Rqlite 6.0: the evolution of a distributed database design | Hacker News Reader

Rqlite 6.0: the evolution of a distributed database design

philipotoole.com

180 pointsotoolep37 comments

Not to be mistaken for high-availability Dqlite[1], which is one of the options one can run the k3s kubernetes distribution on (instead of etcd), via the Kine etcd shim[2]. Ultimately though the K3s team replaced Dqlite with an embedded etcd to get high-availability[3].

[1] https://dqlite.io/

[2] https://github.com/k3s-io/kine

[3] https://rancher.com/docs/k3s/latest/en/installation/ha-embed...

For anyone else facing a website crash error message: I had to turn off “Reader Mode” on Safari iOS to be able to view the DQLite website

Very interesting, I wonder how it’d hold against Aphyr’s db claim invalidating machine.

Related Github ticket: https://github.com/rqlite/rqlite/issues/94 (labeled as 'help wanted')

its a very simple request log in front of SQLite so unless there is a problem with the (Paxos/raft) algorithm used for replicating the logs it should hold very well.

actually it's using "github.com/hashicorp/raft" and not its own version.

So since Hashicorp survived Aphyr test, it should be fine.

Not quite as simple as that. I could have still used the Hashicorp code wrongly.

And once I did: https://github.com/rqlite/rqlite/issues/5

aphyr himself chimed in.

Relevant doc: https://github.com/rqlite/rqlite/blob/master/DOC/CONSISTENCY...

May I ask how users generally use Rqlite? Distributed db for web apps? Embedded? Thanks

I'm not sure how folks use it, but I think the sweet spot is for simple-to-run relational storage for a smallish set of data.

Some people don't use it for the distribution, but just like a HTTP API in front of SQLite.

https://github.com/rqlite/rqlite/blob/master/DOC/FAQ.md#why-...

Thats close to our use case. We use it to sync a small amount of configuration data across a small amount of servers (2-50, depending client).

RQlite was perfect as our software is an addon for a legacy platform. We needed an easy low-access way of installing a distributed datastore.

Cool. Did you use read-only nodes, by any chance?

https://github.com/rqlite/rqlite/blob/master/DOC/READ_ONLY_N...

"I'm not sure how folks use it"

Suggestion: have you considered running office hours and inviting your users to chat to you about what they're doing with it?

I've been doing that for six months for my Datasette project and I've had over 60 conversations now, it's been a revelation - it almost completely solved the "I don't know how people are using this" problem for me, and gave me a ton of ideas for future directions for the project.

I wrote more about that here: https://simonwillison.net/2021/Feb/19/office-hours/

Interesting! An old colleague of mine, Ben Johnson, also does the same thing for litestream, his latest SQLite replication project. I thought it was just him.

Now that I know two folks do it, I'll have to give it serious thought. Thanks for the blog post ref

https://github.com/benbjohnson/litestream

I believe Ben got the idea from me - I was also his first ever office hours appointment once he started :)

Thanks for the quality work here. Have been considering using this over infinicache or hazelcast for locally replicated caching scenario. Are there any battle testing stories/case-studies out there?

I haven't, I'm not super familiar with those systems TBH.

Have you considered using olric[0]?

Just asking so that I can piggyback on your research :)

[0] https://github.com/buraksezer/olric

Olric did come up in my search but I never payed attention to it due to lack of production usage examples. I usually try to piggy back on existing well tested tools. The exception for rqlite because I understand both SQLite and Raft really well. So even if something goes wrong I will know what needs to be done.

isn't knowing every member of a cluster and the leader a core function of raft?

I feel like this post is leaving out critical pieces of information, like why the URL can't be deterministic or data about comparisons of different approaches.

At what cluster size and concurrency does asking every node break down?

I have been meaning to take a closer look at rqlite and want to understand more about it.

rqlite author here.

Yes, you're right every node knows the Raft network address of every other node. But Raft network addresses are not the network address used by clients to query the cluster. Instead every node also exposes a HTTP API for queries.

So code needs exist to share information -- in this case the HTTP API addresses -- between nodes that the Raft layer doesn't handle.

Also the HTTP API URL isn't deterministic because a) the operator sets it for any given node, and b) over the lifetime of the cluster the entire set of nodes could change as nodes fail, are replaced, etc.

>At what cluster size and concurrency does asking every node break down?

None, a follower only needs to ask the leader. So regardless of the size of the cluster, in 6.0 querying a follower only introduces a single hop to the leader before responding to the client. While this hop was not required in earlier versions, earlier versions had to maintain state -- and stateful systems are generally more prone to bugs.

I am curious about where things broke down with the 301 based solution y'all used earlier.

I included details in the blog post, the 3.x to 5.x design had the following issues:

- stateful system, with extra data stored in Raft. Always a chance for bugs with stateful systems.

- some corner cases whereby the state rqlite was storing got out of sync with the some other cluster configuration. Finding the root cause of these bugs could have been very time-consuming.

- certain failure cases happened during automatic cluster operations, meaning an operator mightn't notice and be able to deal with them. Now those failures cases -- while still very rare -- happen at query time. The operators know immediately sometime is up, and can deal with the problem there and then, usually by just re-issuing the query.

As I was reading I was thinking "Why doesn't the Follower proxy the request to the Leader?" which I see was covered later on in "Transparent request forwarding". Good stuff!

I remember building a janky version of sqlite+raft for Stripe's 2014 CTF. I'm sure others here have made a similar comment when rqlite gets posted to HN.

rqlite author here. Yes, it's coming in a future release and is much easier to do now.

One key principle of rqlite has always been quality, clean design, and simplicity of operation. So I've been reluctant to add a feature -- in this case Request Forwarding -- until I was sure it would be a clear win and not make rqlite less robust. After years of experience with the system now, I'm happy it can be added in a high-quality manner.

It definitely makes working with round robin proxies (e.g. k8s services) much, much simpler.

Having proxies be aware of the leader, or having clients being able to access nodes directly instead of behind said proxies, is a lot more complexity.

It does add quite a bit to network traffic though, which may or may not be an issue.

https://web.archive.org/web/20210610124826/https://www.phili...

Very interesting... Does it support full text search? I scanned a little bit but didn’t find any info either way.

Whatever SQLite exposes is available in rqlite.

Would it be possible to run this in the browser? Via WASM or something?

It wouldn't make too much sense to run a distributed raft-based SQLite system in its entirety in a browser (via WASM). However, you can run an individual SQLite instance in the browser (via WASM) using this: https://sql.js.org/#/

Why wouldn't it make sense in your view?

By the way, when I ask about "the browser" I'm also thinking of stuff like electron apps and progressive web apps.

Cause that's a single node, this is made for clusters. Maybe what you are looking for is a replication of another data? What use case would you have for multiple browsers syncing over a network?

Someone needs to officiate a marriage between this and duckdb.

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