Build a Clojure App with CockroachDB

Build a Clojure App with CockroachDB

This tutorial shows you how build a simple Clojure application with CockroachDB using leiningen and a PostgreSQL-compatible driver.

We have tested the Clojure java.jdbc driver in conjunction with the PostgreSQL JDBC driver enough to claim beta-level support, so that combination is featured here. If you encounter problems, please open an issue with details to help us make progress toward full support.

Before you begin

Make sure you have already installed CockroachDB.

Step 1. Install leiningen

Install the Clojure lein utility as described in its official documentation.

Step 2. Start a single-node cluster

For the purpose of this tutorial, you need only one CockroachDB node running in insecure mode:

$ cockroach start \
--insecure \
--store=hello-1 \
--listen-addr=localhost

Step 3. Create a user

In a new terminal, as the root user, use the cockroach user command to create a new user, maxroach.

$ cockroach user set maxroach --insecure

Step 4. Create a database and grant privileges

As the root user, use the built-in SQL client to create a bank database.

$ cockroach sql --insecure -e 'CREATE DATABASE bank'

Then grant privileges to the maxroach user.

$ cockroach sql --insecure -e 'GRANT ALL ON DATABASE bank TO maxroach'

Step 5. Create a table in the new database

As the maxroach user, use the built-in SQL client to create an accounts table in the new database.

$ cockroach sql --insecure \
--database=bank \
--user=maxroach \
-e 'CREATE TABLE accounts (id INT PRIMARY KEY, balance INT)'

Step 6. Run the Clojure code

Now that you have a database and a user, you'll run code to create a table and insert some rows, and then you'll run code to read and update values as an atomic transaction.

Create a basic Clojure/JDBC project

Create a new directory myapp.
Create a file myapp/project.clj and populate it with the following code, or download it directly. Be sure to place the file in the subdirectory src/test in your project.

(defproject test "0.1"
:description "CockroachDB test"
:url "http://cockroachlabs.com/"
:dependencies [[org.clojure/clojure "1.8.0"]
             [org.clojure/java.jdbc "0.6.1"]
             [org.postgresql/postgresql "9.4.1211"]]
:main test.test)

Create a file myapp/src/test/util.clj and populate it with the code from this file. Be sure to place the file in the subdirectory src/test in your project.

Basic statements

First, use the following code to connect as the maxroach user and execute some basic SQL statements, inserting rows and reading and printing the rows.

Create a file myapp/src/test/test.clj and copy the code below to it, or download it directly. Be sure to rename this file to test.clj in the subdirectory src/test in your project.

(ns test.test
  (:require [clojure.java.jdbc :as j]
            [test.util :as util]))
;; Define the connection parameters to the cluster.
(def db-spec {:subprotocol "postgresql"
              :subname "//localhost:26257/bank"
              :user "maxroach"
              :password ""})
(defn test-basic []
  ;; Connect to the cluster and run the code below with
  ;; the connection object bound to 'conn'.
  (j/with-db-connection [conn db-spec]
    ;; Insert two rows into the "accounts" table.
    (j/insert! conn :accounts {:id 1 :balance 1000})
    (j/insert! conn :accounts {:id 2 :balance 250})
    ;; Print out the balances.
    (println "Initial balances:")
    (->> (j/query conn ["SELECT id, balance FROM accounts"])
         (map println)
         doall)
    ;; The database connection is automatically closed by with-db-connection.
    ))
(defn -main [& args]
  (test-basic))

Run with:

lein run

Transaction (with retry logic)

Next, use the following code to again connect as the maxroach user but this time execute a batch of statements as an atomic transaction to transfer funds from one account to another, where all included statements are either committed or aborted.

Copy the code below to myapp/src/test/test.clj ordownload it directly. Again, preserve the file name test.clj.

Note:
CockroachDB may require theclient to retry a transaction in case of read/write contention. CockroachDB provides a generic retry function that runs inside a transaction and retries it as needed. You can copy and paste the retry function from here into your code.

(ns test.test
  (:require [clojure.java.jdbc :as j]
            [test.util :as util]))
;; Define the connection parameters to the cluster.
(def db-spec {:subprotocol "postgresql"
              :subname "//localhost:26257/bank"
              :user "maxroach"
              :password ""})
;; The transaction we want to run.
(defn transferFunds
  [txn from to amount]
  ;; Check the current balance.
  (let [fromBalance (->> (j/query txn ["SELECT balance FROM accounts WHERE id = ?" from])
                         (mapv :balance)
                         (first))]
    (when (< fromBalance amount)
      (throw (Exception. "Insufficient funds"))))
  ;; Perform the transfer.
  (j/execute! txn [(str "UPDATE accounts SET balance = balance - " amount " WHERE id = " from)])
  (j/execute! txn [(str "UPDATE accounts SET balance = balance + " amount " WHERE id = " to)]))
(defn test-txn []
  ;; Connect to the cluster and run the code below with
  ;; the connection object bound to 'conn'.
  (j/with-db-connection [conn db-spec]
    ;; Execute the transaction within an automatic retry block;
    ;; the transaction object is bound to 'txn'.
    (util/with-txn-retry [txn conn]
      (transferFunds txn 1 2 100))
    ;; Execute a query outside of an automatic retry block.
    (println "Balances after transfer:")
    (->> (j/query conn ["SELECT id, balance FROM accounts"])
         (map println)
         (doall))))
(defn -main [& args]
  (test-txn))

Run with:

lein run

After running the code, use the built-in SQL client to verify that funds were transferred from one account to another:

$ cockroach sql --insecure -e 'SELECT id, balance FROM accounts' --database=bank

+----+---------+
| id | balance |
+----+---------+
|  1 |     900 |
|  2 |     350 |
+----+---------+
(2 rows)

What's next?

Read more about using the Clojure java.jdbc driver.

You might also be interested in using a local cluster to explore the following CockroachDB benefits:

Was this page helpful?
Yes No