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Scalding at Etsy (mcfunley.com)
74 points by mcfunley on March 4, 2014 | hide | past | favorite | 19 comments

I was the one at Adtuitive who chose Cascading.JRuby. It was a pre-existing DSL on top of Cascading. It needed some work, but it was a pretty nice concise way to generate Cascading jobs. However, with half a dozen to a dozen people hacking features into it over time at Etsy, and with no real design coordination, things got pretty out of hand. There were a couple of fundamental problems:

1. Type system mismatch. Ruby is dynamic and not type checked; Java is static and type checked; Cascading is somewhere in between and unfortunately doesn't seem to use Java's type system as well as it could.

2. User-defined functions in strings. For some reason Cascading lets you write user-defined functions as strings and compiles them dynamically during job execution. This was the way to write user-defined code in Cascading.JRuby.

Cascading requires a compilation step, yet since you're writing Ruby code, you get get none of the benefits of static type checking. It was standard to discover a type issue only after kicking off a job on, oh, 10 EC2 machines, only to have it fail because of a type mismatch. And user code embedded in strings would regularly fail to compile – which you again wouldn't discover until after your job was running.

Each of these were bad individually, together, they were a fucking nightmare. The interaction between the code in strings and the type system was the worst of all possible worlds. No type checking, yet incredibly brittle, finicky and incomprehensible type errors at run time. I will never forget when one of my friends at Etsy was learning Cascading.JRuby and he couldn't get a type cast to work. I happened to know what would work: a triple cast. You had to cast the value to the type you wanted, not once, not twice, but THREE times.

Scalding fixes both problems since it is statically typed and lets you write real user-defined functions instead of stuffing them in strings. To me, the main moral is never, ever design an API that involves writing code in strings. It's just bound to be a disaster. Also, if you're going to have a compilation step anyway, you might as well get some static checking for obvious problems like type errors as part of the bargain.

One other unforced error with c.j was having almost every function take a dictionary of args instead of a sane argument list. That left "grep the codebase" and "read the entire function definition and god help you if it passes on the argument dict" as the two horrible options for figuring out how to even call most of the functions.

But we probably could have erected a blast shield around that mess if only we could have written functions and aggregators in ruby.

Yeah that right there was our biggest error I think. We should have had a "compilation pass" that used JRuby's ruby parser to do program transformation on the original source, extracting the inline operator code and doing "something" with it to generate fast JVM bytecode (probably using something like Mirah) and then transforming the call site into something that would use this. It would have been a few months of nightmarish debugging but would have also provided the leverage we needed to do data flow-level type checking/annotation as well, in addition to not having to write our UDFs in another language.

I think the thing to remember with the c.j stuff is at the time Cascading's Java API was pretty much state of the art, and we had to shore up a lot of what was in c.j in order to get to a viable DSL. And at that point, it was a huge leap forward from a readability standpoint. (Cascading's API had the same shortcomings as our c.j API wrt types etc, fwiw)

At that point you're just implementing a crappy statically type language inside of a Ruby DSL. Using a real statically typed JVM language that's more expressive than Java – i.e. Scala – would be much better. So, that's basically Scalding. Of course, Scalding wasn't even close to existing when we chose Cascading.JRuby, so yeah.

Yeah I'm just talking counterfactually. At the time there were no solid cascading DSLs whatsoever, cascalog wasn't even around for another 2 years or so iirc. Cascading itself was the new hotness vs writing raw map reduce, so c.j was kind of on the edge of what was out there. The decision to keep UDFs outside of Ruby was made at the outset (out of concern for performance, and being icing on the cake, etc.) and not really revisited.

Yeah, that's an unfortunate part of Ruby's design and culture though and not really specific to c.j. Since Ruby doesn't have real keyword arguments, you don't get any checking (even at run time) that the things you passed in aren't completely wrong (and thus silently ignored). We could have fixed that bit with some API redesign, so it's not nearly as dire a problem. The other two things couldn't really be fixed without changing languages entirely.

I'd like to get in a plug for my personal favourite Scala / Hadoop productivity framework, Scoobi.[0] If you're already familiar with Scala, Scoobi has the more familiar and intuitive API, and it leverages the type system to provide stronger guarantees before the code is even run. (For example, if Cascading / Scalding don't know how to serialize your data you'll get a runtime error; in Scoobi, it complains at compile-time. This is really useful when your compile / deploy / run / error loop may take several minutes...) I've also run into a few bugs in Scalding, while I've found Scoobi to be much more solid.

OTOH, Scalding has the edge in terms of community size and ecosystem. I haven't found this to be a big issue -- shimming an existing Hadoop input / output to work with either project is quite simple -- but YMMV.

[0] https://github.com/NICTA/scoobi

Yeah, for those same reasons I much prefer using Scalding's typed API [1], which feels very similar to Scoobi. The tuple API shown in these slides is great for places like Etsy that already have a large investment in Cascading, but otherwise you're better off getting the added type safety and similarity to the standard Scala API.

[1] https://github.com/twitter/scalding/wiki/Type-safe-api-refer...

I'm familiar with the typed API, but it still doesn't quite bring me as far as Scoobi does. I recognize your name from the Scalding code, so I'll say that this is meant as helpful criticism and not a complaint.*

- There a couple types for datasets in the Scalding API: TypedPipe, and KeyedList and subclasses. Scoobi subsumes both of these under DList; thanks to the usual Scala wizardry, this has all the methods to operate on key-value pairs without loss of typesafety. This isn't a huge deal, but it removes the tiny pains of constantly converting back and forth between the two. - Scoobi's other abstraction, DObject, represents a single value. These are usually created by aggregations or as a way to expose the distributed cache, and have all the operations you'd expect when joining them together or with full datasets. You can emulate this in Cascading / Scalding, but it's a bit less explicit and more error-prone. - There's no equivalent to the compile-time check for serialization in Scalding, AFAICT. - Scoobi has less opinions about the job runner itself... there are some helpers for setting up the job, but all features are available as a library. For some reason, I found the two harder to separate in Scalding? - IIRC, Scalding did job setup by mutating a Cascading object that was available implicitly in the Job. In Scoobi, you build up an immutable datastructure describing the computation and hand that to the compiler. This suits my sense of aesthetics better, I suppose...

* Also, thanks to you guys for Algebird! That's a really fantastic little project, and I use it all the time.

Quick point:

1) Scalding has a DObject like type: ValuePipe[+T]. 2) The reason you must explicitly call .group to go to a keyed type is that is costly to do a shuffle, this makes it clear to people when they do trigger a shuffle. If you don't like that, make an implicit def from TypedPipe[(K, V)] to Grouped[K, V] 3) You can easily use scalding as a library, but most examples use our default runner. We use it as a library in summingbird. But you are right, a nice doc to help people see what to do might help people (hint: set up an implicit FlowDef and Mode, do your scalding code, then call a method to run the FlowDef).

1) Ah, the ValuePipe is (relatively) new; thanks for the pointer. 2) You have to explicitly `.group` in Scoobi as well; it transforms a DList[(K,V)] to a DList[(K, Iterable[V])] or similar. You don't have to call `.toTypedPype` to get map and friends, though, since it's just a DList. 3) I've actually written this exact integration, so I'm glad it's the approved method! The global, mutable Mode made me nervous, IIRC.

The global Mode is gone in 0.9.0. And there is an implicit from Grouped to TypedPipe, so you don't need to call .toTypedPipe (that directly seems less likely to cause problems, especially given we have mapValues and filter on Grouped, so we should avoid needlessly leaving the Grouped representation).

Neat! It looks like things are changing fast; I'll have to do another read-through.

I'm really excited to see scoobi listed here as it's something I have really grown to love over the last couple of years. I find that it's very flexible and end up using it for jobs that do more unconventional mapred things.

Totally read this as scaling at Etsy. Still interesting though. And somewhat accurate.

It's nothing, I read this as "Soldering is easy" and then spent about 5 seconds trying to understand why the material is something different from what I expected. Probably should sleep more.

We are all in this together.

I thought Etsy was test-buying some of the kitchen items listed there and finding them painfully unfit for use.

(No, really. I'm not trying to be clever. That honestly was my first thought.)

Is anyone aware of psychological research into why some people are prone to this type of error?

This is a problem I have and often find bemusing but sometimes alarming. At least a few times a week I encounter text that I initially grossly misread by automatically inserting my own words instead of what was actually written.

I am inclined to think it may have something to do with doing a lot complicated programming. Basically after years of programming and having to deal with detail, you survive by being good at applying triage to problems, ie knowing what to ignore and what to pay attention to. Perhaps misreading text is your brain attempting to apply this type of triage.

I don't think it's likely to have anything to do with 'some people' or any kinds of tasks you perform. It's probably more to do with word shapes [1], which are probably at least part of how you read.

[1] http://en.wikipedia.org/wiki/Bouma

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