Adding Optimistic Locking to an API

[valenterry]

> Looking at our request traffic made choosing optimistic locking fairly easy. We expect the majority of ledger operations to be reads, and we didn't want reads to block writes (and vice versa).

I don't get it. If they use an SQL database that supports ACID already, why not just lock all the ledger rows necessary with an exclusive row access when writing and otherwise just with a shared access so that the write waits until the read finishes?

[cormacrelf]

I think the missing context is that the db reads go all the way to an HTTP client, get modified on the client, and are sent back hoping nobody else has edited the row in the meantime. Pessimistic = check row is not locked on READ; Optimistic = check lock_version matches on WRITE.

For a table where single row ops are all you can do, this is basically enough to let API users read and update rows concurrently. SQL transactions don’t survive longer than a single batch of statements sent in one request, so inadequate.

Edit: Turns out ActiveRecord’s pessimistic locks don’t use a column at all, they use builtin row locking from MySQL/Postgres (ie ‘inadequate’). So you can’t use it for multi HTTP request transactions at all.

Final edit, if you read about how databases implement ACID in the first place, optimistic locking is one of the building blocks. You can use it to reproduce the various isolation levels of SQL transactions in HTTP APIs, manually, in SQL. (Also look at MVCC.) This does not sound fun. Distributed locking is kinda like doing a group project at university, in that you’re going to have to do it all yourself, but in another sense if one writer goes MIA you all fail.

[paulddraper]

They wanted to add locking to their API, not internally.

[hcarvalhoalves]

> Since the protocol doesn't specify how the ETag should be generated, we could have passed in our "lock_version" version numbers. But because it seemed strange to only honor the ETag headers for a single resource in our API, we decided against it.

Odd choice. There's a standard, but the developers still chose to re-implement w/ specific semantics. There's nothing on the standard saying you have to support ETags for all the resources.

[paulddraper]

Yep.

The HTTP standard is rich with a caching, idempotence, etc.

You really have to craft a set of requirements to not find what you need there.

[traceroute66]

Optimistic locking in a stateless environment is actually not difficult to implement. The hardest part is actually making sure #devops are aware of the concept !

Tom Kyte, he of Oracle fame had a particularly good discussion of the concept in one of his books (Effective Oracle by Design ... IIRC).

IIRC, the Oracle way is to enable rowdependencies for the table(s) in question and then use ora_rowscn.

But in reality, you can use almost anything that changes in a defined fashion (timestamps, version numbers etc.). Then all you need to do is test for it in your database stored procedures (or elsewhere in your middleware if you are not using sprocs).

[durbatuluk]

We had similar problem (banking service). Instead of "moving" directly to destination we include a safe account in the middle.

A > safe > B

Safe account is called this way because we don't risk the misuse of money in case of rollback.

[cbetti]

Generally in banking you either separate preauthorization (or reservation of funds) from authorization, or you separate transaction from settlement and you compute an acceptable rate of loss.

Why did you decide to go with an escrow model in your use case?

[mdellavo]

optimistic locking is a useful technique that seems unfamiliar to many - it's worth looking into

[karmakaze]

I thought I didn't know what it was when I heard of them, then realized that it's not a lock at all, it's just a row version column, with the basic rule don't overwrite without having seen what you're about to overwrite.

Same goes for `git push --force`, always use `git push --force-with-lease` instead.

[nesarkvechnep]

How could it be familiar to people as most of them hopped on the GraphQL hype train and treat HTTP as a dumb pipe.

[TuringNYC]

I understand each solution has trade-offs, but I've traditionally done this with an in-db proc (we used stored procedures extensively on Wall Street for txns). How does this compare against having the entire transaction functionality inside the DB?

[beachy]

Using stored procedures is old fashioned and won't earn you any points on your resume if you're being interviewed by 20 year olds. It's a solid solution though unless you are at FAANG volumes.

[mirekrusin]

Tradeoff is that it has very poor performance for frequently changed data, you can starve clients etc.

It's much better to create transfer api which atomically debits one and credits other account instead of low level individual ops.

[ram_rar]

if you're solely relying on "lock_version" numbers, you may wanna watch out for integer overflows (not sure how ruby handles it). I had faced a similar problem in the past, it can have massive ripple effects with downstream services.

Also, if you're using postgres. Its worth looking into advisory locks [1] for similar use cases. They are pretty light weight compared to other locking mechanisms.

[1] https://www.postgresql.org/docs/9.4/explicit-locking.html

[ianpurton]

Modern Treasury is a Ruby on Rails shop

I really wouldn't want to write a service that deals with money in an unsafe language. It's so easy to make a mistake in ruby there's no compiler to help you.

[quadrature]

It definitely a scary proposition. But it's worth mentioning that two of the world's biggest money moving platforms (Stripe and Shopify) are written in ruby.

[mflamespin]

There's lots of Java development that deals with money movement that is suspect. All languages and frameworks are just tools. It depends on how you use them. Compilers are no guarantee for correctness if how you're modeling types is improper.