When trying to enable HTTP/2 on Apache 2.4.29 (Ubuntu 16.04), many developers hit a fundamental compatibility wall. The issue stems from Apache's Multi-Processing Module architecture:

# Check current MPM
apache2ctl -V | grep -i mpm
# Typical output for problematic setups:
Server MPM:     prefork

HTTP/2 requires multiplexed connections handled by a threaded/event-driven MPM. mpm_prefork's process-based model can't support this. Here's the technical breakdown:

# This configuration WON'T work:

    StartServers        5
    MinSpareServers     5
    MaxSpareServers     10
    MaxRequestWorkers   150
    MaxConnectionsPerChild 0

You have two viable approaches:

Option 1: Switch to mpm_event

The modern recommended approach:

sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo a2enmod http2
sudo systemctl restart apache2

Option 2: PHP-FPM Alternative

For shared hosting environments where mod_php is required:

# Install PHP-FPM
sudo apt-get install php-fpm
# Configure Apache to use FastCGI

    SetHandler "proxy:unix:/run/php/php7.0-fpm.sock|fcgi://localhost"

After making changes, verify with:

curl -I --http2 https://yourdomain.com
# Look for:
HTTP/2 200

Here's a working httpd.conf snippet:

    Protocols h2 h2c http/1.1
    H2Direct on



    ServerLimit             16
    StartServers            2
    ThreadsPerChild         25
    MaxRequestWorkers       400
    MaxConnectionsPerChild  10000

---

When trying to enable HTTP/2 on Apache 2.4.29 (Ubuntu 16.04), many developers hit a wall despite following the standard procedure:

sudo a2enmod http2

And adding the protocol directive:

Protocols h2 http/1.1

The primary issue stems from using mpm_prefork module. HTTP/2 requires a threaded MPM (Multi-Processing Module) to function properly. Here's why:

• HTTP/2 multiplexing requires threads to handle concurrent streams
• Prefork creates separate processes (not threads) for each request
• Modern protocols need modern server architectures

The solution is to migrate to mpm_event module:

sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo systemctl restart apache2

Verify the change with:

apache2ctl -M | grep mpm

For PHP on shared hosting, you'll need to consider alternative handlers since php-fpm is the recommended approach:

sudo apt install php-fpm
sudo a2enmod proxy_fcgi setenvif
sudo a2enconf php7.0-fpm  # Adjust version as needed

Here's a working configuration for HTTP/2 with PHP support:

<VirtualHost *:443>
    ServerName example.com
    DocumentRoot /var/www/html
    
    Protocols h2 http/1.1
    
    SSLEngine on
    SSLCertificateFile /path/to/cert.pem
    SSLCertificateKeyFile /path/to/key.pem
    
    <FilesMatch \.php$>
        SetHandler "proxy:unix:/run/php/php7.0-fpm.sock|fcgi://localhost"
    </FilesMatch>
</VirtualHost>

Confirm HTTP/2 is working:

1. Browser DevTools: Check Network tab protocol column
2. Command line: curl -v --http2 https://yoursite.com
3. Online tools: KeyCDN HTTP/2 Test

If you're limited to mod_php on shared hosting and can't change MPM modules:

• Contact your host about enabling HTTP/2
• Consider upgrading to a VPS with more control
• Implement HTTP/2 via CDN (Cloudflare, etc.)