Apache rewrite with fallback - apache

Question

In our AngularJS project I have a set of HTML templates for reports that are currently part of the deployment but this means that whenever our client requests a change to one of the HTML templates I need to make update the complete application.
Is it possible to do the following for a request for these HTML templates:
Assume the request for assets/reports/report1.html.
Use the file <external-dir>/assets/reports/report1.html if that file exists. The external directory is not directly accessible from the internet.
Otherwise, use the initial request.
Most mod_rewrite solutions I could find seem to stop after the first rewrite.
Update: I've added the configuration changes below to the complete virtual host definition but the Alias seems to disrupt things:
<iFmODule mod_ssl.c>
<VirtualHost _default_:443>
ServerAdmin webmaster#localhost
DocumentRoot /var/www/html
Alias /advisor/report /var/www/html/report
Alias /report-templates /opt/reports
#<Directory /var/www/html/report>
RewriteEngine On
# Preventing direct access to /report-templates
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^/?report-templates(/.*)?$ - [R=404,L]
# If the request was not already rewritten,
RewriteCond %{ENV:REDIRECT_STATUS} ^$
# and the file do really exist in /advisor/report/assets/reports
RewriteCond %{REQUEST_FILENAME} -f
# rewrite the request from /advisor/report/assets/reports to /report-templates
RewriteRule ^/?advisor/report/assets/reports/(.*)$ /report-templates/$1 [L]
# If the file do not exist in /report-templates/
RewriteCond %{REQUEST_FILENAME} !-f
# rewrite the request back
RewriteRule ^/?report-templates/(.*)$ /advisor/report/assets/reports/$1 [L]
#</Directory>
LogLevel warn rewrite:trace8
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
</IfModule>
There are 2 AngularJS applications, the advisor portal located under /var/www/html/advisor and the reporting application located under /var/www/html/report. The latter is accessed through https:/example.com/advisor/report and is the reason for the Alias.
I changed the Alias to:
<Directory /var/www/html/advisor>
RewriteEngine On
RewriteRule ^/?advisor/report/(.*)$ /report/$1 [L]
</Directory>
This way the Alias should not interfere. After this I enabled the <Directory> directive for /var/www/html/report and removed the advisor/ part from the rewrite rules and conditions but I didn't see the rewrite happening in the logs.

Answer 1

I assume you have a directory called external-dir in your root (public_html) folder.
If you have full access to your server, this directory could be also somewhere else on the file system, but than you have to add a Alias in your Virtualhost configuration.
Alias "/external-dir" "/absolute/path/to/external-dir"
and the Apache Module mod_alias must be enabled:
sudo a2enmod alias
Now the main Part: This procedure will work for all existing files in the folder (or sub folder) /assets. First we check if the same file exist in /external-dir/assets and if so we serve it, if not we serve the original one. Its not possible to get a file from /external-dir/assets that do not exits in /assets. If you need more restrictions you can add them as your need it.
You can put this it in a .htaccess file in your root (public_html) folder or in your Virtualhost configuration, than I would use a Directory Direktive for your root directory
RewriteEngine On
# Preventing direct access to /external-dir
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^/?external-dir(/.*)?$ - [R=404,L]
# If the request was not already rewritten,
RewriteCond %{ENV:REDIRECT_STATUS} ^$
# and the file do really exist in /assets
RewriteCond %{REQUEST_FILENAME} -f
# rewrite the request from /assets to /external-dir/assets/
RewriteRule ^/?assets/(.*)$ /external-dir/assets/$1 [L]
# If the file do not exist in /external-dir/assets/
RewriteCond %{REQUEST_FILENAME} !-f
# rewrite the request back
RewriteRule ^/?external-dir/assets/(.*)$ /assets/$1 [L]
I tested this positive on an Ubuntu 16.04.03 LTS Server with Apache 2.4.27.