Access PostgreSQL

Connecting to the database requires that you know three pieces of information: The internal postgreSQL database’s username and password, and the host machine’s IP address.

The IP addresses associated with each application unit can be found using the juju status command. Since we will use the leader unit to connect to PostgreSQL, we are interested in the IP address for the unit marked with *, like shown in the output below:

Unit           	  Workload  Agent  Address   Ports  Message
postgresql-k8s/0*  active	idle   10.1.110.80     	Primary

The user we will connect to in this tutorial will be ‘operator’. To retrieve its associated password, run the Charmed PostgreSQL K8s action get-password:

juju run postgresql-k8s/leader get-password

The command above should output something like this:

Running operation 1 with 1 task
  - task 2 on unit-postgresql-k8s-0

Waiting for task 2...
password: 66hDfCMm3ofT0yrG

In order to retrieve the password of a user other than ‘operator’, use the option username:

juju run postgresql-k8s/leader get-password username=replication

At this point, we have all the information required to access PostgreSQL. Run the command below to enter the leader unit’s shell as root:

juju ssh --container postgresql postgresql-k8s/leader bash

which should bring you to a prompt like this:

 root@postgresql-k8s-0:/#

The following commands should be executed from this remote shell you just logged into.

The easiest way to interact with PostgreSQL is via PostgreSQL interactive terminal psql, which is already installed on the host you’re connected to.

For example, to list all databases currently available, run the command below. When requested, enter the password that you obtained earlier.

psql --host=10.1.110.80 --username=operator --password --list

You can see below the output for the list of databases. postgres is the default database we are connected to and is used for administrative tasks and for creating other databases.

   Name    |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges
-----------+----------+----------+-------------+-------------+-----------------------
 postgres  | operator | UTF8     | en_US.UTF-8 | en_US.UTF-8 |
 template0 | operator | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/operator          +
           |          |          |             |             | operator=CTc/operator
 template1 | operator | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/operator          +
           |          |          |             |             | operator=CTc/operator
(3 rows)

In order to execute queries, we should enter psql’s interactive terminal by running the following command, again typing password when requested:

 psql --host=10.1.110.80 --username=operator --password postgres

The output should be something like this:

psql (14.10 (Ubuntu 14.10-0ubuntu0.22.04.1))
Type "help" for help.

postgres=# 

Now you are successfully logged in the interactive terminal. Here it is possible to execute commands to PostgreSQL directly using PostgreSQL SQL Queries. For example, to show which version of PostgreSQL is installed, run the following command:

postgres=# SELECT version();
                                                             version
---------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 14.10 (Ubuntu 14.10-0ubuntu0.22.04.1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, 64-bit
(1 row)

We can see that PostgreSQL version 14.10 is installed. From this prompt, to print the list of available databases, we can simply run this command:

postgres=# \l

The output should be the same as the one obtained before with psql, but this time we did not need to specify any parameters since we are already connected to the PostgreSQL application.

For creating and connecting to a new sample database, we can run the following commands:

postgres=# CREATE DATABASE mynewdatabase;
postgres=# \c mynewdatabase

You are now connected to database "mynewdatabase" as user "operator".

We can now create a new table inside this database:

postgres=# CREATE TABLE mytable (
	id SERIAL PRIMARY KEY,
	name VARCHAR(50),
	age INT
);

And insert an element into it:

postgres=# INSERT INTO mytable (name, age) VALUES ('John', 30);

We can see our new table element by submitting a query:

postgres=# SELECT * FROM mytable;

 id | name | age
----+------+-----
  1 | John |  30
(1 row)

You can try multiple SQL commands inside this environment. Once you’re ready, reconnect to the default postgres database and drop the sample database we created:

postgres=# \c postgres

You are now connected to database "postgres" as user "operator".
postgres=# DROP DATABASE mynewdatabase;

When you’re ready to leave the PostgreSQL shell, you can just type exit. This will take you back to the host of Charmed PostgreSQL K8s (postgresql-k8s/0). Exit this host by once again typing exit. Now you will be in your original shell where you first started the tutorial. Here you can interact with Juju and MicroK8s.