cpcache is a central cache for pacman, the package manager of Arch Linux. It requires little configuration, does not bother you with 404 errors and it allows you to utilize the full bandwidth even as multiple clients download the same file at the same time. Combined with clyde, it also allows you to download updated packages in advance so that most requests can be served from cache when running pacman -Syu.

For each incoming GET request, cpcache checks if it can serve the request from cache. If so, the file is sent to the client while it is being read from the local file system. Otherwise, the file is sent while it is being downloaded from an ordinary pacman mirror. In either case, the client will get an immediate response, i.e., files need not be downloaded from the server or read from the file system entirely before a response is sent to the client. No caching is done for database files, cpcache will send a redirect response instead.

Most importantly, cpcache allows you to share bandwidth when multiple clients are downloading the same file at the same time. For instance, suppose a second client requests a file that has been downloaded to 30% by the first client. The second client will obtain at least 30% of that file with whatever speed your LAN provides. Afterwards, both clients continue to download with the full speed provided by your ISP.

A number of different caching methods are listed in the wiki, let's compare cpcache with each of them.

• Read-only cache using a web server such as darkhttp: This is messy since it will return lots of 404s. With cpcache, uncached packages will be downloaded as if you were downloading them directly from the remote mirror, while also making them available for subsequent requests.
• Read-write caches such as pacserve or paccache: pacserve and paccache are distributed while cpcache is centralized. A distributed solution is your only option if you don't have a device in your LAN which is running 24/7. If, on the other hand, you do have such a device, you may prefer a centralized solution that keeps all your cached packages at one place. This allows you to just set pacman's CacheDir to a tmpfs instead of storing packages redundantly. Also, packages that are cached once are always available, not only if the machine that cached it happens to be online.
• Reverse proxy cache using NGINX: Apart from the fact that cpcache can utilize the full bandwidth even with multiple concurrent downloads, the setup described in the wiki is quite similar to cpcaches approach. However, cpcache provides additional features. For instance, it obtains the most recent list of official mirrors and attempts to choose a fast mirror for you. This means you will not have to maintain a mirror list yourself.
• Proxy cache using squid: From the perspective of the client, cpcache acts like an additional mirror, so that pacman will try another mirror if the machine running cpcache is unreachable. Configuring a proxy cache using squid on the other hand means that all HTTP requests first have to be routed through the machine running squid, which may be undesirable if you cannot rely on that machine being always available. Especially on laptops, you'd have to change the http_proxy environment variable whenever you're on the move.

cpcache runs on all platforms supported by Erlang, which includes x86_64 and most ARM platforms. This also means that cpcache does run on a Raspberry Pi, but you may find that cpcache does not work flawlessly on such low-end devices due to the high CPU requirements of multiple concurrent up- and downloads. Only x86_64 clients are supported by cpcache, which means that while cpcache can be installed on an ARM device, it cannot serve files to clients which run anything other than the official Arch Linux distribution.

A package for Arch Linux is available on AUR. Install cpcache on your server. Then, start and enable the cpcache service:

systemctl start cpcache.service
systemctl enable cpcache.service

Set the new mirror in /etc/pacman.d/mirrorlist on all clients. For instance, if the server running cpcache can be reached via alarm.local, add the following to the beginning of the mirrorlist file:

Server = http://alarm.local:7070/$repo/os/$arch

cpcache expects a configuration file in /etc/cpcache/cpcache.toml. You can copy the example configuration file from conf/cpcache.toml to /etc/cpcache/cpcache.toml and adapt it as required.

In case you want to use NGINX as reverse proxy, keep in mind that it uses caching by default, which will cause timeouts in pacman since downloads then require a few seconds to start. Use proxy_buffering off; to prevent this. Here's an example config that can be used for NGINX:

server {
    server_name archlinux.myhost.org;
    listen [::]:80;
    listen 80;

    location ~ ^/(core|extra|community|multilib)/ {
        proxy_pass http://127.0.0.1:7070;
        proxy_buffering off;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

In case you want to avoid storing packages redundantly you can set the CacheDir in your pacman.conf to a tmpfs. This will clear the pacakge cache when shutting down. More information about these methods can be found here.

Add CacheDir = tmp/pacman/pkg in /etc/pacman.conf.

NOTE: This may produce warnings when the directory does not exist.

This will create a new tmpfs instead of using /tmp

Add this line to your /etc/fstab file:

tmpfs  /var/cache/pacman/pkg  tmpfs  nodev,nosuid,size=2G  0  0

You can customize the size of the tmpfs by modifying size=2G.

To mount the new tmpfs you just need to run mount -a as root. The tmpfs will be mounted automatically on every boot.