I was once a full-time Rails dev and really loved the framework (I don’t write as many user facing applications these days). Most of the Omakase trade offs didn’t bother me. But I never understood the disdain for foreign keys. For 99.99% of web apps, you want them (dare I say, need them).

Even in Rails 3 I would add them by hand in the migration files. Very few applications will ever actually care about sharding. We were pulling in millions at the last Rails company I worked for and were just fine with a master and a single replica.

If you get to a point where sharding is best for your company you hopefully have enough revenue coming in to fund a data transition. Your goal should be to outgrow what MySQL (or Postgres) can do for you in master/replica mode. If you do, you’ll likely be independently wealthy...

If data integrity matters at all, model-based checks (or periodic queries for orphaned data) will not suffice. Just put a foreign key in the table where it belongs and let your DB do what it does best. ACID is an amazing thing...

Maybe originally, but lack of foreign key usage is certainly not Rails specific today. Large MySQL shops generally don't use foreign keys, full stop, for the exact reasons Shlomi described in the original comment.

Facebook does not use foreign keys either. In my experience, same thing is true at all the other large MySQL-based companies. And those companies make up a majority of the largest sites on the net btw -- including most of the consumer-facing social networking and user-generated content products/apps out there.

This does not mean that, for example, Facebook is full of data that would violate constraints. There are other asynchronous processes that detect and handle such problems.

> If you get to a point where sharding is best for your company you hopefully have enough revenue coming in to fund a data transition.

Do you mean, stay with your current RDBMS of choice and shard while also removing the FKs? Or do you mean transition to some other system? (and if so, what?)

The former path is bad: sharding alone is very painful even without introducing a ton of new application-level constraint logic at the same time.

The latter path is also bad: only very recent NewSQL DBs have sharding built-in, and you probably don't want to bet your existing rocket ship startup on your ability to transition to one under pressure. Especially given much higher latency profiles of those DBs, meaning a very thorough caching system is also required, and now you have all the fun of figuring out multi-region read-after-write cache consistency while also transitioning to an entirely new DB :)

I typically espouse app logic to check state and use foreign keys to ensure enforcement (eg, race conditions that are very hard to ensure at the app level but are built into many RDBMS). Foreign key failures are just treated like any other failure mode.

But honestly, I haven’t been part of a company that have really hit those upper limits that require sharding. They exist, yes. But most companies will never need to worry about it. Which is my point.

Social networking and user-generated content products (including GitHub) need to be built with scale somewhat in mind: if the product reaches hockey-stick growth, a lot of scalability work will need to be completed very quickly or the viral opportunity is lost and the company will fail. I'm not saying they should pre-shard, but it does make sense to skip FKs with future sharding in mind.

This was nearly a decade ago, but in one case I helped shard my employer's DBs (and roll out the related application-level changes) with literally only a few hours to spare before the main unsharded DB's drives filled up. If we also had to deal with removing FKs at the same time, we definitely wouldn't have made it in time and the product literally would have gone read-only for days or weeks, probably killing the company. Granted, these situations are incredibly rare, but they really do happen!

Trying to make sure everything is where it needs to be at any given time, everything is inserted in the right order and the data is always consistent brings exponential amount of conplexity when it could all be checked at the end and pruned for invalid data. And usually DB integrity will not be enough, you’ll want business level validation that all is OK, so there will be app level checks anyway.

In particular splitting commits allows to ingest data in parralel (for instance if we import stores and store owners, both could be ingested separately without caring at first if each references a valid entity)

Application code has bugs. Application code can fail in ways that result in corruption. A primary job of the database is to keep itself from getting corrupted. Enforcing foreign key violations is only something the database can do correctly. Punting the responsibility to a higher layer will result in corruption.

That would be where the road splits. If you handle the database as a very fast and structured storage application, a lot of these assumptions go away, with then a different set of tradeoffs.

Some data corruption could be fine if you can guarantee the critical cases, just as bugs in the code are fine as long as the useful cases are covered.

I remember a database with scheduling entries in it that could get duplicated depending on the sharding, but it didn’t matter because the app handled the case gracefully.

I think understanding the tradeoffs that match the best the use case is the most important point, always assuming that a DB has to guarantee integrity can be a burden preventing from looking at all the options.

They have engineered for, and understand the implications of, foreign key violations. Typically, it's as simple as "This row can be deleted", and that can cascade - at a totally different rate than you'd find in a database and with totally different performance characteristics.

Reporting and bi data might get hosed.

Account management might get hosed.

Who knows what happens when FK rules are violated because by definition they should never be violated. It puts all applications on top into a undefined state, leading to bugs and god knows what else.

Foreign key violations are 100% data corruption.

You're missing an important point, it violates a certain set of rules of how the data relates. That certain set of rules typically makes it easier to write most general data applications.

If your rules define a foreign key as optional, you can easily engineer an application around it.

Great examples include NoSQL and Ruby duck-typing. In both cases, you infer actions based on the data structure, rather than making explicit assumptions.

Or you could, you know, simply add a foreign key constraint and never, ever, ever have corrupt table relations.

Why people fight their tools is beyond me. There is almost zero reason to defend not using foreign keys.

i suggest being open to the concept that other perfectly capable humans may, in fact, design systems with different underpinning assumptions than those you appear to presuppose always must apply to everyone and everything.

It's important to remove the DBA or Developer hat and realize that you are working together on a singular system (or maybe it's even the same person, etc.)

"corruption" implies that the desired results have not been achieved, that essential data has been lost or compromised, and this clearly is not the case with such a deliberate design decision. one should be prepared to accept this possibility in order to not be merely a fanatic and unreasonable.

Sure, yes, of course, who cares?

These are self-imposed rules, so breaking them is wholly up to yourself. And there are trade-offs involved that may often make it acceptable relax them.

You seem to be rather invested in one set of arbitrary definitions. If people do fine even in situations where they "by definition" shouldn't, it's the definitions that have been found lacking, not the people.

The stakeholder from whom you got the requirement on which the logical FK constraint is based, for one.

Nobody suggests taking an app / db with foreign keys and removing them that is a recipe for disaster.

In what way is letting the database ensure it isn't getting fed crap any different?

Why do developers constantly think it is okay to let unvalidated user input hit their database? Any client calling your database is a hostile client that will feed your database bad data. Arguing otherwise is complete ignorance.

Because one canonical validation layer is generally enough. You can see how having two separate partial validation layers could cause problems, right? And you can't put all the validation in the database, for non-trivial apps.

And "clients" shouldn't be talking to the database, no matter if you have foreign keys or not. That's a totally separate issue.

> Account management might get hosed.

LOL, if I read the data directly from the db instead of via the application's API then sure, I lose the application's guarantees. But, y'know, same might be said if I just go and read the DBs files from a sidecar shell script or something.

> It puts all applications on top into a undefined state

...that's just an assumption that's false.

Will result in more fault tolerant software, also yes.

It's a trade off and one I make willingly at every scale.

I stopped using foreign keys after university and have never wanted them since.

Non nullable database fields are far more useful than worrying about fks.

More fault tolerant because you have to waste time debugging the faults and monkeypatching them in code just to avoid FKs?

> Non nullable database fields are far more useful than worrying about fks

Can’t even count how many non-nullable fields I’ve seen packed with “” empty strings to get around that requirement

Further to that, instead of having lazy cascading deletes moving the goal posts on you. Data missing its parent for example is an indication something is wrong. It's a useful diagnostic.

2. Most string data is safe to represent as empty. Handling null on the other hand is a different situation and often induces warnings or other side effects depending on the language. Forcing the null checking to your boundaries is much like forcing your state/mutation to the boundaries in FP. Hell eveb though I think it's insane, hexagonal arch works on this idea as well.

One of the nice things you get from database FK constraints is when something messes up the data, you get an exception thrown in the code that's messing it up, complete with a backtrace and whatever inputs and timestamps you care to log.