6 ms·
Show HN: Honker – Postgres NOTIFY/LISTEN Semantics for SQLite
- russellthehippo 6mo agoHey HN, I built this. Honker adds cross-process NOTIFY/LISTEN to SQLite. You get push-style event delivery with single-digit millisecond latency without a damon/broker, using your existing SQLite file. A lot of pretty high-traffic applications are just Framework+SQLite+Litestream on a VPS now, so I wanted to bring a sixer to the "just use SQLite" party. SQLite doesn't run a server like Postgres, so the trick is moving the polling source from interval queries on a SQLite connection to a lightweight stat(2) on the WAL file. Many small queries are efficient in SQLite (https://www.sqlite.org/np1queryprob.html https://www.sqlite.org/np1queryprob.html) so this isn't really a huge upgrade, but the cross-language result is pretty interesting to me - this is language agnostic as all you do is listen to the WAL file and call SQLite functions. On top of the store/notify primitives, honker ships ephemeral pub/sub (like pg_notify), durable work queues with retries and dead-letter (like pg-boss/Oban), and event streams with per-consumer offsets. All three are rows in your app's existing .db file and can commit atomically with your business write. This is cool because a rollback drops both. This used to be called litenotify/joblite but I bought honker.dev as a joke for my gf and I realized that every mq/task/worker have silly names: Oban, pg-boss, Huey, RabbitMQ, Celery, Sidekiq, etc. Thus a silly goose got its name. Honker waddles the same path as these giants and honks into the same void. Hopefully it's either useful to you or is amusing. Standard alpha software warnings apply.
- hk1337 6mo agoI love the name!
- russellthehippo 6mo agohonk
- arowthway 6mo agoNice, I had no idea that stat() every 1 ms is so affordable. Aparently it takes less than 1 μs per call on my hardware, so that's less than 0.1% cpu time for polling.
- WJW 6mo ago"Syscalls are slow" is only mostly true. They are slower than not having to cross the userspace <-> OS barrier at all, but they're not "slow" like cross-ocean network calls can be. For example, non-VDSO syscalls in linux are about 250 nanoseconds (see for example https://arkanis.de/weblog/2017-01-05-measurements-of-system-call-performance-and-overhead/ https://arkanis.de/weblog/2017-01-05-measurements-of-system-...), VDSO syscalls are roughly 10x faster. Slower than userspace function calls for sure, but more than affordable outside the hottest of loops.
- slashdev 6mo agoThat’s ignoring the other costs of syscalls like evicting your stuff from the CPU caches. But I agree with the conclusion, system calls are still pretty fast compared to a lot of other things.
- vlovich123 6mo agoSmall correction on ambiguous wording - syscalls do not evict all your stuff from CPU caches. It just has to page in whatever is needed for kernel code/data accessed by the call, but that’s no different from if it was done in process as a normal function call.
- Polizeiposaune 6mo agoDepending on implementation details of your CPU and OS, the syscall path may need to flush various auxillary caches (like one or more TLBs) to prevent speculation attacks, which may put additional "drag" on your program after syscall return.
- andersmurphy 6mo agoIs the main use case for this for languages that only have access to process based concurrency? Struggling to see why you would otherwise need this in java/go/clojure/C# your sqlite has a single writer, so you can notify all threads that care about inserts/updates/changes as your application manages the single writer (with a language level concurrent queue) so you know when it's writing and what it has just written. So it always felt simpler/cleaner to get notification semantics that way. Still fun to see people abuse WAL in creative ways. Cool to see a notify mechanism that works for languages that only have process based concurrency python/JS/TS/ruby. Nice work!
- infogulch 6mo agoHe mentions Litestream, maybe this also works for litestream read-only replicas which may be in completely different locations?
- russellthehippo 6mo agoWhoa I really hadn’t considered this. Do a litestream read replica, trigger across machines with S3 as the broker essentially. But you’re still stuck with the litestream sync interval. Maybe interesting for cross server notify?
- infogulch 6mo agoI guess the idea is to have all writes go through a central server with local read replicas for improved read perf. The default litestream sync interval is 1s. I bet many use-cases would be satisfied with a few seconds delay for cross-region notifications.
- russellthehippo 6mo agoIt's good for pubsub but not for claim/ack workflow unless you do If-None-Match CAS semantics on a separate filesystem which, actually, yeah that's probably fine. Feels heavy on S3 ops. But! you do save on inter-AZ networking, the Warpstream hypothesis.
- infogulch 6mo agoNeat idea! Would it help if subscriber states were also stored? (read position, queue name, filters, etc) Then instead of waking all subscription threads to do their own N=1 SELECT when stat(2) changes, the polling thread could do Events INNER JOIN Subscribers and only wake the subscribers that match.
- noveltyaccount 6mo agoThis is really interesting. I'm building something on Postgresql with LISTEN/NOTIFY and Postgraphile. I'd love to (in theory) be able to have a swappable backend and not be so tightly coupled to the database server.
- ncruces 6mo agoProbably missing something, why is `stat(2)` better than: `PRAGMA data_version`? https://sqlite.org/pragma.html#pragma_data_version https://sqlite.org/pragma.html#pragma_data_version Or for a C API that's even better, `SQLITE_FCNTL_DATA_VERSION`: https://sqlite.org/c3ref/c_fcntl_begin_atomic_write.html#sqlitefcntldataversion https://sqlite.org/c3ref/c_fcntl_begin_atomic_write.html#sql...
- psadri 6mo agoFor one it seems to be deprecated.
- ncruces 6mo agoIt's not.
- russellthehippo 6mo agoYep, definitely still in use. Do yall above have an opinion if the pragma is better than the syscall? What are the trade offs there? Another comment thread mentioned this as well and pointed to io uring. I was thinking that dism spam is worse than syscall spam.
- ncruces 6mo agoDepends on what to mean by better. I may be wrong, but I think you wrote somewhere that you're looking at the WAL size increasing to know if something was committed. Well, the WAL can be truncated, what then? Or even, however unlikely, it could be truncated, then a transaction comes and appends just enough to it to make it the same size. If SQLite has an API it guarantees can notify you of changes, that seems better, in the sense that you're passing responsibility along to the experts. It should also work with rollback mode, another advantage. And I don't think wakes you up if a large transaction rolls back (a transaction can hit the WAL and never commit). That said, I'm not sure what's lighter on average. For a WAL mode database, I will say that something that has knowledge of the WAL index could potentially be cheaper? That file is mmapped. The syscalls involved are file locks, if any.
- rich_sasha 6mo agoPretty cool! I have a half baked version of something similar :) Can you use it also as a lightweight Kafka - persistent message stream? With semantics like, replay all messages (historical+real time) from some timestamp for some topics? As with pub/sub, you can reproduce this with some polling etc but as you say, that's not optimal.
- russellthehippo 6mo agoAbsolutely! That’s the durable pubsub angle for sure.
- grumbelbart2 6mo agoVery cool! Another maybe stupid question, would something like inotify(7) help to get rid of any active polling?
- GangstaAgents 6mo ago[flagged]
- ArielTM 6mo agokqueue/FSEvents is tempting here, but Darwin drops same-process notifications. If you've got a publisher and listener in the same process the listener just never fires. Nasty thing to chase. stat polling looks gross but it's the only thing that actually works everywhere. What happens on WAL checkpoint? When the file shrinks back, does that trigger a wakeup, or does the poller filter size drops?
- russellthehippo 6mo agoActually need to test this. Will report back
- xenadu02 6mo agoThis comment is completely incorrect. kqueue VNODE events are delivered so long as your process has access to the file. There is no "same-process" notification filter.
- Retr0id 6mo agoCouldn't you use inotify (and/or some cross-platform wrapper) to watch for WAL changes without polling?
- russellthehippo 6mo agoBreaks cross-platform, specifically Macs swallow silently. stat just works
- Retr0id 6mo agoI don't believe this to be true.
- russellthehippo 6mo agoSee comment below - Darwin silently drops same-process notifs. I could change the behavior depending on same vs cross process and platform but I wanted to”just one thing to worry about”. Potentially a good optimization later. Would help reduce syscalls.
- Retr0id 6mo agoI believe you are mistaken. If you are referring to the comment from ArielTM, that's an LLM bot regurgitating your readme.
- russellthehippo 6mo agoApologies for not being specific. The specific thing I'm talking about is this: write events don't fire until the file handle is closed. [1] I didn't validate this myself btw, but my original design was certainly trying to use notify events rather than stat polling. My research (heavily AI assisted of course) led me away from that path as platforms differ in behavior and I wanted to avoid that. [1] https://github.com/notify-rs/notify/issues/240 https://github.com/notify-rs/notify/issues/240
- 6mo ago
- 0x1da49 6mo ago[flagged]
- PunchyHamster 6mo agoWouldn't processes on same machine be able to use different IPCs that don't even touch file ? It's neat but I have feeling in vast majority of cases just passing address to one of the IPC methods would be faster and then SQLite itself would only be needed for the durable parts.
- blacklion 6mo agoThis extension piggyback SQLite native transactions. For example, queueing data will be rolled back if transaction is rolled back due to some constrains violations. It is possible to achieve with external IPC, but require a lot of very careful programming.
- PunchyHamster 6mo agohonestly I used those types of extensions so little I didn't even consider notifications being transactional and firing only when transaction completes
- tuo-lei 6mo agoatomic commit with the business data is the selling point over separate IPC. external message passing always has the 'notification sent but transaction rolled back' problem and that gets messy. one thing i'm curious about: WAL checkpoint. when SQLite truncates WAL back to zero, does the stat() polling handle that correctly? feels like there's a window where events could get lost.
- russellthehippo 6mo agoThe WAL file sticks around but gets truncated so that counts as an update. Though I don’t have tests for this. Good input, thanks I’ll make sure
- kippinsula 6mo agothe atomicity is the whole game. we burned time on a Postgres+SQS setup where the enqueue happened in a trigger that fired before the commit was visible to other connections. added retry logic, then polling on the worker side, then eventually moved the enqueue inside the transaction. at that point you're basically reinventing what Honker does, just with more moving parts. the 'notification sent, row not committed' class of bug is usually silent and timing-dependent, which makes it brutal to track down.
- nzoschke 6mo agoThanks for this! I have a proliferation of small apps backed by SQLite. And most of these need a queue and scheduler. I home rolled some stuff for it but was always pining for the elegance of the Postgres solutions. Will give this a spin very soon
- russellthehippo 6mo ago"a small proliferation" is a nice way to describe the cluster that is my side project habit. if you bump into any issues pls pull a PR or drop an issue on the repo!
- mastermanas1234 6mo ago[dead]
- nodesocket 6mo agoAwesome. I’m currently using AWS SQS which invokes lambda functions for asynchronous tasks like email sends, but Honker seems like a great local replacement. Any conflicts or issues when running Litestream as well?
- russellthehippo 6mo agoNope! The extension just functions as a shortcut for raw SQL. Litestream edits the wal file but only like a normal checkpoint. So not too bad. Although I haven’t tested it directly. Probably need to
- JoelJacobson 6mo agoShameless plug: In the upcoming release of PostgreSQL 19, LISTEN/NOTIFY has been optimized to scale much better with selective signaling, i.e. when lots of backends are listening on different channels, patch: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=282b1cde9 https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit...
- russellthehippo 6mo agoGreat plug very relevant
- russellthehippo 6mo agolink for all others - this commenter fixed a bug in core Postgres that fixed a scaling issue in actual listen/notify https://www.recall.ai/blog/postgres-listen-notify-does-not-scale https://www.recall.ai/blog/postgres-listen-notify-does-not-s...
- yasirlatif 6mo ago[dead]
- robertlagrant 6mo agoIf I'm using SQLAlchemy, can this integrate? It seems to want to make the db connection itself.
- russellthehippo 6mo agoI've added examples for many ORMs and web frameworks to the docs. See here: https://honker.dev/guides/orm/#sqlalchemy--sqlmodel https://honker.dev/guides/orm/#sqlalchemy--sqlmodel
- robertlagrant 6mo agoThank you.
- faraway9911 6mo ago[dead]
- agentbonnybb 6mo agoThis is the kind of skill-level tool I wish existed earlier — I hit the exact pain point running a daily-chronicle site off SQLite + a static deploy a week ago. Ended up with a crude polling loop because the alternatives all wanted me to install Postgres for a single notification semantic. Question: any thoughts on what breaks first when a single process has 10k+ concurrent listeners? I'm curious whether the SQLite side can sustain what Postgres does cheaply.
- russellthehippo 6mo ago10k listeners is a lot. Thundering herd issue at stat(). SQLite may not be your best choice at this scale.
- russellthehippo 6mo agoalso this is designed for a single machine. 10k listeners on one machine seems like a lot!
- adrianojunior 6mo ago[dead]
- zbentley 6mo agoVery neat! I like this a lot, nice work. After peeking the source, a few possible areas of improvement: - You can use `fstat` and keep a file handle around, likely further improving performance (well, reducing the performance hit to other users of the filesystem by not resolving vfs nodes). If you do this, you'll have to check for file deletions. - If you do stick with stat(2), it might be a good idea to track the inode number from the stat result in addition to the time,size tuple. That handles the "t,s = 1,2; honker gets SIGSTOPped/CRIU'd; database file replaced; honker started again", as well as renameat/symlink-swap fiddling. Changing inode probably should just trigger a crash. - Also check the device number from the stat call. It sounds fringe, but the number of weird hellbugs I've dealt with in my career caused by code continually interacting with a file at the same time as something else mounted an equivalent path "over" the directory the file was originally in is nonzero. - It's been a few years since I fought with this, but aren't there edge cases here if the system clock goes backwards? IIRC the inode timestamp isn't monotonic--right? There are various strategies for detecting clock adjustment, of various reliability, that you could use here, if so. Just checking if the mtime-vs-system-clock diff is negative is a start. That covers the more common of the "vanishingly uncommon but I've still seen 'em" cases related to file modification detection. Whether you choose to cope with people messing with the file via utime(2) is up to you (past a point, it feels like coping with malicious misuse rather than edge cases). But since your code runs in a loop, you're well-positioned to do that (and detect drift/manipulations of the system clock): track a monotonic clock and use it to approximate the elapsed wall time between honker poller ticks (say it fast with an accent, and you get https://www.bbc.com/news/world-latin-america-11465127 https://www.bbc.com/news/world-latin-america-11465127); if the timestamp reported by (f)stat(2) ever doesn't advance at the same rate, fall back to checksumming the file, or crashing or something. But this is well into the realm of abject paranoia by now. It's been a decade or so since I worked in this area, so some of that knowledge is likely stale; you probably know a lot more than I do after developing this library even before considering how out-of-date my knowledge might be. When I worked on this stuff, I remember that statx(2) was going to solve all the problems any day now, and then didn't. More relevant, I also remember that the lsyncd (https://github.com/lsyncd/lsyncd https://github.com/lsyncd/lsyncd) and watchman (https://github.com/facebook/watchman https://github.com/facebook/watchman) codebases were really good sources of "what didn't I think of" information in this area. But seriously, again, nice work! Those are nitpicks; this is awesome as-is!
- aldielshala 6mo agoNice project. I'm also working on something that pushes SQLite well beyond its typical use case. It's encouraging to see more people exploring what SQLite can really do.
- jiusanzhou 6mo ago[dead]
- Oxlamarr 6mo agoVery cool. Is the bottleneck under load mostly SQLite write throughput, or the WAL notification layer?
- russellthehippo 6mo agowrites and claim/ack flow. really depends on your journal mode and synchrnous mode as well. notifs are extremely cheap, either in the old stat(2) mode or the new PRAGMA page_version (see my update on feeback comment). Some other comments mentioned that stat(2) is about 1µs.
- russellthehippo 6mo ago[Response to feedback] Thanks all for your feedback, responses, and discussion. I've done a PR here taking your suggestions into account: https://github.com/russellromney/honker/pulls/1 https://github.com/russellromney/honker/pulls/1 The PR implements a three-layer polling architecture: - PRAGMA data_version every 1ms - stat every 100ms - retry connection to handle blips 1. PRAGMA data_version every 1ms replaces stat-based (size, mtime) change detection. This is SQLite's own commit counter: monotonic, immune to clock skew, correctly handles WAL truncation and rolled-back transactions. ~3µs nonblocking query. Credit to ncruces for pointing to this. This is not done for performance but for correctness as it is slightly slower. tuo-lei also pointed out truncation risk, which turned out to be more real than i thought. Interesting note: I found in testing that the C API's SQLITE_FCNTL_DATA_VERSION does not work cross-connection. So for now honker continues paying the cost of going through the VFS layer which vlovich123 pointed out and now we tradeoff explicitly. 2. Reconnect-on-error: if the data_version query fails (disk blip, NFS hiccup, corrupted connection), honker tries to reconnect and wakes subscribers as a precaution. zbentley pointed me in this direction. 3. stat identity check every 100ms: compares (dev, ino) against startup values to detect file replacement (atomic rename, litestream restore, volume remount). data_version can't catch this because it polls through the open fd, which follows the original inode even after replacement. Credit to zbentley for the file-replacement scenarios. Again - thanks for the discussion, honker got better because of it and I learned some stuff. See you round
- MarcelinoGMX3C 6mo ago[dead]
- davisphan04 6mo agoInteresting approach—using WAL file stat instead of polling queries is clever. Curious how it behaves under high write contention?
- hizihic 6mo ago[dead]
- valkdb 6mo ago[dead]
- vogje01 6mo agoCool! Could I use it as a CRUD message stream? Just in case I need to inform some consumers.
- nawi 6mo agoThe 1 ms pragma data version polling is a nice way to bypass the lack of a server process. didyou measure the impact on cpu or battery life for applications running on lower power devices or laptops? i curious if the polling that prevents the CPU from entering deeper sleep states
- aleken 6mo agoI have used update_hooks to achieve similar behaviour in my projects. But perhaps this is intended as a higher level layer? It looks like the implementation is not using hooks? https://sqlite.org/c3ref/update_hook.html https://sqlite.org/c3ref/update_hook.html