FediMeteo, HAProxy, and the art of not wasting snac threads

How FediMeteo uses HAProxy caching, static pages, and small FreeBSD jails to keep snac quiet and serve ActivityPub traffic efficiently.

https://it-notes.dragas.net/2026/05/18/fedimeteo-haproxy-and-the-art-of-not-wasting-snac-threads/

#FediMeteo #snac #snac2 #haproxy #freebsd #it #sysadmin #ITNotes

When I wrote about FediMeteo (https://it-notes.dragas.net/2025/02/26/fedimeteo-how-a-tiny-freebsd-vps-became-a-global-weather-service-for-thousands/) for the first time, I told the story from the beginning: the idea born almost by chance while checking the weather for a holiday, the memory of my grandfather, who for years had been my personal meteorologist, the decision to build something small and useful, and then the surprise of seeing people actually use it. What began as a personal experiment quickly became a small global service, still running with the same philosophy: FreeBSD, jails, simple scripts, snac, text, emoji, and a lot of small pieces doing their work quietly.

That article was mostly about the birth and growth of the project. This one is about one of the less romantic parts of the same story, although I have to admit that I find a certain beauty in it too: keeping the service light as it grows.

FediMeteo (https://fedimeteo.com) is still intentionally simple from the outside. A homepage, some numbers, a list of countries, and many ActivityPub accounts publishing weather forecasts. The posts are text and emoji. There is no JavaScript requirement to read the pages, no heavy frontend, no unnecessary media attached to every forecast, and no dynamic homepage recalculated at every visit just to show the same numbers. This is not accidental. It is the way I wanted the service to behave from the beginning.

But the more the service is used, the more the small details matter. A request that looks harmless when there are ten followers may become a repeated request when there are thousands of followers, remote instances, crawlers, previews, and other servers fetching the same public objects. In the Fediverse, the same small thing can be asked many times by many different places, each one with a perfectly legitimate reason. The backend doesn't care: it just needs to deal with the requests.

And in FediMeteo, the backend is snac (https://codeberg.org/grunfink/snac2).

I like snac very much precisely because it is small, clear, and efficient. It is not a giant application that tries to be everything. It does a focused job and does it well. But this also means that I want to respect its shape. I do not want to waste its threads on work that the reverse proxy can safely do. A snac thread serving the same public avatar again and again is not a tragedy, but it is still a waste. A snac thread answering the same public ActivityPub object several times in the same minute is doing real work, but often not necessary work.

This is the reason behind the HAProxy (https://www.haproxy.org) tuning I am currently using in front of FediMeteo.

It is not about making the configuration look clever. It is about keeping snac quiet.

A continuation of the same idea

This is especially important because snac uses a limited number of threads. I like that. Limits are healthy. They force us to understand what the service is doing, and they prevent a small program from pretending to be an infinite resource. But limits also make waste visible. If a few threads are busy serving files that could have been served from cache, those threads are not available for something more useful.

With FediMeteo the implementation is different because the reverse proxy is HAProxy, but the reasoning is the same. I have many small snac instances, each one in its own FreeBSD (Bastille (https://github.com/BastilleBSD/bastille)) jail, and one public entry point that has to route, terminate TLS, compress, cache, and generally remove as much repetitive work as possible from the backends.

This is, in a way, the natural continuation of the original FediMeteo design. In the first article I wrote that I wanted to manage everything according to the Unix philosophy: small pieces working together. This is another piece of that same puzzle. HAProxy does the edge work. snac does the ActivityPub work. Scripts generate forecasts. cron launches updates. ZFS gives me snapshots. FreeBSD jails keep countries separated. Nothing is particularly heroic by itself, but the whole system becomes pleasant because each part has a clear responsibility.

Why there is almost no media

FediMeteo does not use media in its forecasts.

No images attached to the posts, no generated weather cards, no maps for each city, no decorative banners. The forecasts are text and emoji. This was a deliberate decision. Weather information does not become more useful just because it is put inside an image, and every media file used by the service would become something to store, serve, cache, federate, expire, back up, and occasionally debug.

Text and emoji are enough. They are accessible, light, readable in text browsers, friendly to timelines, and understandable even when someone does not know the local language perfectly. This was one of the original design principles of FediMeteo, and it also helps the infrastructure. Less media means less work, fewer cache entries, fewer repeated fetches, fewer surprises.

There is one exception: the avatar.

All FediMeteo accounts use the same avatar, and this is also intentional. I could have used a different avatar for each country, or for each city, or created something visually richer. It would have been nicer in some screenshots, perhaps. It would also have been operationally worse.

With one shared avatar, the reverse proxy has one very useful object to cache. It is public, identical for everyone, small, requested often, and therefore almost always hot in cache. HAProxy can serve it directly instead of asking each snac instance to return the same file. Since avatars are requested by remote instances, browsers, profile previews, and all sorts of federation-related fetches, this single decision removes a surprising amount of pointless backend traffic.

So the avatar is not only a visual identity. It is part of the architecture.

This is the kind of optimization I like most, because it starts before the software. It starts with deciding not to create a problem.

The homepage is static because it can be static

It is a static HTML page generated from a template. Once per hour, a cron script updates the numbers and statistics. It counts the data I want to show, regenerates the page, and then the page remains static until the next run.

This is not because I cannot make a dynamic page. It is because I do not need one. Boring is good.

The homepage does not need to query all the country instances on every visit. It does not need a database request for each user who opens it. It does not need to ask snac anything in real time. The numbers are useful, but they do not need to be updated every second. Once per hour is enough, and it also fits the spirit of the whole project: do the work when it is needed, then serve the result cheaply.

I have seen too many small services become heavy because the first implementation was convenient rather than appropriate. A cron job and a template are not fashionable, but they are often exactly what a page like this needs.

Many countries, one entry point

fedimeteo.com
www.fedimeteo.com
it.fedimeteo.com
uk.fedimeteo.com
jp.fedimeteo.com
us.fedimeteo.com
usa.fedimeteo.com
can.fedimeteo.com
canada.fedimeteo.com

At the beginning, it is always tempting to write one ACL after another in the HAProxy frontend. It is quick, it is explicit, and for five hostnames it is perfectly fine. But FediMeteo did not remain at five hostnames. As countries and aliases grew, a long chain of ACLs would have turned the frontend into a list of names instead of a description of how the proxy behaves.

So I moved the hostname to backend mapping into a map file:

fedimeteo.com        backend_fedimeteo
www.fedimeteo.com    backend_fedimeteo
it.fedimeteo.com     backend_it
uk.fedimeteo.com     backend_uk
jp.fedimeteo.com     backend_jp
us.fedimeteo.com     backend_us
usa.fedimeteo.com    backend_us
can.fedimeteo.com    backend_ca
canada.fedimeteo.com backend_ca

use_backend %[req.hdr(host),field(1,:),lower,map(/usr/local/etc/fedimeteo.map,backend_fedimeteo)]

I like this because it keeps the configuration honest. The frontend contains the policy. The map contains the data. Adding a country means adding an entry to the map and defining a backend. I do not need to make the frontend more complicated every time the service grows.

Backends as small compartments

backend backend_it
    mode http
    http-reuse safe
    server srv1 10.0.0.2:8001 maxconn 30
backend backend_uk
    mode http
    http-reuse safe
    server srv1 10.0.0.7:8001 maxconn 30
backend backend_jp
    mode http
    http-reuse safe
    server srv1 10.0.0.32:8001 maxconn 30

The maxconn 30 value is not a magic number. It is a ceiling. I want each small backend to have a visible limit in front of it. If something starts hammering a country instance, I prefer the pressure to appear at the HAProxy layer instead of becoming unlimited concurrent work inside snac.

http-reuse safe lets HAProxy reuse backend connections where appropriate. This is another small reduction in unnecessary work. Opening connections repeatedly is not the biggest problem in the world, but avoiding it is still better, especially when many small services sit behind the same proxy.

The front door

frontend https_in
    bind :::443 v4v6 ssl crt /usr/local/etc/certs/ alpn h2,http/1.1
    mode http
    option http-keep-alive

ssl-default-bind-ciphersuites TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256
ssl-default-bind-options no-sslv3 no-tlsv10 no-tlsv11 no-tls-tickets

acl letsencrypt-acl path_beg /.well-known/acme-challenge/
http-request redirect scheme https code 301 unless letsencrypt-acl
use_backend letsencrypt-backend if letsencrypt-acl

http-request set-header X-Real-IP %[src]
http-request set-header X-Forwarded-Proto https

http-response set-header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"

Two caches, because the requests are different

cache mediacache
  total-max-size 128
  max-object-size 10000000
  max-age 3600
  process-vary on
  max-secondary-entries 12
cache jsoncache
  total-max-size 16
  max-object-size 1000000
  max-age 60
  process-vary on
  max-secondary-entries 12

The media cache is larger and has a longer maximum age. In FediMeteo, this mostly means the shared avatar and a few static-looking objects. Since there is intentionally almost no media, the important cached object is requested very often and remains warm.

The JSON cache is smaller and short-lived. It is there for public ActivityPub GET requests, not to store federation state forever. A 60 second cache is enough to collapse many repeated requests that arrive close together in time, without pretending that ActivityPub responses should be treated like immutable files.

This distinction is important. Caching is not one decision. It is a set of small decisions about what a response means, who can see it, how often it changes, and what happens if it is served again.

Recognizing media

acl is_media path_end -i .jpg .jpeg .png .gif .webp .svg .ico .mp4 .webm .mp3 .ogg .wav .flac .mov .avi .mkv .m4v

http-request set-var(txn.is_media) bool(true) if is_media

http-request cache-use mediacache if { var(txn.is_media) -m bool true }

http-response set-header Cache-Control "max-age=3600, public" if { var(txn.is_media) -m bool true }
http-response del-header Set-Cookie if { var(txn.is_media) -m bool true }
http-response del-header Vary if { var(txn.is_media) -m bool true }
http-response cache-store mediacache if { var(txn.is_media) -m bool true }

This is aggressive only if removed from its context. In this service, with this media policy, it is a reasonable choice. FediMeteo is not serving private media under these paths. It is mostly serving the same public avatar over and over.

For the same reason, I clean the request before it reaches the backend:

http-request del-header Authorization if { var(txn.is_media) -m bool true }
http-request del-header Cookie        if { var(txn.is_media) -m bool true }

The result is exactly what I want: the shared avatar becomes an almost perfect cache object. Small, public, repeatedly requested, and served by HAProxy instead of snac.

ActivityPub JSON microcaching

acl is_ap_json   req.hdr(Accept),lower -m sub application/activity+json
acl is_ap_ldjson req.hdr(Accept),lower -m sub application/ld+json
acl is_outbox    path_end /outbox
acl is_get       method GET
acl has_auth     req.hdr(Authorization) -m found
acl has_cookie   req.hdr(Cookie) -m found

So I only mark public ActivityPub GET requests as cacheable:

http-request set-var(txn.is_activitypub) bool(true) if is_get !is_outbox is_ap_json !has_auth !has_cookie
http-request set-var(txn.is_activitypub) bool(true) if is_get !is_outbox is_ap_ldjson !has_auth !has_cookie

It must be a GET, because I am not caching deliveries or anything that changes state. It must not be /outbox, because outbox collections are not the traffic I want to cache here. It must not have Authorization, and it must not have cookies, because authenticated or user-specific requests do not belong in a shared public cache.

Then the cache can be used and populated:

http-request cache-use jsoncache if { var(txn.is_activitypub) -m bool true }
http-response set-header Cache-Control "max-age=60, public" if { var(txn.is_activitypub) -m bool true }
http-response cache-store jsoncache if { var(txn.is_activitypub) -m bool true }

This is microcaching in the most practical sense. It reduces repeated work without changing the nature of the service.

Static media paths

acl is_short_path path_reg ^/[^/]+/s/
http-request cache-use mediacache if is_short_path

In FediMeteo this is less central than on a normal social instance, because I deliberately do not use media except for the avatar and basic static objects. Still, the rule fits the general policy: let HAProxy handle repeatable edge work, and let snac spend its threads where they are actually needed.

Vary, but not without limits

process-vary on
max-secondary-entries 12

For media, I remove Vary before storing the response. A shared avatar does not need to vary by Accept. For ActivityPub JSON, I am more careful because the representation matters.

Again, the important thing is not the number itself. It is the decision to make variation explicit and limited.

Seeing whether it works

http-response set-header X-Cache-Status HIT if !{ srv_id -m found }
http-response set-header X-Cache-Status MISS if { srv_id -m found }

Did this request reach snac?

A test can be as simple as:

curl -I https://it.fedimeteo.com/path/to/avatar.png
curl -I https://it.fedimeteo.com/path/to/avatar.png

For ActivityPub JSON, the test must use the right Accept header:

curl -I \
  -H 'Accept: application/activity+json' \
  https://it.fedimeteo.com/some/activitypub/object

curl -I \
  -H 'Cookie: test=value' \
  -H 'Accept: application/activity+json' \
  https://it.fedimeteo.com/some/activitypub/object
curl -I \
  -H 'Authorization: Bearer fake' \
  -H 'Accept: application/activity+json' \
  https://it.fedimeteo.com/some/activitypub/object

Compression and operational paths

filter compression
compression algo gzip
compression type text/css text/html text/javascript application/javascript text/plain text/xml application/json application/activity+json

There is also a local Prometheus exporter:

frontend prometheus
  bind 127.0.0.1:8405
  mode http
  http-request use-service prometheus-exporter
  no log

What this changes in practice

The map keeps hostname routing manageable. The backend definitions keep each country isolated and limited. The static homepage avoids dynamic work for something that changes once per hour. The shared avatar gives HAProxy one very hot media object to serve directly. The media cache keeps public files away from snac. The JSON microcache absorbs short ActivityPub bursts. Header cleanup prevents useless variation. Connection reuse avoids unnecessary backend connection churn.

But all of this is only a longer way of saying one thing:

fewer requests reach snac.

That is the metric I care about here.

Not because snac is slow. If anything, FediMeteo exists in its current form because snac is efficient enough to make this kind of project possible on a very small VPS. But precisely because the whole architecture is small and pleasant, I do not want to waste resources where there is no need.

This is also consistent with the rest of the project. Forecasts are serialized by scripts. Updates happen every six hours. The homepage is regenerated hourly. Countries live in separate jails. Snapshots and backups are handled outside the application. No single component tries to be the entire system.

HAProxy is just another small piece, but it sits in the right place to remove a lot of repeated work.

Caveats

It matches FediMeteo as it is now: almost no media, one shared avatar, static homepage, public forecasts, many small snac instances, and ActivityPub traffic that can benefit from a short public cache when there are no cookies or authorization headers.

If I decide one day to use media in forecasts, the media cache rules will need to be reviewed. If I use different avatars for each city or country, the cache will still work, but I will lose the very nice property of one shared, always-hot avatar. If ActivityPub responses become actor-dependent, public JSON caching must be reconsidered. If one country grows a very different traffic pattern from the others, it may deserve a different limit or policy.

This is why I do not like presenting configurations as magic. A good configuration is a written form of the assumptions behind a service. When the assumptions change, the configuration must change too.

Conclusion

The HAProxy layer follows this idea. It terminates TLS, routes hostnames through a map, reuses backend connections, serves the shared avatar from cache, microcaches public ActivityPub JSON, avoids authenticated and cookie-based traffic, and gives me a small diagnostic header to see what is happening.

There is no single brilliant directive here. There is only the usual work of matching infrastructure to reality.

FediMeteo publishes weather forecasts as text and emoji. The homepage is static HTML updated every hour. The accounts share the same avatar because it is enough, and because it is better for the cache. Each country has its own snac instance in its own FreeBSD jail. HAProxy stands in front of them and tries, quietly, not to bother them unless it has to.

I like this kind of infrastructure.

Not because it is invisible, but because when it works well, it leaves very little to say.

https://it-notes.dragas.net/2026/05/18/fedimeteo-haproxy-and-the-art-of-not-wasting-snac-threads/

#ITNotes #NoteHUB #fediverse #freebsd #haproxy #hosting #jail #networking #ownyourdata #server #snac #snac2 #social #web

Second question: Which software should I use? Note: If someone can help me get #snac running on OS/2 there's obviously gonna be no contest... #Retrocomputing #OS2 #Snac2

@wec That is hideously fantastic. I spent some time trying to compile #snac on OS/2 a while back, but in the end I had to give up. I hope to pick it up again one day... Someone knowing what they're doing might get it done in 20 minutes, using EMX or kLibC or whatnot. I got to the linker step..

Announcing FediMeteo – Weather in the Fediverse!

UPDATE: I have created an account for updates and other information on FediMeteo - follow the account @admin to stay updated!

UPDATE: Ireland, Poland, Portugal and Switzerland have just been added

Weather has always influenced our lives: from agriculture to outdoor activities, to extreme events that, thanks to modern technology, can now be predicted with greater reliability. Personally, weather plays a significant role in my daily decisions, which is why I decided to create a service tailored for the Fediverse.

FediMeteo uses Open-Meteo data to publish updates every 6 hours, including current weather conditions, forecasts for the next 12 hours, and predictions for the upcoming days. Each country is served by its own dedicated instance (e.g., it.fedimeteo.com for Italy), managed through snac to ensure simplicity and efficiency in publishing.

You can follow FediMeteo directly in the Fediverse (on Mastodon and compatible platforms), via RSS, or by visiting the dedicated page for your city (e.g., fr.fedimeteo.com/paris).

Currently supported countries include:
Austria, Germany, France, Ireland, Italy, Netherlands, Poland, Portugal, Spain, Switzerland and the United Kingdom, – with many more regions coming soon!

FediMeteo is hosted on a FreeBSD-based VPS, with each country isolated in its own jail to ensure security and scalability.

Visit the main site to explore the national instances and start following your local weather updates today:
https://fedimeteo.com

Happy weather monitoring to all! 🌦️

FediMeteo is dedicated to my grandfather, who every evening would give me the weather forecast based on TV, radio, and his personal experience. He would convince me that the weather would be bad, so he had an excuse to accompany me to school instead of me going alone.

#FediMeteo #Announcements #FreeBSD #FediMeteo #WeatherForecasts #Weather #Meteo #snac #Fediverse #Mastodon

#FediMeteoUpdates #FediMeteoAnnouncements

The #BSDCafe #snac instance has been updated to 2.92

#BSDCafeServices #BSDCafeUpdates

Changed default: for newly created instances, disable_inbox_collection is set to true (see snac(8) for more information). The reason is because it seems to be used for harrasing people.

Changed default: for newly created instances, disable_history is set to true (see snac(8) for more information). The reason is because archived history files don't reflect reality after posts are deleted or modified (they always have been an ugly kludge).

Changed default: in previous versions, posts with a scope of unlisted were shown in public pages and RSS feeds. Now, they are no longer shown. If you want to get back to previous behaviour, use a new toggle in the User Settings section (see snac(1) for more information).

New admin configuration option: if the purge_static value is set to true in server.json, each user's static directory is explored and those files there that are no longer attached to any post or referenced anywhere are deleted. See snac(8) for more information about those cases where you may not want to enable this option.

Allow serving files from subdirectories of the static/ subdirectory (contributed by la_ninpre).

Minor tweak to webfinger code to handle Hubzilla's peculiarities.

Fixed a search case where URLs to GotoSocial statuses were misidentified as accounts.

Accounts that follow you are now marked with a thumb-up emoji, because followers are adorable people.

Fixed some account export errors.

Fixed an incorrect hash in post links.

Show an account's location link in the people page, if they have one.

Mastodon API: Fixed hashtags loosing the link after editing a post, minor tweak in access token processing (contributed by trondd555).

Drop usage of PATH_MAX (contributed by sergiodj).

New Polish translation (contributed by kpm).

Updated German and Czech translations (contributed by zen and pmjv).

https://comam.es/what-is-snac

If you find #snac useful, please consider buying grunfink a coffee or contributing via LiberaPay.

#snacAnnounces #FrugalFediverse

• Light on resources. I can run this on a potato if I wanted and it would run great (I'm not, but I could.)
• Backups are easy. Tar up the directory. Done.
• Migrations are easy. Install nginx, build snac, untar backup. Done.
• Quirky web interface. It takes a little to get used to it, but it is very functional and it's got character. I thought I would use Phanpy with it more, but I'm not. I enjoy the snac web interface.
• Works (mostly) with Mastodon clients.
• No crazy caching of photos, avatars and other media. This is a really big deal because my disk usage is so much lower than when I ran Mastodon.
• No database. NO DATABASE. So good.
• Fast. snac is really fast doing everything. It's nice.
• @grunfink@comam.es is awesome. Friendly and responsive.

Looks like it has basic Markdown -> HTML converting. Neat.

That

works

for

me.

• (I'm not gonna show it all)

And of course, no post or poll length limit :)

#Snac #SnacServer #SelfHosting

Also I think it would be even cooler if it was a CGI binary

https://encrypted.tesio.it/2024/12/18/how-to-run-your-own-social-network.html

CC: @nick@treffen.geekyschmidt.com

Because snac2 is a standalone daemon and does not natively speak FastCGI

See the snac(8) manpage for more information.

I sincerely love you weirdos. <3

#Fediverse #Mastodon #GoToSocial #snac #snac2 #honk #meme #AVIF

Edit: added transparency to image. Scratch that, GtS seems to not like transparency in AVIF, so I restored the original and uploaded a black background variant as well

Alt...

"X all the Y" guy: I want an amazing community of noble-minded people to study ethics with and change the world!! Pawn Stars with the faces of the fediverse logo, the mastodon logo, and the logos for snac2 GoToSocial, and Honk in the background: Best we can do is a bunch of cool weirdos with similar nerdy interests "X all the Y" guy's reply: I'll take it!!! (light background)

Alt...

"X all the Y" guy: I want an amazing community of noble-minded people to study ethics with and change the world!! Pawn Stars with the faces of the fediverse logo, the mastodon logo, and the logos for snac2 GoToSocial, and Honk in the background: Best we can do is a bunch of cool weirdos with similar nerdy interests "X all the Y" guy's reply: I'll take it!!! (dark background)

Great news, you can now post a picture to https://subversive.pics interactively from your favorite internet browser!

You can achieve this by making a picture post inside the fediverse and tagging @subversive_pics (this works with snac, lemmy, mastodon, but not all of its forks, your mileage may vary)

If your picture makes it past the horp censors, your picture will appear on the subversive.pics website and the rss feed!

Incredible.

#theworkshop #subversivepics #snac

Alt...

HAVE YOU EVER WANTED TO CONTRIBUTE TO SUBVERSIVE.PICS BUT DONT SPEAK COMPUTER?! metal vaccine angel says sweet

#mastoblaster #snac #fediverse

Predictably the resource utilization is dramatically different than Mastodon.

At times there have been over a half million message objects in my snac profile directory. Snac has always handled it like a champ.

#snac ... Snac's Not A Crappie.

Snac2 software sources: https://codeberg.org/grunfink/snac2

I come out to see that the marvelous @lisamelton@mastodon.social gave it a boost! Thank you Lisa!

Then I went to check on my VPS and it is showing:

load average: 0.07, 0.05, 0.05

New #snac instance survived a Lisa boost. Yay! (Lisa Melton Boost Certified)

#fediverse

The #BSDCafe #snac instance has been updated to 2.91

#BSDCafeInstance #BSDCafeServices #BSDCafeUpdates #BSDCafeAnnouncements

Fixed some local posts not being shown if the replied post is deleted.

Fixed web UI incorrect links to public posts for some configurations.

Fixed encoding bug in Mastodon-style share links.

Fixed an error that made imposible to leave empty the following hashtags or blocked hashtags from the web UI.

Include post attachments in the RSS feed entries.

Added some OpenBSD-specific documentation (contributed by oxzi).

Fixed some timezones (contributed by matoken).

Fixed some crashes (special thanks to Louis Merlin for helping me with this).

Updated Docker scripts to avoid generating a useless log file.

Fixed several memory leaks, key generation errors and HTML inconsistencies (contributed by dandelions).

Added ostatus.org and FEP-3b86 data to Webfinger objects.

Added a new CONTRIBUTING.md file. Among other guidelines, I explicitly say there that AI contributions are NOT accepted.

snac is now available as a Yunohost app. Thank you very much to Bruno Cesar Rocha for this.

https://comam.es/what-is-snac

If you find #snac useful, please consider buying grunfink a coffee or contributing via LiberaPay.

#snacAnnounces #FrugalFediverse

@Paulatics I heard lots of stories about servers with bad moderators, both ones that ignore abuse complaints and ones where the mods were abusive, so I decided to run my own.

Had #snac (fediverse software) been around when I started, I’d already be on it - it’s super simple to set up. I’ll move to snac from Akkoma some time soon.