What Happens When You Install PostgreSQL 16 on Ubuntu

This document explains in detail what happens when you run sudo apt install postgresql-16 on Ubuntu. We'll examine everything from command processing to having a running PostgreSQL server.

Note: Skip to the end for a quick overview.

Command Processing

When you hit Enter after typing sudo apt install postgresql-16:

The shell processes the command:

Tokenizes input into: sudo, apt, install, postgresql-16
Locates sudo executable in PATH (typically /usr/bin/sudo)

Creates a child process to run this command:

pid = fork()
if (pid == 0) {
   // Execute `/usr/bin/sudo` in child process
   execve("/usr/bin/sudo", 
      [
         "sudo",             // argv[0] - program name
         "apt",              // argv[1] - first argument
         "install",          // argv[2] - second argument
         "postgresql-16",    // argv[3] - third argument
         NULL               // argv[4] - array terminator
      ],
      env                    // Current environment variables
   );
} else {
   // Parent process (bash)
   waitpid(pid, &status, 0);
}

sudo verifies user permissions and elevates the process privileges to root:
1. Reads main sudoers file /etc/sudoers:
```
# User privilege specification
root    ALL=(ALL:ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# Example user entry
username ALL=(ALL:ALL) ALL

@includedir /etc/sudoers.d
```
2. Reads additional configurations /etc/sudoers.d/*:
```
# User rules for ubuntu
ubuntu ALL=(ALL) NOPASSWD:ALL
```
3. If the user has sufficient privileges to run this command, the system changes the UID (user ID) from current user (UID=1000) to root (UID=0), giving it root privileges for the duration of the process.

Sudo executes apt:

pid_t childPid = fork();
if (childPid == 0) {
   // Execute `/usr/bin/apt` in child process
   execve("/usr/bin/apt",
      [
         "apt",              // argv[0] - program name
         "install",          // argv[1] - first argument
         "postgresql-16",    // argv[2] - second argument
         NULL               // argv[3] - array terminator
      ],
      cleaned_env           // Sanitized env vars with updated $USER (root)
   );
} else {
   // Parent process (sudo)
   waitpid(childPid, &status, 0);
}

Note:

Parent process waits for child process to finish with waitpid()
Child process inherits stdout/stderr, allowing output to terminal

Process Hierarchy

# Before sudo:
bash (UID=1000)            # bash running with current user ID (1000)
└── sudo (UID=1000 → 0)    # sudo starts with current UID (1000) but escalates to root (0)

# After sudo:
bash (UID=1000)            # Bash running with current UID (1000)
└── sudo (UID=0)           # sudo running with root UID (0)
    └── apt (UID=0)        # apt starts with root UID (0)

APT Package Resolution

To install PostgreSQL, APT (Advanced Package Tool) needs to:

Find where to download it from
Make sure the download source is trustworthy
Figure out what additional software (dependencies) is needed
Check if there's enough space
Prepare for download and ask for user input
Download the packages and verify checksums

1. Repository Configuration Processing

APT reads its "address book" of software sources. APT needs to know which servers have the software it needs to install.

Read sources configuration:

# Primary source file
/etc/apt/sources.list.d/ubuntu.sources:

Types: deb
URIs: http://archive.ubuntu.com/ubuntu/
Suites: noble noble-updates noble-backports
Components: main universe restricted multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

Build repository URLs:

# URL Pattern:
{URI}/dists/{Suite}/{Component}/binary-{Architecture}/Packages.gz

# Actual URLs:
http://archive.ubuntu.com/ubuntu/dists/noble/main/binary-amd64/Packages.gz
http://archive.ubuntu.com/ubuntu/dists/noble/universe/binary-amd64/Packages.gz
http://archive.ubuntu.com/ubuntu/dists/noble-updates/main/binary-amd64/Packages.gz
...

2. Repository Authentication and Metadata

APT downloads a Release file and verifies it's really from Ubuntu.

The InRelease file contains repository metadata (checksums) and is signed by a GPG key to ensure its authenticity. APT fetches this file from the repository and verifies its signature before trusting the metadata for package installation or upgrade.

Download and verify release files:

# Download InRelease file
# URL: http://archive.ubuntu.com/ubuntu/dists/{Suite}/InRelease
curl http://archive.ubuntu.com/ubuntu/dists/noble/InRelease

# Content excerpt of InRelease:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Origin: Ubuntu
Label: Ubuntu Noble
Suite: noble
Version: 24.04
Codename: noble
Date: Thu, 14 Mar 2024 12:04:32 UTC
Architectures: amd64 arm64 armhf i386
Components: main restricted universe multiverse
Description: Ubuntu Noble 24.04

MD5Sum:
   262e2d...          7165069 main/binary-amd64/Packages
   61fdc2...          1808488 main/binary-amd64/Packages.gz
   d9a7b0...          1401160 main/binary-amd64/Packages.xz
SHA1:
   b81dce...          7165069 main/binary-amd64/Packages
   41679a...          1808488 main/binary-amd64/Packages.gz
   cfb5fc...          1401160 main/binary-amd64/Packages.xz
SHA256:
   8f6f71...          7165069 main/binary-amd64/Packages
   e0d7e4...          1808488 main/binary-amd64/Packages.gz
   2a6a19...          1401160 main/binary-amd64/Packages.xz
-----BEGIN PGP SIGNATURE-----
[signature data]
-----END PGP SIGNATURE-----

Verify repository InRelease file signature:

# Using Ubuntu's keyring
gpgv \
   --keyring=/usr/share/keyrings/ubuntu-archive-keyring.gpg \            # Official Ubuntu public keys to sign package metadata
   /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_noble_InRelease    # Location of InRelease file

# If verification fails:
E: Release file not valid yet (invalid for another Xh XXmin XXs)
# or
E: GPG error: http://archive.ubuntu.com noble InRelease: Invalid signature

3. Package List Processing

APT downloads and reads detailed information about all available packages to find what exactly it needs and its specifications.

APT downloads the Packages.gz to /var/lib/apt/lists and verifies the data integrity using checksums in InRelease file.

Parse Packages.gz repository downloaded above:

# Example package entry - like a product description:
# Entry taken from http://archive.ubuntu.com/ubuntu/dists/noble/main/binary-amd64/Packages.gz
Package: postgresql-16
Architecture: amd64
Version: 16.2-1ubuntu4
Priority: optional
Section: database
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Debian PostgreSQL Maintainers <team+postgresql@tracker.debian.org>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 43800
Provides: postgresql-16-jit-llvm (= 17), postgresql-contrib-16
Depends: locales | locales-all, postgresql-client-16, postgresql-common (>= 252~), ssl-cert, tzdata, debconf (>= 0.5) | debconf-2.0, libc6 (>= 2.38), libgcc-s1 (>= 3.3.1), libgssapi-krb5-2 (>= 1.14+dfsg), libicu74 (>= 74.1-1~), libldap2 (>= 2.6.2), libllvm17t64, liblz4-1 (>= 0.0~r130), libpam0g (>= 0.99.7.1), libpq5 (>= 16~~), libselinux1 (>= 3.1~), libssl3t64 (>= 3.0.0), libstdc++6 (>= 5.2), libsystemd0, libuuid1 (>= 2.16), libxml2 (>= 2.7.4), libxslt1.1 (>= 1.1.25), libzstd1 (>= 1.5.5), zlib1g (>= 1:1.1.4)
Recommends: sysstat
Breaks: dbconfig-common (<< 2.0.22~)
Filename: pool/main/p/postgresql-16/postgresql-16_16.2-1ubuntu4_amd64.deb
Size: 15452168
MD5sum: a0ddd38a3ce650de38cbd1fa4960861f
SHA1: 55098507be06aa3bbdb7d0fef855338c6195e0fd
SHA256: 7f25383e8f39ca99f4886b7b2cbb31c62534ac924e3a61da3f0e8e6456091e69
SHA512: 181d452698e0895c5f54d916a4ee2f2bf6899175e9f5a7d0049723026db9360da0a3f4001f34e3015bc0e0d90d92a63eb161b50d62085cc2c87f0a559e317fe2
Homepage: http://www.postgresql.org/
Description: The World's Most Advanced Open Source Relational Database
Task: postgresql-server
Postgresql-Catversion: 202307071
Description-md5: d7367291c15795e56aa78252604f9ced

4. Dependency Resolution

APT figures out what other software PostgreSQL needs to work properly.

Build dependency graph:

postgresql-16
├── postgresql-client-16    # Needed to connect to database
│   ├── libpq5             # Core PostgreSQL library
│   └── ssl-cert           # For secure connections
├── postgresql-common      # Shared PostgreSQL utilities
└── System libraries       # Basic system requirements

Version Resolution: Check the availability and details of the package postgresql-16 in the APT repositories configured on the system:

$ apt-cache policy postgresql-16
# Output format
postgresql-16:
   Installed: <installed_version>     # Installed version (if any)
   Candidate: <candidate_version>     # Version chosen to install (higher priority or version)
   Version table:                     # Lists all available versions of postgresql-16
      <version_num> <priority>        # Default priority is 500
         <priority> <repository_url> <distribution>/<repository_section> <architecture> <file>

# Actual output
postgresql-16:
   Installed: (none)
   Candidate: 16.4-0ubuntu0.24.04.2
   Version table:
      16.4-0ubuntu0.24.04.2 500
         500 http://ap-south-1.ec2.archive.ubuntu.com/ubuntu noble-updates/main amd64 Packages
      16.4-0ubuntu0.24.04.1 500
         500 http://security.ubuntu.com/ubuntu noble-security/main amd64 Packages
      16.2-1ubuntu4 500
         500 http://ap-south-1.ec2.archive.ubuntu.com/ubuntu noble/main amd64 Packages

5. Package Download Preparation

APT calculates total download size and checks if you have enough space.

Calculate requirements:

# Create download queue
Need to get 23.1 MB of archives.
After installation: 85.3 MB will be used.

Download Queue:
1. postgresql-16          (16.2 MB)
2. postgresql-client-16   (4.8 MB)
3. postgresql-common      (2.1 MB)

Check free space:

# Verify space in:
/var/cache/apt/archives/     # Temporary download storage
/var/lib/postgresql/         # Where PostgreSQL will live
/etc/postgresql/            # Where settings will be stored

Prepare download locations:

# Create/verify directories
mkdir -p /var/cache/apt/archives/partial

# Set up download resume markers
touch /var/cache/apt/archives/partial/postgresql-16_16.2-1ubuntu1_amd64.deb.dl

At this point, APT has:

Verified repository authenticity
Located all required packages
Resolved dependencies
Calculated disk space requirements
Prepared for download

The next phase begins once the user confirms the installation prompt.

6. Package Download

Download packages:

# Download to cache
/var/cache/apt/archives/
└── partial/                  # Temporary download location
    └── postgresql-16_16.2-1ubuntu1_amd64.deb

# After download completion, move to
/var/cache/apt/archives/postgresql-16_16.2-1ubuntu1_amd64.deb

Verify packages:

# Check SHA256 sum against Packages file
sha256sum postgresql-16_16.2-1ubuntu1_amd64.deb

# Verify package signatures if available
dpkg-sig --verify postgresql-16_16.2-1ubuntu1_amd64.deb

Installation Process

Pre-installation:

# Create system user/group
useradd --system \
        --home /var/lib/postgresql \
        --shell /bin/bash \
        postgres

# Create directories
mkdir -p /etc/postgresql/16/main
mkdir -p /var/lib/postgresql/16/main
mkdir -p /var/log/postgresql

Package Extraction:

# Extract package contents
dpkg-deb -x postgresql-16_16.2-1.deb /

# Main directories:
/etc/postgresql/16/          # Configuration files
/usr/lib/postgresql/16/      # Binaries
/usr/share/postgresql/16/    # Shared files
/var/lib/postgresql/16/      # Data directory

Database Cluster Creation:

# pg_createcluster execution
1. Initialize data directory (initdb)
2. Generate configuration files
3. Set up system tables
4. Configure authentication

System Service Setup

Systemd Configuration:

# Create service files
/lib/systemd/system/postgresql@.service    # Service template
/lib/systemd/system/postgresql.service     # Main service

# Enable service
systemctl enable postgresql@16-main
systemctl start postgresql@16-main

Process Hierarchy:

systemd
└── postgres (postmaster)
    ├── postgres: logger process
    │   └── Writes to /var/log/postgresql/postgresql-16-main.log
    ├── postgres: checkpointer process
    │   └── Flushes dirty buffers to disk
    ├── postgres: background writer
    │   └── Writes dirty shared buffers
    ├── postgres: WAL writer
    │   └── Writes WAL buffers to disk
    ├── postgres: autovacuum launcher
    │   └── Cleans up dead tuples
    ├── postgres: stats collector
    │   └── Collects database statistics
    └── postgres: logical replication launcher
        └── Handles logical replication

Resource Allocation

Memory Allocation:

# For 1GB RAM system:
shared_buffers       = 128MB   # 25% reduced for small RAM
work_mem             = 4MB     # Per operation memory
maintenance_work_mem = 64MB    # For maintenance operations
wal_buffers          = 4MB     # WAL writing buffer

Disk Organization:

/var/lib/postgresql/16/main/
├── base/                    # Databases
│   ├── 1/                  # template1
│   ├── 13067/              # template0
│   └── 13068/              # postgres
├── global/                 # Shared objects
├── pg_wal/                # Write-ahead logs
├── pg_xact/               # Transaction status
└── postgresql.conf        # Configuration

Database Initialization

System Catalog Creation:

# Creates system databases
CREATE DATABASE template0;
CREATE DATABASE template1;
CREATE DATABASE postgres;

# Creates system tables
CREATE TABLE pg_class;    # Relation catalog
CREATE TABLE pg_attribute; # Column catalog
CREATE TABLE pg_proc;     # Function catalog

User Authentication Setup:

# pg_hba.conf configuration
local   all   postgres               peer
local   all   all                    peer
host    all   all     127.0.0.1/32   md5

Final State

After installation completes:

PostgreSQL Server State:
- Running as service (postgresql@16-main)
- Listening on port 5432
- Managing shared memory segments
- Processing client connections
- Writing to WAL files
- Performing background maintenance
System Configuration:
- Default databases created
- System catalog populated
- PostgreSQL superuser account configured
- Basic security settings applied
- Logging enabled

The installation process involves multiple components:

APT handles package management
dpkg handles package extraction
pg_createcluster initializes the database
systemd manages the service
postmaster manages the running instance

System Requirements and Performance:

Installation time: 2-5 minutes on a system with 1 vCPU
Initial disk usage: ~100MB
Runtime memory usage: ~200MB
Idle CPU usage: <1%

TODO

Add a sequence diagram.
Review postgres preinst, installation, postinst steps and add more details.

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you_install_postgres_on_ubuntu.md

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What Happens When You Install PostgreSQL 16 on Ubuntu

Command Processing

Process Hierarchy

APT Package Resolution

1. Repository Configuration Processing

2. Repository Authentication and Metadata

3. Package List Processing

4. Dependency Resolution

5. Package Download Preparation

6. Package Download

Installation Process

System Service Setup

Resource Allocation

Database Initialization

Final State

TODO

Files

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you_install_postgres_on_ubuntu.md

File metadata and controls

What Happens When You Install PostgreSQL 16 on Ubuntu

Command Processing

Process Hierarchy

APT Package Resolution

1. Repository Configuration Processing

2. Repository Authentication and Metadata

3. Package List Processing

4. Dependency Resolution

5. Package Download Preparation

6. Package Download

Installation Process

System Service Setup

Resource Allocation

Database Initialization

Final State

TODO