I've heard of even Unicorns throwing consistency out the window. Apparently Netflix has a bunch of "cleanup jobs" that comb the database for various inconsistencies that inevitably show up.
You can't have consistent microservices without distributed transactions. If a service gets called, and inside that call, it calls 3 others, you need to have a roll back mechanism that handles any of them failing in any order.
If you write to the first service and the second two fail, you need to write a second "undo" call to keep consistent.
Worse, this "undo state" needs to be kept transactionally consistent in case it's your service that dies after the first call.
In reality, nobody does this, so they're always one service crash away from the whole system corrupting the hell out of itself. Since the state is distributed, good luck making everything right again.
Microservices are insane. Nobody that knows database concepts well should go near them
Data consistency is one of those things that sounds like it matters but often doesn't. There's not much in Netflix's platform that screams data consistency is an ultra high priority. Any application that deploys active/active multi-region is by definition going to encounter scenarios where data loss is possible. There's just no way around CAP.
I'd venture a guess that most applications have all sorts of race conditions that could cause data corruption. The fact of the matter is that almost nobody notices or even cares.
A little birdie who worked at Grab once told me that they pretty much don't use transactions anywhere. So... maybe there are problem domains where you can get away without transactions, but I'm quite sure that a marketplace that arranges transportation for fee is not one of them. The next time you're standing in a monsoon waiting for the "coming" car that never comes, remember this post :-)
Aside from that, I've found that even in non-mission-critical scenarios ("it's just porn!") it's incredibly convenient to have a limited number of states the system can be in. It makes debugging easier and reduces the number of edge cases ("why is this null??") you have to handle.
> maybe there are problem domains where you can get away without transactions, but I'm quite sure that a marketplace that arranges transportation for fee is not one of them.
I think you'd be surprised/alarmed at how little transactions actually get used in the software world. Not just on small systems where it doesn't matter but I've seen a complete absence of them in big financial ones handling billions of dollars worth transactions (the real world kind) a day. Some senior, highly paid people even defend this practice for performance reasons because they don't realize the performance cost of implicit transactions. And this is just the in process stuff where transactions are totally feasible, it get's even worse when you look at how much is moved around via csv files to FTP and excel sheets attached to emails. I've spent the last 2 weeks being paid to fix data consistency issues that should never have been issues in the first place.
Maybe when we're teaching database theory we shouldn't start at select/join but at begin transaction/commit/rollback?
Having spent a good amount of time fixing bugs related to code that fetches data with a consistency level of READ_UNCOMMITTED for "performance" reasons I can appreciate what transactions give you as an application developer.
However, I would argue that transactions are overkill. What's the worst case scenario if I book a ride for Grab and my request gets corrupted? I'm guessing I'll see an error message and I'll have to re-request my ride.
Re: I've heard of even Unicorns throwing consistency out the window. Apparently Netflix has a bunch of "cleanup jobs" [to fix "bad" transactions]
If your business model is to be cheap with high volume sales, then corrupting say 1 out of 10,000 customer transactions may be worth it. If you give customers a good price and/or they have no viable alternative, you can live with such hiccups, and the shortcuts/sacrifices may even make the total system cheaper. You are like a veterinarian instead of a doctor: you can take shortcuts and bork up an occasional spleen without getting your pants sued off. But most domains are NOT like that.
You can't have consistent microservices without distributed transactions. If a service gets called, and inside that call, it calls 3 others, you need to have a roll back mechanism that handles any of them failing in any order.
If you write to the first service and the second two fail, you need to write a second "undo" call to keep consistent.
Worse, this "undo state" needs to be kept transactionally consistent in case it's your service that dies after the first call.
In reality, nobody does this, so they're always one service crash away from the whole system corrupting the hell out of itself. Since the state is distributed, good luck making everything right again.
Microservices are insane. Nobody that knows database concepts well should go near them