How we built a serverless SQL database

We have opted for YugabyteDB table spaces to solve the isolation problem. We allocate a number of dedicated tablet servers to a tenant so that their db objects are close to each other. YugabyteDB executes queries only on those tablet servers where the data relevant to the query is. A resource heavy tenant doesn’t have an impact on other tenants. It’s not really “serverless” but does seem to solve the problem at a cost.

Tenants receive complete databases with grant permissions so they can do as they please.

Because YugabyteDB uses Postgres 11 under the hood, we have an additional sanity layer in form of a Postgres process utility hook which prevents the tenant from escaping the sandbox.

Google says that Cloud Spanner should get you around 7,000 read queries per second and 1,800 write queries per second for $650/mo. If a simple indexed read is 1 RU, $650 on Cockroach Serverless would get you around 2,500 reads/second. Of course, I think it's completely reasonable for a Serverless option to cost a bit more given that you'd need to over-provision Cloud Spanner (even if you smartly increased/decreased the amount of compute allocated based on demand).

Planet Scale charges $15 per 100M rows read and $15 per 10M rows written. If an RU is a row read, then Cockroach Serverless would be $10 per 100M rows read. If a write takes 10 RUs, Cockroach would cost $10 per 10M rows written. Both of those would be less than Planet Scale's cost - but it's possible that a row read will cost more than 1 RU. Let's say that an indexed lookup of a row costs 5 RUs. Then Cockroach Serverless starts costing 3.3x more than Planet Scale.

AWS DynamoDB charges $1.25 per million write request units and $0.25 per million read request units. If I can get 10M reads from Cockroach Serverless for $1 and 4M reads from DynamoDB for $1, Cockroach's pricing looks pretty good. Of course, if I need 5 RUs to do an indexed read, the pricing doesn't look as good anymore.

I do respect Cockroach Labs somewhat ambiguous description here. Planet Scale's $15 per 10M rows written feels like something that could become bad. What if I define hundreds of indexes on the table? What if I'm inserting very large blob/text columns that are 50MB in size? Likewise, what if I index no columns and end up forcing a full table scan, but only 10 rows are returned? Do they consider that I "read" 10 rows or "read" all the rows in the table? If it's the former, I'm putting a lot of strain on their system without paying for it. If it's the latter, I'm going to just define indexes that might not be worth it if I were paying for the IO needed to do all that writing.

Still, it would be nice if Cockroach Labs offered some indication of what could be accomplished with 1 RU. "An indexed read of 1 row or an indexed read of a few rows in sequence; for example, 'SELECT * FROM people WHERE age > 18 ORDER BY age, name LIMIT 10' where there exists an index on (age, name)." That would let me know what to expect. "A write of a row under 4KB in size with no secondary indexes will cost 3 RUs; expect secondary indexes to increase the cost by 1 RU each" would give me an idea of what's going on.

I think there are definitely cases that one can't easily enumerate. For example, "UPDATE people SET age = 18 WHERE EXISTS (SELECT * FROM legacy_info WHERE people.id = legacy_info.people_id AND legacy_info.is_adult = true)". That's potentially going to require lots of stuff that's harder to predict. However, at this point I don't know if an indexed read of a row costs 1 RU or 10 RU. If an indexed read of a single row costs 1 RU, if I read 10 rows sequentially will that mean 10 RUs or will 1 RU have enough IO to cover that since they're sequential (or will the billing just over-charge since there's tangibly more rows and that's easy to explain)?

I think a decent amount of the value depends on the pricing and it's hard to judge that right now.

One thing I will note is that the storage seems expensive. It's slightly cheaper than Planet Scale, but a lot more than the $0.30/GB of Cloud Spanner or the $0.23/GB of FaunaDB or $0.25/GB of DynamoDB. I've been wondering a bit about the storage pricing of Planet Scale since $1.25/GB seems expensive. Cockroach Serverless is coming in at $1/GB which also seems expensive compared compared to alternatives. If Cloud Spanner is basically offering a third the price, is the "Serverless" flexibility worth it given that Spanner can be scaled up/down pretty easily in very granular increments.

Actually, one thing that could be useful might be noting how many request units per month one of the dedicated instances would have. A 2 vCPU CockroachDB instance costs $350-400. Would that be 4 billion request units per month (assuming you were fully utilizing the box)? Would it be more like 15 billion request units per month since you're presumably paying a premium for the Serverless flexibility?

Now, if you setup your DB using public cloud's flavor of PostgreSQL... That's a different story.

In your k8s example, by running a k8s cluster when you don't need one, a cost you pay is the overhead.

In the Cockroach serverless case, costs that come to mind include vendor lock-in when you evolve a pattern of production traffic that is hard to migrate to other solutions, and security and compliance challenges due to the virtualized instances running on shared clusters. In many cases these tradeoffs may be worthwhile. My point is that looking at it only in the dimension of scaling up and down, doesn't tell the whole story. The OP doesn't talk about tradeoffs, so in the comment section we must.