Rxdb: A reactive database where you can subscribe to the result of a query | Hacker News Reader

Rxdb: A reactive database where you can subscribe to the result of a query

github.com

288 pointsdsun180122 comments

I have been developing something that provides this functionality for PostgreSQL: https://github.com/supabase/realtime

It's still in very early stages (although I am using it in production for my company)

It's very similar to Debezium (mentioned in another comment), but it's built with Phoenix (elixir), so great for listening via websockets.

Basically the Phoenix server listens to PostgreSQL's replication functionality and converts the byte stream into JSON which it then broadcasts over websockets. This is great since you can scale the Phoenix servers without any additional load on your DB. Also it doesn't require wal2json (a Postgres extension). The beauty of listening to the replication functionality is that you can make changes to your database from anywhere - your api, directly in the DB, via a console etc - and you will still receive the changes via Phoenix.

I still have to document a lot of how it works and how to use it, but if anyone is interested then I will make it a priority over the weekend

There is a big difference between the changestream of many SQL and noSQL databases, and what RxDB does. Having a stream of changes is useful but not the whole solution. RxDB is capable of using single document changes of a stream and recalculate the new results of an existing query. This saves you not only much IO performance but makes developing much easier. See https://rxdb.info/query-change-detection.html

Yeah perhaps "same functionality" is an ambitious statement. I just saw debezium mentioned below and thought i'd throw this out in case someone reading this and it fits their needs!

I would love to use this just for the offline and query change detection capabilities alone - but the latter is currently beta and disabled by default. Is there a reason for that?

I do this sort of thing with triggers and table-based materializations of views.

RethinkDB might be another database to look at if this is functionality which you are interested in. My only gripe with doing this sort of thing is that if you offer a public API, it becomes separate to what you are using for the application internally. There are also some GraphQL databases like Prisma that offer this sort of functionality.

I actually used Firebase at the start but this implementation is so that we can migrate 100% to Postgres. I agree though, Rethink is awesome

Just out of curiosity, are you moving off of Firebase because of technical limitations, or because of pricing?

Mainly because I want to keep my team small so it’s easier if we all commit to Postgres. But We also hard a hard time with Firebase’s filtering on sub collections (for their new Firestore)

Fair enough.

Make this good enough and you could very well cut into a huge chunk of Firebase's business model.

I'd seriously love to have query subscriptions, especially if the library is robust enough for use on mobile.

Ine large differentiator of Firebase's DB offerings is the subscription functionality, mobile and web, but using that means buying into NoSQL. A proper subscribable SQL database would be amazing.

Of course the conflict resolution on writes, and the local caching is another key benefit Firebase offers. Not sure how that could be done with a real relational DB!

Thanks for the comments. I agree, the conflict resolution of FB on mobile is pretty awesome.

FYI we are using this on our mobile (react native) in production, so I have no doubts it will be robust enough with a bit more polish

Did you evaluate gRPC or gRPC-Web when considering your websockets implementation?

I'm using gRPC in other languages/frameworks and I'd really like to migrate one of those to Elixir in the near future, but I'm curious to know if anyone else has given it a try.

There's a prominent warning on the repo:

https://github.com/elixir-grpc/grpc

I didn’t try, to be honest i think the implementation of this might be beyond my elixir capabilities. But I’ll check it out - thanks for the link!

Interesting, thanks for sharing how you're doing it.

Is there any mileage in doing this with triggers? I have a _very_ legacy system which needs caching adding. Rather than dig through the code to invalidate the cache every time a record is updated/deleted in 20+ tables, I am thinking that being able to listen to the SQL executed and invalidate the cache based on the tables involved would be a clean approach.

But not found any way to make that possible - yet.

Yeah I also used triggers at the start. There are 2 things that this implementation achieve over triggers.

1. Triggers have an 8000 byte limit. I ran against these limits pretty quickly

2. You need to attach the trigger each time you create a new table. With this you can set and forget

There's no 8k limit with triggers? Could you expect on what you mean by that?

One of the motivating factors for not using triggers is that the implied overhead is very significant. By logging changes separately which triggers the write volume is roughly doubled, and the overhead of insertions is much much higher (a lot of fast/bulk path can't be used, the trigger processing needs to be performed).

Sorry I meant I was using triggers to sent a NOTIFY which has an 8k byte limit.

Very true what you mention about trigger overhead. Also you don’t get guaranteed atomicity

I wrote a node.js package a few years ago that gets past the 8000 bytes by sending multiple notifications.

https://github.com/numtel/pg-live-select

You can use apache ignite as a caching layer over your db. (Or as a db itself). You can then register a ContinuousQuery to listen for changes. A grpc service can be stood up to stream these changes to other clients, even front end ones (via grpc-web).

Having client state just be a replica of server state solves so many problems I don't understand why the concept never caught on. Pouchdb/couchdb are still the only ones doing it afaik.

Instead we have a bajillion layers of CRUD all in slightly different protocols just to do the same read or write to the database.

How would that work, with, say Twitter? Copy all tweets ever to each browser?

I mean, that's why it didn't catch on right :-) It's hard :-)

Same way you scale replication for any server, by sharding and only replicating the shards you care about.

The "shard" could just be that users own feed in this case. Then you get offline for free where user adds a tweet and it appears immediately, replicating back to server when he goes back online. The server replica side will need to be a lot more complicated to deal with broadcasting but I don't see why it won't work.

I attempted something similar on a current project, the problem is with inital data loading. If you are hitting the URL/Page for the first time you are waiting minutes or more for non trivial data sets.

Why not just load what is needed and hydrate the data over time? What about datasets where you need pagination/ordering etc. And the only way to guarantee order is to pull the whole set?

In twitter-like applications, the default ordering is usually just ORDER BY timestamp DESC. You could rely on this default ordering to load the first few dozen items on first visit, and load the remainder asynchronously. Sort of like automatic infinite scrolling.

Of course, users with limited RAM and metered connections won't like that. Which is another reason why it didn't catch on.

I've heard somewhere that Twitter maintains a copy of all tweets in a user's timeline for that user.

If I had to make a Twitter clone with CouchDB, I would probably have one timeline document per user, and maybe one per day to limit the syncing bandwidth.

There is a limit on what data can be replicated with the client. For a simple chat-app you can replicate all messages of a user. But you would never replicate the whole state of wikipedia to make it searchable.

When your data is public and immutable, this approach is very pleasant. The client becomes just another caching layer and worst case it's presenting a historical version of the truth. You can even extend this across tabs with things like local storage.

This breaks down quickly once you have data that could become private or mutate rather than append.

I think the Firebase Realtime Database and Firestore have a good model for offline and being able to have private and mutable data. It does get complex but the Firebase SDKs do the heavy lifting for you here.

Can you share more about the private and mutable data functionality for Firebase? I used them a few years back, and never really understood how to do private data without building my own ACL inside Firebase.

Here is a couple of specific examples: https://medium.com/firebase-developers/patterns-for-security... https://firebase.google.com/docs/firestore/solutions/role-ba...

You do need to have you ACL data also stored in the database, which can be a hassle if you have existing ACL system already built outside of Firebase.

xrd, do you have a specific question about private data in Firestore?

Thanks for the response. I suppose my biggest question is not how I can store private data (I can just make it inaccessible via the right rules). But, it seems like I am then layering my own ACL system onto those rules. And, I never got a sense there was an easy way to write a test that simulated my rules against my data and made sure I was not accidentally creating a leaky rule.

In so many ways it is SO much easier to use Firebase because all the pieces are right there as compared to a DB + Server + Front End + Tooling. But, I still always worried that I would somehow leave a gaping hole in my data and not know about it.

And, I was never really sure how I can easily do joins across data without writing my own bespoke metalanguage inside Firebase. A link posted today on HN talked about XML does turn out to be good for nested data (hence the reason it is used for UIs), and it feels like Firebase being more or less JSON loses in this respect.

That's just my experiences, and I say that loving Firebase.

Those two examples made me think: Firebase removes a lot of complexity for me, but it forces me to write my own layer of complex access and DB logic which I never felt fully qualified to do, and as such, just went back to using databases with an ORM and a backend server.

This what stopped me exploring couchdb further.

The "one db per user" model for private data made using other features like views etc more difficult when you have to upgrade,edit,remove them.

Mutability wasn't really a problem, either present the conflicts to user and pick one or write code to merge if possible.

Couchbase (with its sync gateway) uses "channels" to sync data. It even lets you change the channel of a doc, and to the sync client it shows as if the doc was deleted (if the user is not part of the new channel)

Yeah, couchdb more or less requires you to replicate data for individual users if you need complex permissions and want the user to access the couchdb directly.

Permissions in general need to be handled by custom reconciliation functions (dropping unauthorized changes) or some kind of nanny system that can react to changes.

For example, imagine blog posts as documents, and a list of comments inside that document. Instead of the user adding/changing the comment list, the user would add a record to a comment request list, and either the reconciliation process or a nanny service checks the requests and updates the comment list.

The much simpler solution of course is to not let the users have any write access to the couchdb and just use a REST API. But then you loose much of the benefits of couchdb...

In reality data can't become private again, after being available. You may try to contact all users to delete their copy, but they may not respect that.

That's in theory. In actual reality, if you're making an app that has private data and is not crawled by bots, most of the time users don't save everything that they see.

I think the parent means clients (user agents), not actual human users. As soon as any state that needs to be hidden is exposed to clients, that's a security breach regardless of whether any human eyeballs have seen it.

This is true, too, but I meant users, too. It's all too common for users to screenshot things, etc. I see it a lot of the time on Twitter for example. You can delete tweets, but oftentimes it's pointless. But I also regularly see my gf taking photos with her phone of various apps she uses on her notebook, just to have the info around on her phone. I suspect it's pretty common, because it's much more low-tech then saving pages or API scraping.

> RxDB can do a realtime replication with any CouchDB compliant endpoint and also with GraphQL endpoints.

PouchDB is mentioned a couple of times, including one “PouchDB compatible” mention. Wondering what unique use cases RxDB supports?

Firestore is doing something very similar and it is very easy to use.

Data security is a huge issue, Facebook.com has very specific whitelisted access patterns encoded as CRUD endpoints. 1) Users want to make sure their data is used in non-creepy or non-stalky ways; 2) Facebook's business needs to control the access point so they can serve you adds or otherwise monetize. So the API exposes only limited access patterns, the API TOS disallows caching, and they go to great lengths to prevent scrapers.

If immutable fact/datom streams with idealized cache infrastructure becomes a thing (and architecturally i hope it does) it's going to need DRM to be accepted by both users and businesses.

Security is a huge deal

That's exactly the model that Apollo Client library uses (GraphQL-based data store for react), and teams I spoke that tried it are quite enthusiastic for this reason.

> Having client state just be a replica of server state solves so many problems I don't understand why the concept never caught on.

As soon as your server state is larger than whatever your client can handle, the whole metaphor breaks down.

I'm assuming that the client state would be a lazy representation of the back end state, that only pulls data as needed. The result being that local and server state must both be treated only as asynchronously accessible.

MeteorJS tried it and failed miserably. It doesn't scale.

Wrong. This is popular anti-Meteor FUD spread by people who don't know how to use its features properly or have the engineering/computer science background to design a system to be able to manage computational complexity or scalability.

In 2015, my business implemented a Meteor-based real-time vehicle tracking app utilising Blaze, Iron Router, DDP, Pub/Sub

Our Meteor app runs 24hrs/day and handles hundreds of drivers tracking in every few seconds whilst publishing real-time updates and reports to many connected clients. Yes, this means Pub/Sub and DDP.

This is easily being handled by a single Node.js process on a commodity Linux server consuming a fraction of a single core’s available CPU power during peak periods, using only several hundred megabytes of RAM.

How was this achieved?

We chose to use Meteor with MySQL instead of MongoDB. When using the Meteor MySQL package, reactivity is triggered by the MySQL binary log instead of the MongoDB oplog. The MySQL package provides finer-grained control over reactivity by allowing you to provide your own custom trigger functions.

Accordingly, we put a lot of thought into our MySQL schema design and coded our custom trigger functions to be selective as possible to prevent SQL queries from being needlessly executed and wasting CPU, IO and network bandwidth by publishing redundant updates to the client.

In terms of scalability in general, are we limited to a single Node.js process? Absolutely not - we use Nginx to terminate the connection from the client and spread the load across multiple Node.js processes. Similarly, MySQL master-slave replication allows us to spread the load across a cluster of servers.

For those using MongoDB, a Meteor package named RedisOplog provides improved scalability with the assistance of Redis's pub/sub functionality.

Hey Vlad, very cool to hear you're still using this stuff. I've seen the notications on the latest with Zongji. [0] Kudos for keeping it going!

https://github.com/nevill/zongji

In short, server data is more normalized than the data client needs. As you get closer to view layer, your data gets denormalized further and further. Client-server interaction sits somewhere in the middle to both minimize the bytes-over-wire as well as the round-trips to the backend to get up-to-date.

Take a look at GraphQL, its central promise is to let client choose what's the optimal data it needs (that often denormalized through nested GraphQL queries), and send it in one batch.

It is not to say there shouldn't be a simple replica. It is just if we want it to be a simple replica, we should have a server-side mirrored some-what-denormalized representation rather than just the raw server-data models.

If you want to subscribe to changes in a Postgresql database, whether produced by your app or not, you can also use Hasura or PostGraphile:

https://docs.hasura.io/1.0/graphql/manual/subscriptions/inde...

https://www.graphile.org/postgraphile/realtime/

There is a big difference between the changestream of many SQL and noSQL databases, and what RxDB does. See https://news.ycombinator.com/item?id=21353971

I tried out Postgraphile Realtime and it's pretty cool.

For anyone who wants something similar that's not GraphQL, then I'm in the early stages of developing a Phoenix (Elixir) implementation which broadcasts changes over websockets: https://github.com/supabase/realtime

See comments here: https://news.ycombinator.com/item?id=21354039

Neat; happy to have inspired you!

You did Benjie! Postgraphile is really an awesome piece of tech. I use PostgREST a lot, and I see graphile as an awesome graphql equivalent.

You can do this in datomic! I made an end-to-end proof of concept of this using datomic and websockets. The client can open a websocket and subscribe to a query and then the server will react to changes in the database and automatically update the model and therefore the UI (no eventing code required).

Demo vid https://v.usetapes.com/85knuAPruB Longer description in github readme https://github.com/jtmarmon/hackerthreads

Also see https://github.com/metasoarous/datsync - note that this project stalled out due to technical challenges, though i hear they're stealthily working on it again and coordinating with the Declarative Dataflows project https://github.com/comnik/declarative-dataflow

I think you have not understood what RxDB is. It is a client side database. It works also when the client is offline. It does not need a stable internet connection over a websocket.

Your example is similar to RethinkDB and others. A websocket streaming json.

A more common solution includes using Datascript[1] as the client side DB and then using something like Posh[2] or Datsync[3] to handle query/pull subscription changes.

[1]: https://github.com/tonsky/datascript

[2]: https://github.com/mpdairy/posh

[3]: https://github.com/metasoarous/datsync

Another DB that plays in that field is Watermelon DB (https://github.com/Nozbe/WatermelonDB) which supports react-native based on SQLite and the web based on the LokiJS in memory DB.

General question about reacting to database events: It seems like when responding to DB events it would be easy to accidentally create an infinite loop. Is that an issue with this pattern, or is it easy to avoid? Do any of these data subscription tools have safeguards in place to prevent this?

Isn't this the case with any interaction between 2 systems? Between a client and server, you can have logic in the client that reacts to a response from the server that triggers another request to the server.

Very true, maybe I'm overthinking it? In my head: If I update full_name from first_name and last_name whenever a user table row changes, if I do this naively I will update the user on every update and trigger a continuous loop of updates.

I have certainly created infinite loops while using React's componentDidUpdate, maybe it's just important to define triggers on single attributes rather than entire database rows.

This is where command and query (with subscribe) must be neatly separated. An event A launches an updates of the DB, that launches the query part to react, then stop. There is the risk of having an infinite loop, if the event A is launched by the query part, which is very related to the behavior of your app.

Naively it sounds like the halting problem. You would have to verify that no consumer can make a change to the data set you are listening to. You would want to manage yourself when not to respond to an event.

The ideal case is that the DB can work out the difference in the subscribed query caused by the DB update so that the front end can make changes incrementally and doesn't have to rerender the entire list/table.

Meteor do this since inception with MongoDB on server and MiniMongo in client.

Yes, mongodb and meteor and minimongo do something similar but with much more restrictions on what you can do.

RxDB does exactly one thing which is being a client side database. You are not tied to a specific ecosystem or backend database.

Yeah, with RxDB, we are tied with RxJS, PouchDB and CouchDB.

Anyway, database is hard, opensource is great. We love them both, RxDB or Meteor.

Isn't "subscribe to all state-changes like the result of a query" something you'd implement efficiently using a forward chaining inference engine, like something based on Rete?

It is implemented with something called QueryChangeDetection here. https://rxdb.info/query-change-detection.html

This is based on meteors oplog-observe-driver https://github.com/meteor/docs/blob/version-NEXT/long-form/o...

So basically when a change-event comes, RxDB does not run the query against the database again, but instead uses the old results together with the event to calculate the new results.

I don't know too much about database systems, but if we were in an inference (e.g., production) system, then that would make a lot of sense! I build logical inference systems and have implemented this kind of functionality in forward, backward, and bi-directional logical inference.

If you want a change-stream out of your regular MySQL or PostgreSQL database systems, check out Debezium: https://debezium.io/

It basically registers itself as a fake replica server so that it can get updates from the master (like binlog in case of MySQL) and then it forwards those updates to Kafka. The possibilities are endless what you can do with those updates as Kafka consumers.

Sure, that's handy as an extra feature to get better performance, but I see that even in RxDB it's currently in beta and turned off by default.

I suppose this could be the killer feature to warrant using a new database -- because otherwise, from a user point of view, the result is the same.

So like firestore?

Maybe this is an uninformed comment, but how is this different from the Firestore database in Firebase where you can listen for when a document changes?

hey, this is great! I've been building my own version of exactly this concept based on an event-sourced in-memory graph for my visual programming environment and it works really well for creating a collaborative editing system. I'll be investigating this project now to see if I can't use it instead of my hand-rolled solution.

This is something Realm does really well, both locally on the device and also with live subscriptions to data in the cloud.

We used this for our mobile apps and the experience was pretty awesome. The ability to live observe both individual objects in the dB and results of queries makes building reactive UI’s a very pleasant experience.

I was complaining about lack of such a database just today: https://news.ycombinator.com/item?id=21352235

I'm not doing field projects anymore but knowing this exists would have helped me tremendously in the last few years.

Thanks for sharing!

Nice ! reminds me of the Ryzom project in Python/Django/Postgres which implements a meteorjs-like protocol and proposes the same example with a todo list

https://github.com/yourlabs/ryzom

What I personally like about RxDB is that it also works completely offline. However, I don't really understand encryption: As far as I understood, it encrypts on the client side with the db passwort which is sent to the server.

Another issue is Authorization and Authentication, I could not find a good solution for me for CouchDB. Couchbase seems to have better solutions for this but the premium plan seems really expensive and as far as I understood you need a "server" and a "sync server" which don't have low system requirements, at least for me.

No you normally do not send the password to the server. You can ask the user to enter the password when the application is openend.

Authentication is much easier when you use the GraphQL replication. There you are much more flexible on which data you return depending on which user is asking for it.

I personally don't like the idea of that coupling, I prefer to use my own events, and go through websockets for ex..

This is a nice feature for some, but not all situations..

deepstream.io has similar functionality and supports postgres, rethinkdb and other databases. It is a great tool and promotes a very nice paradigm.

I'm not sure why you would use this over something more battle-tested like RethinkDB which was also built with this use-case in mind.

Because rethinkdb development has ceased? Last release was in 2017.

Is there still maintenance?

Why not maintain it rather than writing something new? Open source is frustrating

The community has taken it over as far as I know. Looking at the github there appears to be some active development still. https://github.com/rethinkdb/rethinkdb

"Is RethinkDB dead?"

https://github.com/rethinkdb/rethinkdb/issues/6747#issuecomm...

TLDR: They are hoping to announce a revival in the near future.

RethinkDB is not offline first. It runs queries on the server-side, not on the client. There is horizonDB which can replicate with RethinkDB but the project is dead with no commits since the rethinkdb company gave up.

So I was one of the developers of RethinkDB's Horizon, and I'm really glad to see someone take the database change stream to RxJS concept to its logical conclusion.

This project looks really awesome, and very much has the shape we wished Horizon could have turned into

I have made something similar trying to match more the concept of a materialized view, but just on the client instead of inside the database: https://github.com/tozd/node-reactive-postgres

EDIT: never mind, I should have read the article.

I'm sorry but I think you comment is spam. The product in your article has only one similarity with RxDB which is being a database.

You're right, sorry. I didn't click on the link and thought it was something else. I can't delete my comment, unfortunately.

This project has all of the goals that excited me about RethinkDB, I hope this project is more successful.

Cool. But how is this better than Meteor for building webapps? Meteor would scale better too with Apollo.

I do not think that meteor will scale better then your GraphQL server with whatever database you want to have. With meteor you are bound to a specific ecosystem which is often a pain. RxDB does only one thing, it is a client side database. Everything else in your stack is free to choose.

Oh, I didn't know this was just for the client. Missed that.

Anyone know how this is different from the reactive query layer that Meteor built on top of Mongo?

What is the time complexity of receiving a notification?

Are there triggers that run on every update on the dataset?

Hasura does this for Postgres. What are some areas where I'd use this instead?

RxDB is offline first. You can still query your data even when the user has no internet. Hasura will not make your app workable without a stable connection to the server.

How useful is real-time data really? For example, HN isn't updating in real time and for me that's fine. If real-time updates come at a ridiculous complexity and performance cost, then is it really worth it?

See also Google Wave, which had "real time" as its main novelty, but ultimately failed.

You know that it depends on the situation.

for my project it's very useful for collaborative features. Multiple people need to be able to edit the same graph structures at the same time, which would be difficult to scale out via a notify-and-poll architecture.

I've found that collaborative editing can be very confusing, and prefer a commit/merge step over simultaneous editing. I think all collaborative editing software should at least offer this kind of interface.

agreed - there's a branching model in the roadmap :)

There is also (mine) https://github.com/amark/gun

It is run by Internet Archive, HackerNoon, DTube, Notabug, etc.

Handles about 8M monthly active users!

+ end-to-end encryption, graphQL & graph data, upcoming Svelte support, decentralized, etc.

MIT/Zlib/Apache2 licensed.

I read through your whole readme and I still am not able to answer the question: "what kind of project would I use GUN for?"

or, even, "what is gun replacing?"

I mean, it looks interesting and I would like to play around with it, but I don't understand enough to know what kind of stuff I can build with it.

I tried several times to understand what GUNdb does and how its different or which features it has. I have given up. As far as I can tell it is something between Blockchain, graph-database and sync.

I also tried to understand the codebase which is impossible with obfuscated code like this one: https://github.com/amark/gun/blob/master/lib/store.js#L16

Thunderbong's answer is spot on (it replaces Firebase).

Here is a 5min interactive coding tutorial that shows that using GUN is more powerful than reading about GUN. https://gun.eco/docs/Todo-Dapp

For instance, Archive integrated in 1 week.

Notabug first version was built in 1 week with it. (p2p Reddit)

Organization can go a far ways in a short time.

Gun is essentially similar to Firebase[0]

I've used it in some projects and found it useful. Thanks.

[0]: https://firebase.google.com/

its all just a for loop anyway...you can do the same if you put tiny wrapper on db-local server to avoid network and you'll get the same result.

IMO JavaScript is not compatible with "realtime".

“Real-time” in JS tends to refer to approaches like websockets.

So it's not really realtime... No ?

I noticed an instant downvote. It's not the first time i see this behavior on rxdb comments.

The definition of realtime is vague so saying that something is "not really realtime" just because it is not "realtime computing" is wrong.

In fairness it was a bit pretentious of whoever decided to call it real-time which was a word with a specific meaning and a very hard earned reputation. 'Live' might have been better.

"You see the updates in real time without having to refresh the page" seems like a perfectly reasonable statement.

I submit that being pedantic about such things is frequently its own form of pretension.

Depends on your definition of realtime. Realtime with RxDB is not like "Real-time Computing" but like realtime synchronisation how it is described by firebase https://firebase.google.com/docs/database

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