Puppet caching Edit

Published on May 20, 2014 by daenney

With Puppet 3.6 out the door and the new caching mechanisms it provides I started exploring how to do caching with Puppet. A thank you goes out to Ken Barber of Puppet Labs and Erik Dalén of Spotify for their help on this quest.

The first thing I wanted to do is use the new caching mechanism for directory based environments on Puppet 3.6 in such a way that our production environment would be cached forever. Of course we also need a way to invalidate that cache when we deploy but between two deploys this code never ever changes.

Directory-based environments cache

First order of business, turn on caching in /etc/puppet/puppet.conf:

environment_timeout = unlimited

Restart the Puppet master and there you go. All the *.pp files are now parsed once (instead of being reparsed every 5s or so). Problem is, how do we invalidate this cache?

If you’re running the ‘community’ stack all it takes is to tell Passenger to reload. This can be done by simply touching the tmp/restart.txt file that Passenger looks for. On the next request it receives, Passenger will reload and the Puppet Master will now reparse the *.pp files for the environment.

The location of the tmp/ directory varies but it’s right next to where the config.ru file is located:

├── config.ru
├── public
└── tmp
    └── restart.txt

There is no need to remove the restart.txt file, Passenger looks at the timestamp when a request comes in and makes the right decision.

If you’re deploying with Capistrano you can override the finalize_update task and add something like this in it:

task :finalize_update do
    run "#{try_sudo} touch /usr/share/puppet/ext/rack/tmp/restart.txt"

Nginx cache

The second thing I wanted to do is cache a few more things at the nginx level. If you drill down a bit into Puppet you’ll notice that one of the things the agent does frequently is request things from the following endpoints:

  • /$environment/file_metadata
  • /$environment/file_metadatas
  • /$environment/file_content

These endpoints are computationally expensive; file_metadata for example ends up running md5sum over every file that is being transferred and file_metadatas does the same but in bulk for the plugins that are being synced.

As you can imagine, md5sum over all these files constantly is slow and also pretty useless as we just stated that these files don’t change except for when we deploy. Prime candidate for some fancy caching!

The first thing to do is configure the cache space itself in nginx. This must be done in the http block.

proxy_cache_path /var/cache/nginx levels=1 keys_zone=puppetmaster:10m max_size=500m inactive=60m;
proxy_temp_path /var/cache/tmp;

What we’ve configured here is a cache space who’s files will live in /var/cache/nginx, have a directory structure of 1 level/folder deep, a key_zone size of 10 megabytes, allowed to grow to 500 megabytes of disk space and entries will be removed after 60 minutes if not being hit.

The proxy_temp_path is a filesystem location where temporary files that nginx creates for its own purposes will live. It’s a good idea for these directories to be on the same filesystem.

Up next, configuring the actual cache for the proxy:

location ~ ^/production/file_(metadatas?|content) {
        proxy_redirect             off;
        proxy_cache                puppetmaster;
        proxy_cache_valid          200 302 15m;
        proxy_cache_valid          404 1m;
        proxy_pass                 http://puppetmaster;

This block tells nginx to match certain paths in the request and proxy those to the Puppet Master but by using the cache. A backend response of 200 or 302 is cached for 15 minutes, a 404 is cached for 1m.

Notice that we’re only matching /production, the production environment and not any other. This is done on purpose, the other environments are usually for testing that map to a feature-branch in git. We usually have no need to cache these as they are short-lived.

A somewhat complete configuration looks like this:

upstream puppetmaster {
    server unix:/path/to/passenger/puppetmaster/socket/file.sock;

server {
    listen              8140;
    root                /usr/share/puppet/ext/rack;
    ssl_certificate     /var/lib/puppet/ssl/certs/$FQDN.pem;
    ssl_certificate_key /var/lib/puppet/ssl/private_keys/$FQDN.pem;
    ssl_verify_client   optional;

    # All HTTP API requests, requiring a valid certificate
    location / {
        if ($ssl_client_verify != SUCCESS) {
            return 403;
        proxy_set_header  X-Client-Verify  $ssl_client_verify;
        proxy_set_header  X-Client-DN      $ssl_client_s_dn;
        proxy_set_header  X-SSL-Subject    $ssl_client_s_dn;
        proxy_set_header  X-SSL-Issuer     $ssl_client_i_dn;
        proxy_redirect    off;
        proxy_pass        http://puppetmaster;

    # Requests for cached endpoints, requiring a valid certificate
    location ~ ^/production/file_(metadatas?|content) {
        if ($ssl_client_verify != SUCCESS) {
            return 403;
        proxy_cache_valid  200 302 15m;
        proxy_cache_valid  404 1m;
        proxy_pass         http://puppetmaster;
        proxy_set_header   X-Client-Verify  $ssl_client_verify;
        proxy_set_header   X-Client-DN      $ssl_client_s_dn;
        proxy_set_header   X-SSL-Subject    $ssl_client_s_dn;
        proxy_set_header   X-SSL-Issuer     $ssl_client_i_dn;
        proxy_redirect     off;
        proxy_cache        puppetmaster;

    # Requests for /certificate, used before a valid certificate
    # has been recieved, therefor not requiring $ssl_client_verify
    location /certificate {
        proxy_redirect    off;
        proxy_pass        http://puppetmaster;

Reload your nginx configuration and enjoy. Keep in mind that this cache will not speed up agent run times but will mostly decrease load on your masters.

We are left with one problem though; how do we invalidate this cache at deploy time? Throwing away the cache is pretty easy, just remove all files in /var/cache/nginx and you’re done. Trouble is, you probably don’t have the permissions to do so and you probably don’t want to be sudoing during your deploy to do so.

Enter mod_lua for nginx. This allows us to create an endpoint on the Puppet Master vhost that we can hit, simply with cURL, which will take care of throwing away the cache. Beware that this is a hack, albeit an awesome one:

location /cache_purge {
    limit_except POST {
        allow ::1;
        deny all;
    content_by_lua '
        os.execute("find /var/cache/nginx -type f -delete")
        ngx.status = 204

I’m sure you can guess what this does. You can now post to /cache_purge which in turn, by using the power of Lua, executes the necessary command to clear up the cache.

Going back to the Capistrano example earlier you can now add this too:

task :finalize_update do
    run "#{try_sudo} touch /usr/share/puppet/ext/rack/tmp/restart.txt"
    run 'curl --silent -k -X POST https://localhost:8140/cache_purge'

I personally frown on the -k in the cURL command here so I suggest you alter it to include --cacert and point that to /var/lib/puppet/ssl/certs/ca.pem instead.

Apache cache

The cache configuration for nginx is inspired based on what Erik Dalén has been doing at Spotify and decided to share:

Note that this configuration is not caching the file_metadatas endpoint, I suggest you do. It also expires the cache after 300 seconds, 5m, and can grow up to 1GB.

I’m familiar enough with Apache to be able to tell you that this stores the cache in RAM but I have no idea how to invalidate it at deploy time.