build-a-clojure-app-with-cockroachdb.md

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Build a Clojure App with CockroachDB	Learn how to use CockroachDB from a simple Clojure application with a low-level client driver.	false	true

This tutorial shows you how build a simple Clojure application with CockroachDB using leiningen and a PostgreSQL-compatible driver. We've tested and can recommend the Clojure java.jdbc driver in conjunction with the PostgreSQL JDBC driver, so that driver is featured here.

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.

{% include app/common-steps.md %}

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

1. Create a new directory myapp.

2. 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.

   {% include app/project.clj %}

3. 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.

{% include app/basic-sample.clj %}

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 or download it directly. Again, preserve the file name test.clj.

{{site.data.alerts.callout_info}}With the default SERIALIZABLE isolation level, CockroachDB may require the client 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.{{site.data.alerts.end}}

{% include app/txn-sample.clj %}

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 core CockroachDB features:

• Data Replication
• Fault Tolerance & Recovery
• Automatic Rebalancing