Mass redirects with Apache RewriteMaps

I've taken the decision to not reuse the same URI structure as my old Wordpress blog, but cool URIs don't change, so I'm going to have to redirect all those old links — or at least the ones I think are marginally "cool".

Aside: Could I not have just kept the same URIs?

Well, yeah, it was technically possible of course, and generally desirable, but I've decided not to for a variety of reasons, some of which I might have to go into on an as-yet-to-be-written "About" page or something.

Markdown

The new software uses Markdown. Wordpress export and conversion to Markdown has got me 95% of the way there. Surprisingly far actually — even tables have come out mostly right — but still every article I move over needs a minor rewrite. I can't do that all at once and I'd rather not spend years doing it without deploying any of it.

New thing must be used

I'd like to be writing new articles in the new site. A big part of why this is happening is that due to vision problems I can't actually read my old blog's light theme any more. If I'm going to take the time to fiddle with a theme I would rather take the opportunity to change away from Wordpress as well.

Parallel running

So that leaves me in a situation where two blogs are running concurrently. I think that's okay. The old Wordpress has been going since 2006 so is probably okay to do so for a while longer. I might try to find a way to archive it to a static version of itself but the main thing is, any time I find the time to move an article over I'm going to have it redirect from its old URI to the article on this blog.

Simple redirects

Doing a redirect in Apache is simple enough. Since all my old blog articles start with https://strugglers.net/~andy/blog/2… (They're all laid out like YYYY/MM/DD/slug) I could just have a heap of:

Redirect permanent /~andy/blog/2024/04/28/musings-on-link-shorteners-uuidv7-base58 https://strugglers.net/posts/2024/musings-on-link-shorteners-uuidv7-base58

…but that's rather verbose and clutters up my Apache config file.

Enter the RewriteMap

It took me a bit of time to understand the documentation, but basically it's possible to put key:value pairs in a separate file and have just one RewwriteRule act on that:

RewriteMap blogmap "txt:/etc/apache2/legacy-blog-map.txt"
RewriteCond %{REQUEST_URI}            ^/~andy/blog/(2.*)$
RewriteCond ${blogmap:%1|NOT_PRESENT} !NOT_PRESENT
RewriteRule .?                        https://strugglers.net/posts/${blogmap:%1} [R,L]

How it works

1. The first RewriteCond only proceeds if the request URI is in the format of a legacy blog article. If so it captures that part of the URI starting with the date, so like 2024/04/28/slug and saves it in the first back reference (%1).
2. The second RewriteCond only proceeds if there is an entry for that %1 in the map. If there's no entry this lookup returns NOT_PRESENT which fails the condition.
3. Assuming both those conditions passed, the URI is rewritten to the new prefix plus the lookup from the map.

An example of that map file for one migrated article is:

2024/04/28/musings-on-link-shorteners-uuidv7-base58 2024/musings-on-link-shorteners-uuidv7-base58/

It's not amazing but it's slightly smaller and tidier. I suppose I could go a bit further as the slug part (the bit after the YYYY/MM/DD/) is I think unique without the date, so in theory I could lop off the date from the start of the key also. One for another day.