A Weird Imagination

Pelican publish without downtime

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The problem

My existing script for publishing my blog has Pelican run on the web server and generate the static site directly into the directory served by nginx. This has the effect that while the blog is being published, it is inaccessible or some of the pages or styles are missing. The publish takes well under a minute, so this isn't a big issue, but there's no reason for any downtime at all.

The solution

Instead of serving the output/ directory, instead generate it and then copy it over by changing the make publish line in schedule_publish.sh to the following:

make publish || exit 1
if [ -L output_dir ]
then
    cp -r output output_dir/
    rm -rf output_dir/html.old
    mv output_dir/html output_dir/html.old
    mv output_dir/output output_dir/html
fi

where output_dir/ is a symbolic link to the parent of the directory actually being served and html/ is the directory actually being served (which output/ previously was a symbolic link to).

The details

Double buffering

This is effectly double buffering the output/ directory.

The work is organized such that invalidating the old site and installing the new site can be executed in quick succession. As both operations are done as simple renames (mv within a single filesystem), they will be nearly instant, so it's extremely unlikely a web request will arrive while the site is down. If we wanted to avoid even that, we could tell nginx check both directories (html/ and html.old/) for every request, but that seems unnecessary and without extra work would mean files that were deleted in the most recent version of the site would still be accessible until the next update as nginx would find them in html.old/.

To ensure the timing works out, the script assumes output_dir/ may be a symbolic link to a different filesystem and therefore copies output/ over first as that may take time. Also, instead of deleting the old site, it is just moved out of the way, which doubles as a backup in case the new site ends up being corrupted somehow. The previous old site is deleted before moving the current site out of the way as deleting files also takes time.

Error handling

Adding the || exit 1 after make publish ensures that if the publish fails, the old site will stay in place. This also skips scheduling future posts to be published, but if the site is unable to be published, that would be a waste of time anyway.

Permissions

In order to make sure the files are generated with the right permissions to be readable by the web server, output/ and the directory the output_dir/ link points to both have their group set to www-data and have the g+s bit set with chmod so files/directories created in them will inherit the group and therefore also be readable by www-data.

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