}

On the very first line, a SmsGatewayMock is created and on another line, we tell the :sms.services.message/service to use the SmsGatewayMock as a Sender.

Now if we restart the REPL or call (reset) in the REPL, the code will be refreshed and we should be able to call the API e.g. via curl:

curl localhost:3000/messages -X POST -H 'Content-Type: application/json' -d '{"receiver": "+4207000000000", "text": "Hej Clojure!"}'

If everything went well the output will  look like this:

{"receiver":"+4207000000000","text":"Hej Clojure!","id":"097bd803-c3fb-4d4a-beeb-bc381ec4e4d8"}

As you can see we have sent the message and a new id has been put into that message. Because we added the SmsGatewayMock to the dev folder the mock will not be part of the production JAR (all files from the dev folder will not be included too).

In this chapter, we made a component that wraps calling the 3rd party API. By defining the actual call in the component we made it an explicit sign that this is the edge of the system. This approach allowed us to simply create a stub in tests [XUnit] and test your system without calling the real 3rd party API. Also, we have got Dependency Injection [BobCA] for free, because we can change the actual implementation of Sender without changing its usage e.g. in sms.domain.message.impl.

## The end of the second part

In this part, we have discussed how to implement the API, how Duct helps with Dependency injection and we introduced a concept of boundaries. In the next part, we will finish the project for production and discover some magic.

Next part