fastcgi_cache with conditional purging

Check if your nginx has fastcgi_cache_purge module

Support for fastcgi_cache_purge should be already there. You can test it by running following command:

nginx -V 2>&1 | grep nginx-cache-purge -o

If you see nginx-cache-purge in output then you already have it.

Otherwise, if you are on Ubuntu with default Nginx installation, you can run following commands to install nginx with fastcgi_cache_purge module.

Reinstall nginx with fastcgi_cache purge module support

sudo add-apt-repository ppa:rtcamp/nginx
sudo apt-get update
sudo apt-get remove nginx*
sudo apt-get install nginx-custom

Install Nginx Helper Plugin

Above step ensures that Nginx can purge a page from its fastcgi_cache selectively. But Nginx cannot automatically find out which page to purge and when to purge?

So install Nginx helper plugin from WordPress plugin repository and activate it. Apart from other features, it provides cache purging options. Just activate it, go to its settings and turn on “Enable Cache Purge” option.

If you want more control over your cache purging rules, you can play with different purging options it provides.

Using ramdisk (tmpfs) for cached content

This step is optional. You need give Nginx a folder store fastcgi_cache content. I will recommend using /var/run on Ubuntu as its mounted as tmpfs (in RAM). If you do not have ample RAM you can pick any other location.

If you are going with RAM, make sure you check size of /var/run folder. Its generally 20% of your total RAM size.

To verify it, run command df -h /var/run. You will see output like below:

Filesystem      Size  Used Avail Use% Mounted on
tmpfs           6.3G  364K  6.3G   1% /run

It looks like we have more than 6GB at our disposal! Our server has 32GB RAM by the way, so 6.3GB is close to 20%

Nginx Config

No matter how you are using WordPress, i.e. single or Multisite (with subdirectory/subdomain/domain-mapping) fastcgi_cache related configuration will remain similar.

Now make changes to /etc/nginx/sites-available/example.comfile so it looks like one below:

# move next 4 lines to /etc/nginx/nginx.conf if you want to use fastcgi_cache across many sites
fastcgi_cache_path /var/run/nginx-cache levels=1:2 keys_zone=WORDPRESS:100m inactive=60m;
fastcgi_cache_key "$scheme$request_method$host$request_uri";
fastcgi_cache_use_stale error timeout invalid_header http_500;
fastcgi_ignore_headers Cache-Control Expires Set-Cookie;
server {
    server_name example.com www.example.com;

    access_log   /var/log/nginx/example.com.access.log;
    error_log    /var/log/nginx/example.com.error.log;

    root /var/www/example.com/htdocs;
    index index.php;

    set $skip_cache 0;

    # POST requests and urls with a query string should always go to PHP
    if ($request_method = POST) {
        set $skip_cache 1;
    }
    if ($query_string != "") {
        set $skip_cache 1;
    }

    # Don't cache uris containing the following segments
    if ($request_uri ~* "/wp-admin/|/xmlrpc.php|wp-.*.php|/feed/|index.php|sitemap(_index)?.xml") {
        set $skip_cache 1;
    }

    # Don't use the cache for logged in users or recent commenters
    if ($http_cookie ~* "comment_author|wordpress_[a-f0-9]+|wp-postpass|wordpress_no_cache|wordpress_logged_in") {
        set $skip_cache 1;
    }

    location / {
        try_files $uri $uri/ /index.php?$args;
    }

    location ~ \.php$ {
        try_files $uri =404;
        include fastcgi_params;
        fastcgi_pass 127.0.0.1:9000;

        fastcgi_cache_bypass $skip_cache;
            fastcgi_no_cache $skip_cache;

        fastcgi_cache WORDPRESS;
        fastcgi_cache_valid  60m;
    }

    location ~ /purge(/.*) {
        fastcgi_cache_purge WORDPRESS "$scheme$request_method$host$1";
    }

    location ~* ^.+\.(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|rss|atom|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf)$ {
        access_log off; log_not_found off; expires max;
    }

    location = /robots.txt { access_log off; log_not_found off; }
    location ~ /\. { deny  all; access_log off; log_not_found off; }
}

The line fastcgi_cache_use_stale is what makes caching on Nginx-side unique. This line tells Nginx to use old (stale) cached version of page if PHP crashes. This is something not possible with WordPress caching plugins.

Don’t Forget

Always test your Nginx configuration and then reload it. All changes to Nginx config must be followed with these commands:

nginx -t && service nginx reload