Apache server -- multiple directories, different error logs

Why do you set Directory options for / in the VirtualHost context? Use <Directory /var/www/app1> instead of <Directory />

Due to the Apache ErrorLog directive docs its context is server config, virtual host - which means that it's only possible to define ErrorLog for the whole server or for a VirtalHost, not for a Directory. So if you want to send different logs to different files, try to use SetEnvIf to set an Env variable. Depeding on the directory where you are, it should be something like SetEnvIf Request_URI ^\/a1\/ a1 and SetEnvIf Request_URI ^\/a2\/ !a1. Then write logs depending on the a1 environment variable.

Set custom ID for every Directory and you can separate logs by directories like this:

<Directory app1>
    SetEnv app1
</Directory>
<Directory app2>
    SetEnv app2
</Directory>
CustomLog ${APACHE_LOG_DIR}/site1.log combined env=subwebsite1
CustomLog ${APACHE_LOG_DIR}/site2.log combined env=subwebsite2

Finally I did it, first create internal subdomains per folder and with proxypass pass the subdomain content.

Enable apache mods:

a2enmod authz_core dir proxy proxy_http

/etc/hosts

127.0.0.1       localhost
127.0.0.1       a.localhost
127.0.0.1       b.localhost

/etc/apache2/sites-available/default.conf

<VirtualHost *:80>
        ServerName localhost
        ServerAdmin [email protected]
        DocumentRoot "/dev/null"

        ProxyPass /a http://a.localhost/
        ProxyPassReverse /a http://a.localhost/
        ProxyPass /b http://b.localhost/
        ProxyPassReverse /b http://b.localhost/

        LogLevel debug

        ErrorLog ${APACHE_LOG_DIR}/default-error.log
        CustomLog ${APACHE_LOG_DIR}/default-access.log combined

</VirtualHost>

/etc/apache2/sites-available/a.conf

<VirtualHost *:80>
        ServerName a.localhost
        ServerAdmin [email protected]
        DocumentRoot "/Publikoa/a"

        <Directory "/Publikoa/a">
                DirectoryIndex index.html
                Require all granted
        </Directory>

        LogLevel debug

        ErrorLog ${APACHE_LOG_DIR}/a-error.log
        CustomLog ${APACHE_LOG_DIR}/a-access.log combined

</VirtualHost>

/etc/apache2/sites-available/b.conf

<VirtualHost *:80>
        ServerName b.localhost
        ServerAdmin [email protected]
        DocumentRoot "/Publikoa/b"

        <Directory "Publikoa/b">
                DirectoryIndex index.html
                Require all granted
        </Directory>

        LogLevel debug

        ErrorLog ${APACHE_LOG_DIR}/b-error.log
        CustomLog ${APACHE_LOG_DIR}/b-access.log combined

</VirtualHost>

Enable sites:

a2ensite default a b

Restart apache:

/etc/init.d/apache2 restart