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Configuring Database Connection Pooling, Migration and Reloaded Workflow

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In the last blog post, we bootstrapped the Clojure project using Mount and Aero. We are going to continue from we left off and configure database connection pooling, migration & Reloaded Workflow in this blog post.

This blog post is a part 2 of the blog series Building an E-Commerce Marketplace Middleware in Clojure.

Configuring Hikari-CP

Let's get started by adding the hikari-cp, a Clojure wrapper to HikariCP, and the Postgres driver dependencies in the project.clj.

(defproject wheel "0.1.0-SNAPSHOT"
; ...
:dependencies [; ...
[org.postgresql/postgresql "42.2.6"]
[hikari-cp "2.8.0"]]
; ...

NOTE: If you already have a running (and jacked in) REPL, you need to stop and start it again after adding any dependencies in the project.clj file.

To configure the Hikari connection pool, Let's create a new file database.clj in the infra directory.

> touch src/wheel/infra/database.clj

Then define a mount state datasource to manage the life-cycle of the connection pool.

(ns wheel.infra.database
(:require [wheel.infra.config :as config]
[mount.core :as mount]
[hikari-cp.core :as hikari]))

(defn- make-datasource []
(hikari/make-datasource (config/database))) ;<1>

(mount/defstate datasource
:start (make-datasource)
:stop (hikari/close-datasource datasource))

<1> Retrieves the database configuration and creates the datasource object.

Now if we start the application through Mount, we will get the following output.

wheel.infra.database=> (mount/start)
{:started ["#'wheel.infra.config/root"

As the state datasource depends on the config's root state, Mount starts it first and then it starts the datasource.

Database Migration Using Flyway

There are multiple libraries in Clojure (and Java) to perform database migration. Our preference is Flyway, based on our success in other Java projects.

Let's get started by adding the dependency in the project.clj.

(defproject wheel "0.1.0-SNAPSHOT"
; ...
:dependencies [; ...
[org.flywaydb/flyway-core "5.2.4"]]
; ...

Then in the database.clj file, import the Flyway namespace and add a new function migrate

(ns wheel.infra.database
; ...
(:import [org.flywaydb.core Flyway]))

; ...

(defn migrate []
(.. (Flyway/configure) ; <1>
(dataSource datasource)
(locations (into-array String ["classpath:db/migration"])) ; <2>

<1> Creates an instance of Flyway and setup the configuration using the dot special form

<2> Setting the migration files path to db/migration.

This path doesn't exist yet. So, let's create it.

> mkdir -p resources/db/migration

To verify the database migration, let's load the updated database.clj in the REPL and invoke the migrate function.

wheel.infra.database=> (mount/start)
{:started ["#'wheel.infra.config/root"
wheel.infra.database=> (migrate)
wheel.infra.database=> (mount/stop)
{:stopped ["#'wheel.infra.database/datasource"]}

If we check the database now, it will have the flyway_schema_history table.

> psql -d wheel

wheel=# \d

List of relations
Schema | Name | Type | Owner
public | flyway_schema_history | table | postgres
(1 row)

Separating Database Migration From Application Bootstrap

Performing database migration during application bootstrap has certain limitations, and it is a best practice to decouple it. In our application, we did it by having separate entry-points, one to start the application and another to migrate the database.

Let's create a new file core.clj in the infra directory.

> touch src/wheel/infra/core.clj

Then define two functions, start-app and migrate-database, to start the application and perform database migration, respectively.

(ns wheel.infra.core
(:require [wheel.infra.config :as config]
[wheel.infra.database :as db]
[mount.core :as mount]))

(defn start-app []

(defn migrate-database []
(mount/start #'config/root #'db/datasource) ; <1>
(db/migrate) ; <2>
(mount/stop #'db/datasource)) ; <3>

<1> Invokes Mount's start function with that states that we wanted to start. Note that Mount doesn't start the transitive dependent states in this function overload.

<2> Performs the database migration

<3> Stops the datasource after completing the migration.

Let's add the stop-app function as well to stop the application.

(ns wheel.infra.core
; ...
(defn stop-app []

Reloaded Workflow

Reloaded Workflow is one of the common practice in Clojure development to do interactive REPL driven development.

Adding this to our current codebase is straight-forward.

As a first step, add a Leiningen user profile called dev in the 's project.clj file.

(defproject wheel "0.1.0-SNAPSHOT"
; ...
:profiles { ;...
:dev {:source-paths ["dev"] ; <1>
:dependencies [[org.clojure/tools.namespace "0.3.1"]]}}) ; <2>

<1> Adds a new source-path to load the Clojure files from the dev directory.

<2> Adds a dev dependency tools.namespace to reload the modified source files interactively.

Then, create a new file user.clj in the dev directory.

; dev/user.clj
(ns user
(:require [wheel.infra.core :refer [start-app ; <1>
:as infra]
[clojure.tools.namespace.repl :as repl]))

(defn reset [] ; <2>
(repl/refresh :after 'infra/start-app))

<1> Makes start-app, stop-app and migrate-database function to be available in the user namespace.

<2> Adds a reset function, which stops the application and reloads all the modified codes. Using the :after parameter, we are informing the refresh function to start the app after the reload.

To see it in action, stop the current REPL session and start it again. This time profile selection Calva prompt will include the dev profile in the list of options.

When we select the dev profile, it will start the Leiningen REPL along with the configuration specified the dev profile.

Then from the REPL, we can include the user profile and manage the application's life cycle.

wheel.core=> (in-ns 'user)

wheel.core=> (start-app)
{:started ["#'wheel.infra.config/root"

wheel.core=> (stop-app)
{:stopped ["#'wheel.infra.database/datasource"]}

wheel.core=> (migrate-database)
{:stopped ["#'wheel.infra.database/datasource"]}

wheel.core=> (reset)
:reloading (wheel.infra.config wheel.infra.database
wheel.infra.core wheel.core wheel.core-test user)
{:started ["#'wheel.infra.config/root"


In this blog post, we learnt how to configure the database connection pooling using Hikari, database migration using Flyway and Reloaded Workflow using the Mount and Leiningen profile. There are other libraries and ways to do this as well, but the approach that I described here is the one that worked for us.

I would love to learn your preferences and approaches.

The source code associated with this part is available on this GitHub repository.

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