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Show HN: I wrote a new BitTorrent tracker in Elixir
Hello everyone!
I'm currently in a journey to learn and improve my Elixir and Go skills (my daily job uses C++) and looking through my backlog for projects to take on I decided Elixir is the perfect language to write a highly-parallel BitTorrent tracker.
So I have spent my free time these last 3 months writing one! Now I think it has enough features to present it to the world (and a docker image to give it a quick try).
I know some people see trackers as relics of the past now that DHT and PEX are common but I think they still serve a purpose in today's Internet (purely talking about public trackers). That said there is not a lot going on in terms of new developments since everyone just throws opentracker in a vps a calls it a day (honorable exceptions: aquatic and torrust).
I plan to continue development for the foreseeable future and add some (optional) esoteric features along the way so if anyone currently operates a tracker please give a try and enjoy the lack of crashes.
note: only swarm_printout.ex has been vibe coded, the rest has all been written by hand.
- IlikeKitties 1y agoNow that's neat. The Beam VM sounds like a natural fit for a torrent tracker
- dahrkael 1y agoI feel like ETS has been the real killer feature to pull this, being able to concurrently read and write from protected tables makes the whole thing incredibly parallel
- desireco42 1y agoNow this is serious business, congrats on the project! I can see how this is perfect fit for elixir...
- abrookewood 1y agoHey congrats on the launch! Can you provide any details on how it runs compared to opentracker? I'm really interested in the performance etc.
- dahrkael 1y agoFor small trackers opentracker is probably faster and use a bit less memory. Where extracker is gonna shine compared to it is when core count starts having 2 digits. I still have to do a proper benchmark though.
- jhgg 1y agoReally cool! You looking to write Elixir as your main job?
- bavell 1y agoVery cool! Is this suitable for using as a private tracker?
- dahrkael 1y agonot out of the box but it can be done. All the required moving parts are there (hash whitelist support, udp path parsing, peer rejection, etc).
- toomuchtodo 1y agoAwesome work!
- s-mon 1y agoLove Elixir so much, building a kick-ass notification engine with it now. Its so so good.
- mikehostetler 1y agonice, private or OSS? Elixir needs a better notification engine badly
- rhgraysonii 1y agoWhat do you mean exactly? If you need a notification engine, reaching for a pubsub implementation is very easy with phoenix’s popularity and quite battle tested. I’ve implemented notifications at scale a few times in the ecosystem. What problems are you encountering that you don’t feel you have a tool in the shed to work with in this case?
- rhgraysonii 1y agoAnd if you wanna buy over build, check out https://knock.app https://knock.app - also built in elixir
- mikehostetler 1y agoreally cool project! well done
- voicedYoda 1y agoWell done. Couple quick notes, move to a logger instead of using IO.puts. Also consider adding OTel.
- ai-christianson 1y agoWhat's your favorite otel sink?
- solid_fuel 1y agoI'll second this, just using the built in Logger [0] and Telemetry [1] applications would be fine, opentelemetry or anything else can be added to the telemetry hooks easily to export the metrics later. [0] https://hexdocs.pm/logger/1.18.4/Logger.html https://hexdocs.pm/logger/1.18.4/Logger.html [1] https://hexdocs.pm/telemetry/readme.html https://hexdocs.pm/telemetry/readme.html
- dahrkael 1y agoI use Logger through all the project except for the table pretty print. Telemetry is optionally available in prometheus and html formats: https://github.com/Dahrkael/ExTracker/tree/master/lib/ex_tracker/telemetry https://github.com/Dahrkael/ExTracker/tree/master/lib/ex_tra...
- guywithahat 1y agoThere's something about C++ developers that makes them love Go and Elixir (and I include myself in this demographic). I think it's something about the people who are attracted to C++ for performance are attracted to Go/Elixir for its multithreaded performance. Really cool project
- uncircle 1y agoNot sure about C++ devs, but Erlang/Elixir are great to handle parsing of protocols, with its implementation of pattern matching. Also, makes the code much cleaner because pattern matching basically eliminates most branching and thus depth of the code base. The let it crash philosophy allows you to ignore most corner cases with the knowledge that, if they are encountered or a cosmic ray flips a bit, the crash is localised to a single client. I have worked with Elixir almost a decade at this point, and I have never seen an unexpected downtime of the apps I deployed. Aside of maintenance and updates, they all have 100% uptime. How cool is that? This is how I sell it to clients. “Will you be using Python, Go?” Me: “What about Elixir and the promise that your service won’t ever crash? And you get cool dashboards with it.” Them: “Sold.” I wish there was a systems language that allows you to pattern match on structs and enums, and in function signatures like Elixir
- dahrkael 1y agoIndeed. when your daily job is tracking down memory stomps, deadlocks, invalid pointers and unexpected state in very big codebases then using Elixir feels like "why is this so easy? it just works?". Also i'm a network programmer so the binary pattern matching is very much appreciated.
- Thaxll 1y ago"The let it crash philosophy allows you to ignore most corner cases" This is such a dangerous take. Also Elixir is not strongly typed, so...
- ricketycricket 1y agoIt's not though. Processes can be supervised and crashes can just lead to "restart with good state" behavior. It's not that you don't try handling any errors at all, you just can be confident that anything you missed won't bring the system down. And Elixir is strongly typed by most definitions. Perhaps you mean static?
- quechimba 1y agoNice! I'll check this out some point. I wrote a basic tracker in Elixir a few years ago, here's the code: https://github.com/aalin/mr_torrent https://github.com/aalin/mr_torrent
- dahrkael 1y agointeresting! why did you decide on a private tracker specifically?
- arthurcolle 1y agoCheck out https://github.com/sergiotapia/magnetissimo https://github.com/sergiotapia/magnetissimo it was a popular one like 7 years ago
- KomoD 1y agoThat's not a bittorrent tracker.
- arthurcolle 1y ago[flagged]
- vivzkestrel 1y ago- how did you start - did you refer to other projects - how long did it take - how much functionality do you think works compared to say qbittorrent?
- lionkor 1y agoit's a tracker, not a torrenting client.
- NooneAtAll3 1y agowhat does tracker mean?
- devoutsalsa 1y agoA torrent tracker is basically the world’s most antisocial matchmaking service that knows who has what files but refuses to actually store anything itself, like that friend who always knows where the party is but never hosts one. When your BitTorrent client asks “hey who’s got that Linux ISO,” the tracker dumps a list of IP addresses faster than a startup pivoting after their Series A falls through. Your client then connects to these strangers (seeders with complete files and leechers still downloading) and starts exchanging data while the tracker pretends nothing happened. It’s like Tinder but for file sharing, except everyone’s anonymous and probably downloading something weird at 3am.
- vjerancrnjak 1y agonot anonymous at all, while interacting with the tracker can be done with https, all of the communication between peers is unencrypted.
- memoryuns4f3fff 1y ago[dead]
- immibis 1y ago
- KomoD 1y agoI tried it, couldn't get HTTPS to work. Also my console gets spammed with: 04:43:20.160 [warning] invalid 'event' parameter: size: 6 value: "paused" but it seems to work. I would've liked to see HTTP stats too but I guess UDP is fine (though I have it disabled)
- bill876 1y agoThe "paused" event is part of BEP 21. Clients send it to the tracker to let it know that the client is still incomplete, but won't download anymore. For example, because a user only wants some files from the torrent. Readme of the project shows that support for BEP 21 is not implemented.
- KomoD 1y ago> Readme of the project shows that support for BEP 21 is not implemented. Ah, missed that.
- dahrkael 1y agoTelemetry for the HTTP side is in my ToDo list yes, since I'm using a 3rd party library for the webserver I still need to figure out how to do it right. For HTTPS to work you need to provide a valid certificate path in :https_keyfile but right now I would recommend sticking Caddy or Nginx in front of the tracker if you want HTTPS. I have certbot integration planned but is not a priority since most of the torrent peers use UDP.
- nayuki 1y agoSimilar to: https://news.ycombinator.com/item?id=44265851 https://news.ycombinator.com/item?id=44265851 "Show HN: I wrote a BitTorrent Client from scratch" [2025-06-13], https://github.com/piyushgupta53/go-torrent-client https://github.com/piyushgupta53/go-torrent-client
- lionkor 1y agoNo that's a client
- kat529770 1y ago[dead]
- arch-choot 1y agoInteresting! I'd done something similar in Typescript to learn more about BT, and then redid it in rust to learn rust (https://github.com/ckcr4lyf/kiryuu https://github.com/ckcr4lyf/kiryuu). However I decided to just use redis as the DB. It sounds like your entire DB is in memory? Any interesting design decisions you made and/or problems faced in doing so? (My redis solution isn't great since it does not randomize peers in subsequent announces afaik)
- dahrkael 1y agoin my case using the in-memory ETS has been the best decision, it lets me read&write the peer's data concurrently each on its own process so contention and latency are minimal. the only sequential part is when a new swarm is initially created but that doesn't happen a lot so its fine. there's sadly no native support for taking random rows directly from the tables, so for now i grab the whole swarm and then take a random subset (https://github.com/Dahrkael/ExTracker/blob/master/lib/ex_tracker/processors/announce.ex#L134 https://github.com/Dahrkael/ExTracker/blob/master/lib/ex_tra...)
- toast0 1y agoI don't remember if there's a way to see how many slots an ets table has, but if you're ok with imperfect distribution, you could maybe pick a slot at random and use ets:slot/2 to get all the items in that slot, then select from those. You might be able to get the slot count from eta:table_info(Table, stats), although that's not intended for production use, so the format may change without notice.
- nesarkvechnep 1y agoI really wished to see an OTP-first design. Unfortunately for me, the code is almost procedural as it's touching ETS or Application, which is built on ETS, in nearly every operation. If the author wishes to learn how to design services in Elixir, or any BEAM language, with OTP, they can take a look at "Designing Elixir Systems with OTP" by by James Edward Gray and Bruce Tate, and "Functional Web Development with Elixir, OTP, and Phoenix" by Lance Halvorsen.
- dahrkael 1y agoOn my first try I did write it in a more OTP-y style but the scaling potential for this very specific flow is just not the same. In the end a torrent tracker is just a specialized database and handling the data as fast as possible is the top objective. That said I'll give the books a go.
- throwawaymaths 1y agousing ets is fine.
- salviati 1y agoIf, like me, you don't know what OTP means in this context, here it is: OTP stands for Open Telecom Platform, although it's not that much about telecom anymore (it's more about software that has the property of telecom applications, but yeah.) If half of Erlang's greatness comes from its concurrency and distribution and the other half comes from its error handling capabilities, then the OTP framework is the third half of it. https://learnyousomeerlang.com/what-is-otp https://learnyousomeerlang.com/what-is-otp
- Zarathu 1y agoOut of curiosity, what would an "OTP-first" design look like? ETS is built into OTP, so how is using ETS not "OTP-first"? What's wrong with using ETS? It's just an in-memory store. I looked through the code and didn't find it to be anywhere close to procedural in style.
- andyleclair 1y ago
- eatbitseveryday 1y agoTrackers are not relics - they're used exclusively in private tracker websites. Public-access torrents would more commonly use DHT and PEX for discovery.
- deleted 1y ago[deleted]
- dewey 1y agoBut also private trackers are far from popular these days. Even if I’m a heavy user, but I know it’s a niche.
- TheJoeMan 1y agoCould you please clarify what DHT and PEX are? I'm having trouble searching "tracker PEX".
- yesco 1y ago- https://en.m.wikipedia.org/wiki/Mainline_DHT https://en.m.wikipedia.org/wiki/Mainline_DHT - https://en.m.wikipedia.org/wiki/Peer_exchange https://en.m.wikipedia.org/wiki/Peer_exchange
- atmosx 1y agoDHT (Distributed Hash Table) and PEX (Peer Exchange) let torrent clients find peers without centralised trackers. Hence, you don't need a central place / public tracker anymore
- Thaxll 1y agoYou still need a central server though...
- perching_aix 1y agoYes, they just don't track the individual torrents anymore. They only play a role during the initial peer discovery stage (bootstrapping). Peers find torrent swarms on their own, the bootstrap servers are excluded from all that.
- LtdJorge 1y agoIf you are connected to the DHT network, you don’t. Unless you mean for DNS and such.
- perching_aix 1y agoNo, they mean specifically that in order to connect to such a network, you need to hit some specific central nodes first.
- b0a04gl 1y ago[dead]
- andyleclair 1y agoThis is really good, my dude. I took a peruse through the code and I'm definitely impressed, this is like, code I would expect my senior engineers with elixir experience to turn in. Great job!