Install and configure PostgreSQL

PostgreSQL (also known as Postgres) is an object-relational database system that has the features of traditional commercial database systems with enhancements to be found in next-generation database management systems (DBMS).

Installation

To install PostgreSQL, run the following command in the command prompt:

sudo apt install postgresql

The database service is automatically configured with viable defaults, but can be customised based on your specific needs.

Configuration

PostgreSQL supports multiple client authentication methods. In Ubuntu, peer is the default authentication method used for local connections, while scram-sha-256 is the default for host connections (this used to be md5 until Ubuntu 21.10). Please refer to the PostgreSQL Administrator’s Guide if you would like to configure alternatives like Kerberos.

The following discussion assumes that you wish to enable TCP/IP connections and use the MD5 method for client authentication. PostgreSQL configuration files are stored in the /etc/postgresql/<version>/main directory. For example, if you install PostgreSQL 14, the configuration files are stored in the /etc/postgresql/14/main directory.

Tip:
To configure IDENT authentication, add entries to the /etc/postgresql/*/main/pg_ident.conf file. There are detailed comments in the file to guide you.

By default only connections from the local system are allowed, to enable all other computers to connect to your PostgreSQL server, edit the file /etc/postgresql/*/main/postgresql.conf. Locate the line: #listen_addresses = ‘localhost’ and change it to *:

listen_addresses = '*'

Note:
‘*’ will allow all available IP interfaces (IPv4 and IPv6), to only listen for IPv4 set ‘0.0.0.0’ while ‘::’ allows listening for all IPv6 addresses.

For details on other parameters, refer to the configuration file or to the PostgreSQL documentation for information on how they can be edited.

Now that we can connect to our PostgreSQL server, the next step is to set a password for the postgres user. Run the following command at a terminal prompt to connect to the default PostgreSQL template database:

sudo -u postgres psql template1

The above command connects to PostgreSQL database template1 as user postgres. Once you connect to the PostgreSQL server, you will be at an SQL prompt. You can run the following SQL command at the psql prompt to configure the password for the user postgres.

ALTER USER postgres with encrypted password 'your_password';

After configuring the password, edit the file /etc/postgresql/*/main/pg_hba.conf to use scram-sha-256 authentication with the postgres user, allowed for the template1 database, from any system in the local network (which in the example is 192.168.122.1/24) :

hostssl template1       postgres        192.168.122.1/24        scram-sha-256

Note:
The config statement ‘hostssl’ used here will reject tcp connections that would not use ssl. Postgresql in Ubuntu has the ssl feature built in and configured by default, so it works right away. On your postgresql server this uses the certificate created by ‘ssl-cert’ package which is great, but for production use you should consider updating that with a proper certificate from a recognized CA.

Finally, you should restart the PostgreSQL service to initialise the new configuration. From a terminal prompt enter the following to restart PostgreSQL:

sudo systemctl restart postgresql.service

Warning:
The above configuration is not complete by any means. Please refer to the PostgreSQL Administrator’s Guide to configure more parameters.

You can test server connections from other machines by using the PostgreSQL client as follows, replacing the domain name with your actual server domain name or IP address:

sudo apt install postgresql-client
psql --host your-servers-dns-or-ip --username postgres --password --dbname template1

Streaming replication

PostgreSQL has a nice feature called Streaming Replication which provides the capability to continuously ship and apply the Write-Ahead Log (WAL) XLOG records to some number of standby servers in order to keep them current. Here is presented a very basic and simple way to replicate a PostgreSQL server (main) to a standby server.

First, create a replication user in the main server to be used from the standby server:

sudo -u postgres createuser --replication -P -e replicator

Let’s configure the main server to turn on the streaming replication. Open the file /etc/postgresql/*/main/postgresql.conf and make sure you have the following lines:

listen_addresses = '*'
wal_level = replica

Also edit the file /etc/postgresql/*/main/pg_hba.conf to add an extra line to allow the standby server connection for replication (that is a special keyword) using the replicator user:

host  replication   replicator   <IP address of the standby>      scram-sha-256

Restart the service to apply changes:

sudo systemctl restart postgresql

Now, in the standby server, let’s stop the PostgreSQL service:

sudo systemctl stop postgresql

Edit the /etc/postgresql/*/main/postgresql.conf to set up hot standby:

hot_standby = on

Back up the current state of the main server (those commands are still issued on the standby system):

sudo su - postgres
# backup the current content of the standby server (update the version of your postgres accordingly)
cp -R /var/lib/postgresql/14/main /var/lib/postgresql/14/main_bak
# remove all the files in the data directory
rm -rf /var/lib/postgresql/14/main/*
pg_basebackup -h <IP address of the main server> -D /var/lib/postgresql/14/main -U replicator -P -v -R

After the above this will have done a full single pass copying the content of the main database onto the local system being the standby. In the pg_basebackup command the flags represent the following:

-h: The hostname or IP address of the main server
-D: The data directory
-U: The user to be used in the operation
-P: Turns on progress reporting
-v: Enables verbose mode
-R: Creates a standby.signal file and appends connection settings to postgresql.auto.conf

Finally, let’s start the PostgreSQL service on standby server:

sudo systemctl start postgresql

To make sure it is working, go to the main server and run the following command:

sudo -u postgres psql -c "select * from pg_stat_replication;"

As mentioned, this is a very simple introduction, there are way more great details in the upstream documentation about the configuration of replication as well as further High Availability, Load Balancing, and Replication.

To test the replication you can now create a test database in the main server and check if it is replicated in the standby server:

sudo -u postgres createdb test # on the main server
sudo -u postgres psql -c "\l" # on the standby server

You need to be able to see the test database, that was created on the main server, in the standby server.

Backups

PostgreSQL databases should be backed up regularly. Refer to the PostgreSQL Administrator’s Guide for different approaches.

Resources

As mentioned above, the PostgreSQL Administrator’s Guide is an excellent resource. The guide is also available in the postgresql-doc package. Execute the following in a terminal to install the package:
```
sudo apt install postgresql-doc
```

This package provides further man pages on postgresql ‘dblink’ and ‘server programming interface’ as well as the html guide that you’d find upstream. To view the guide enter xdg-open /usr/share/doc/postgresql-doc-*/html/index.html or point your browser at it.

For general SQL information see the O’Reilly books Getting Started with SQL: A Hands-On Approach for Beginners by Thomas Nield as an entry point and SQL in a Nutshell as a quick reference.
Also, see the PostgreSQL Ubuntu Wiki page for more information.

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