I worked for Etsy for 6.5 years, mostly from Brooklyn. In an office considerably less sparse than this one, I assure you. Mea culpa, that’s “worked” in the past tense. I quit to join a startup last month. After signing up to give this talk. But I left on very good terms so I’m still doing it.
When I was writing this talk this passage from Douglas Adams kept popping into my brain. I do feel like we had scalding thrust upon us at Etsy, rather than choosing it intentionally. Which is not the same as saying that I was personally unhappy with it, exactly. I was not. This is the character that went on to try to insult every being in the cosmos in alphabetical order. So I’m not sure if it was intended as intentional allegory about the scala community.
This is hopefully the only strata-esque slide in the talk. Don’t run for the exits or anything. What I want to communicate with it is that in abstract, we aggregate logs from the live site, put them on hdfs. Then from there we crunch them to build internal tooling and features. For live features we’re putting job outputs into mysql shards; for backend tools we typically use a BI database (vertica) to fill the same need.
Scalding gets used at all points on the hadoop side. Parsing logs, generating recommendations and ranking datasets, and business intelligence is all either done in Scalding or will be ported to Scalding very shortly.
That’s a pretty common question. And so somebody’s thought of it way before you, and they’ve put it on a dashboard. So you can just go look at the dashboard.
We do a lot of A/B testing at Etsy, so much so that we’ve built our own A/B analyzer fronted called Catapult. So for most questions relating to variants in A/B tests you can go to that.
Then there are slightly more complicated questions. Like, how many of the top sellers sell vintage goods? Maybe you’re the first person to ever ask such a question.
And then there are questions that are even farther out there. Cases where you’re probably the first person to ask not just this specifically, but you’re also probably the first person to ask any question even similar to it. Like this one. Etsy gets traffic to items that are sold. How often could we redirect that traffic to items that have close tags and titles?
That’s the kind of thing you’d use scalding to answer today. We have the data in theory, but we haven’t normalized it and put it in BI. Or maybe it’s too big to fit in BI.
That’s questions where the data needed to get an answer is in a relatively raw form, which I’ll wave my hands and call analysis. And then we also build features and systems with scalding, which is more like what I’d call “engineering.” We do work for ranking, for recommendations, and so on in scalding.
It’s pretty big, I guess. When I quit we had about 800 scalding jobs in source control. And if everyone is like me, there are probably twice as many in working directories, not committed. Only about 90 of those, though, run as part of our nightly batch process.
This histogram showing how many jobs people have written is about what you’d expect. There’s a small group of people like me who have written a ton of jobs. And most people have written one or two jobs.
And the way it breaks down across the domains is like this. Most of the people using scalding are using to answer analytics questions. The experts tend to be the people building systems with scalding.
The thing we wanted to get out of that acquisition this feature. Paid promoted listings that you see when you search on Etsy. In the beginning we pretty much just wanted to build whatever we needed to have this.
That decision to make a table for visits, with a row per user session, turned out to be important. Our data is stored as serialized sequences of events inside cascading tuples.
By 2013 we definitely had our own giant analytics stack. It was built, racked, and debugged. And It was right about then that scalding blew the whole thing to smithereens.
The thing that caused this was that we had hired Avi Bryant, who some of you may know as one of the authors of scalding. And something of a group theory crank. And just an all-around amazing smart guy.
And what he did with that cover was that he added scalding to the build. And then he started trying to make things with it. Etsy’s not bureaucratic in any way I understand the word. But in theory there’s supposed to be at least some discussion before you start using a new framework. That didn’t happen at all with Scalding.
And immediately after this, he up and quit. So the force of his intellect and personality doesn’t explain scalding’s runaway success. If that’s all it was about everyone would have stopped using it the minute he left. But the opposite of that happened.
I think it’s simple enough that I can show it to you in a couple of examples. Let’s say that we want to count how many visits searched for any given search term.
The cascading.jruby job is really simple and straightforward. It looks like this. Don’t worry about understanding it or anything, the point is that it’s short and easy.
But you can start to see the difference if you add just one more layer of complexity. Let’s say that we wanted to count up the search terms again, but this time relate them to purchases that happen after them in visits.
And scalding manages to get this done in one mapreduce step again. It’s just unrolling the searches out of the visits like it was before, and grouping with a sum.
I can show you what it does schematically. You make two branches, one for the searches and one for the purchases. Then you cross join them and filter that shit down. And then you wind up with a branch for conversions per search term and a branch for visits per term, and you join those back together to get your answer.
So the pure cascading.jruby solution is more complicated. And it also turns out to be a lot slower, too. Cascading doesn’t have a query optimizer, and this might be a lot closer if it did. But it doesn’t, so jruby winds up being done in many more mapreduce steps and takes like eight times longer.
This here is the feature that killed cascading jruby. We just wrote a cascading user-defined function without even having to realize that that’s what we were doing.
But in reality, even though it comes up constantly, nobody wants to do that. You have to change files, and you have to change programming languages. Those hurdles are enough to make people write slower jobs.
For example we had one job that was a major resource problem in JRuby, which was taking seven hours to run every night. Someone rewrote it in scalding in a day or two and got it down to 20 minutes. The problem wasn’t that anything was impossible in cascading.jruby. The point is merely that scalding makes doing it the right way feel natural.
This is a talk about scalding. So I’m going to spare you my list of cascading gripes. You probably have your own. I will say that if you do, using a DSL on top of cascading doesn’t help with any of them.
Very flippantly, this is basically the problem. Scala is too far from what most of our engineers are using on a daily basis. It’s too weird. I assure you Kellan’s not this crotchety in reality. And he’s probably mad at me for paraphrasing this from memory.
I firmly believe that analytics is for everyone. I don’t mean statistical modeling, or machine learning, or things like that. But I do think that asking straightforward questions about the thing you’re tasked with building should be for everyone.
Etsy’s a relatively enlightened place, by software industry standards anyway. So everyone gets some time at the beginning and the end of that project to do quote-unquote “analysis.” It's "thinking time." And the stuff called "work" gets done in the middle.
But I think this more accurately describes reality. We’re all still carrying the baggage of 20th century software around with us. So analysis up front, which you’d do to see if you can make a case for doing the feature at all, feels like you’re not working. And the stuff in the middle feels like you’re really making progress. Even if it’s progress on something that could never actually work.
That’s how it is everywhere, more or less. This is the social framework everybody’s working inside of. So as somebody who really believes that analytics up front is powerful, I want to give everyone the best chance possible.
And scala is just too different from what other Etsy programmers are using day to day. Don’t mistake this as me saying they’re not smart enough, because they are. And it's not that learning FP wouldn't be good for everyone, because I think it is. And it's not that functional programming is fundamentally too hard, or anything like that. It’s just a statement of fact. Most programmers I know are not experienced with functional programming, and scala shares many functional idioms.
So the analysis process winds up looking like this. Between asking the question and getting an answer there’s this weird period in the middle where you have to learn a bunch of category theory. Sure it’s good for them, or something. But it’s also going to stop them from getting their answer.
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