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Frontend & Backend

The website is designed to run in a frontend/backend scenario. This is to achieve both load distribution and redundancy for the case when the main webserver (known as wwwmaster) goes offline or becomes loaded.

Previous versions of the website used static files on the frontend, that were spidered at regular intervals and then push-rsynced out to the frontends. This made the frontends entirely independent of the backends, and as such very "available". Unfortunately it made a lot of coding difficult, and had very bad performance (a re-spidering including documentation and ftp would take more than 6 hours on a fast machine).

This generation of the website will instead rely on a varnish web cache running on the frontend servers, configured to cache things for a long time. It will also run in what's known as "grace mode", which will have varnish keep serving the content from the cache even if it has expired in case the backend cannot be contacted.

All forms that require login will be processed directly by the master server, just like before. These will always be processed over SSL, and as such not sent through varnish at all. They will still be accessed using the domain, which will then simply proxy the SSL connection to the backend. For the initial deployment we'll just use HAProxy, but we may switch to a more feature-rich proxy server in the future - in which case it's important to maintain the encrypted channel between the frontend and the backend, since they are normally not in the same datacenter.

Requests that require up to the second content but do not require a login, such as a mirror selection, will be sent through the frontends (and served under the name) but without caching enabled. Note that in most cases, these should actually be cached for at least 5 or 10 seconds, to cut off any short term high load effects (aka the slashdot effect).

Normal requests are always cached. There is a default cache expiry that is set on all pages. There is a longer default policy set for images, because they are considered never to change. Any view in the django project can override this default, by specifying the "Cache-control: s-maxage=xxx" http header on the response. This is done by using the @cache() decorator on the view method. Caching should be kept lower for pages that have frequently updating data, such as the front page or the survey results page.

Any model inheriting from PgModel can define a tuple or a function called purge_urls (if it's a function, it will be called and should return a tuple or a generator). Each entry is a regular expression, and this data will be automatically removed from the frontend caches whenever this object changes. The regular expression will always be prepended with ^, and should be rooted with /. It should be made as restrictive as possible (for example, don't purge "/" since that will remove everything from the cache completely). This makes it possible to have frontends react instantly to changes, while maintaining high cacheability.

Finally, there is a form on the admin web interface that lets the administrator manually purge pages from the caches. This may be necessary if changes have been made to static pages and/or site structure that otherwise wouldn't show up until the cache has expired.

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