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Check your performance with ScalaCheck generators

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How fast is your Scala application at responding queries? ScalaCheck generators can help you with that. Here I explain how.

The problem

You have a new feature to implement. The user wants some value and she wants it fast. We know that correctness, simplicity and performance stem all from the same trunk: design. If you want correctness, design so that errors cannot happen (from your code at least). If you want simplicity, have a simple design. If you want performance, design for locality, locality and locality! Slowness comes from latency. A database is slower than memory for accessing data because in that case data travel more layers of software and hardware. For the same reason a database is quicker than a HTTP request. Berkeley University has an interactive picture showing that.

Still, your application (maybe) really needs to store data. Or, worse, you may find yourself stuck with an old design that you cannot change for now. How do you make sure your feature is fast?

It is a problem indeed

Performance testing is an art. I remember the first time I paired with an engineer tuning a Gatling scenario. He was trying to mimic a swarm of users which would start accessing the application in waves. Once written the test, we ran it against the application instances for test, and: flop! The application broke after a couple of waves. Unsurprisingly, the issue was not the test.

Although I got a few performance tests under my belt, I remember the disappointment in that crash. The shifting left movement resonates with my feeling. I think we should catch slowness earlier in the development cycle. Ideally, the tools we use to write software should highlight slow code while we type (I am experimenting with that, but not quite there yet). For now though: what is a quick (and cool) way to check performance for a (Scala) developer?

And there is a solution

Slowness appears in the data. A function may seem alright for the few test cases we test it with. Then it hits production and hell breaks loose.

What has happened? The real worlds hosts the most fearsome data generators: humans. I am not joking. People make data and machines filter it. Any program is just a filter on many data sources. I learned that from Linux and the Unix Philosophy and my experience shows it true.

So I get that what we need is data generators. If possible, these should behave like people. Reproducing people behavior is difficult though. But we can approximate: randomness!

You may have heard of ScalaCheck already. Earlier I wrote about jsverify, its JS counterpart. ScalaCheck is a tool for property testing. That type of testing is fantastic and you should check it out. In this context though, we want only ScalaCheck's generators. Also these are more digestible for your team, if they do not know property testing yet.

Let me show you a generator quickly in Ammonite syntax:

import $ivy.`org.scalacheck::scalacheck:1.14.1`

import org.scalacheck.Gen

val stringGen: Gen[String] = Gen.alphaNumStr
val listOfStringGen: Gen[List[String]] = Gen.listOf(stringGen)

import $ivy.$

import org.scalacheck.Gen

stringGen: Gen[String] = org.scalacheck.Gen\[anon$5@552fffc8 listOfStringGen: Gen[List[String]] = org.scalacheck.Gen\]anon$1@368ff8be res3_4: Option[List[String]] = Some( List( "QIsEOjnHpyIwbsjfc3epMxupwpzjwgducjyYhmgjgd8ncbttthyq4ezLlimeswgaOpieinqavfou", "oplCvfyrvfwpffnppzw4drD2gwcbyYphva0i0bxrzhcrfgkcoawbvslfwaQktwnv4cfuoyo3", "gtOyf0bsbi0mdcgm5llb", "O0gO4qzud9hspnFrMuw3npslpvenh0lLeueAu5cnb5zbhnsc4ggmwsq0crcstgcp3c", "ggcaiffzsTEvrjxcwhavYifIfJx9ouumer7h8oupzxmjat3KAop", "pb4n9wf4rqwpifboazjittgurxvkgaNld6w8wlhdgsqwxF", "ycsj8ekmawmMsj3gauvh4njo7pd7bbxvp6", "g79afe0obfttnxrlf6r6pes2oWswiiixjnvvBu28a1dcbnqqfrqqsjjx4xIwvfhn", "2jvcuslGpPiglbthzawa39vgsprmrqy4lqUeoegYaR5ylQ5pfaauui3TecbqllnuuhboupuK", "qlgfxg8n3q7u8kacrcad0b30Hyb1akuhnhn5bTwon9wykdHqnIe0fZijdiYTqhnrm", "kjgxslQd6wbgfaopeau7oUfa0c", "gibzsbpRlyKsz2iaHeLbmkkddapensUy9DzqBocUdtxjvnomyf9y8qlknbcbthgHI0qohroweNyRcqtfjsDKylwgj", "bquqvW3rvigWi6aW3udmji1vzqenijzpSmqJerevnWd" ) )

In 5 lines of code we generated a list of random strings. That is cool! Now let me show you how to setup the data for a performance test. Say you have an object, how do you make a generator for that?

import $ivy.`org.scalacheck::scalacheck:1.14.1`

import org.scalacheck.{Gen, Arbitrary}

case class SomeObjectToTest(int: Int, maybeString: Option[String])

val someObjectToTestGen: Gen[SomeObjectToTest] = for {
  int <- Arbitrary.arbitrary[Int]
  maybeString <- Gen.option(Gen.alphaNumStr)
} yield SomeObjectToTest(int, maybeString)

Gen.listOfN(2, someObjectToTestGen).sample

import $ivy.$

import org.scalacheck.{Gen, Arbitrary}

defined class SomeObjectToTest someObjectToTestGen: Gen[SomeObjectToTest] = org.scalacheck.Gen$$anon$5@2321c420 res11_4: Option[List[SomeObjectToTest]] = Some( List(SomeObjectToTest(-2147483648, Some("4wPNvt")), SomeObjectToTest(-924389141, None)) )

As you can see, a for comprehension saves you here. We just compose generators. And note Gen.listOfN: you can get N pieces of random data, once you define your generator.

Now it is easy to test your function over a million of inputs!

For performance we want to check time. What I use to record time follows.

def time[R](block: => R): R = {
  val t0 = System.nanoTime()
  val result = block    // call-by-name
  val t1 = System.nanoTime()
  println(Console.GREEN + "Elapsed time: " + (t1 - t0) + "ns, ie, " + ((t1 - t0) / 1000000000) + "s" + Console.RESET)

If you connect to a database in your test, you can also try to store the data. Then you can time how long your database takes as storing/accessing data. If it feels slow, instrument your database and run again to check what queries are slow.

Generators save you a lot of time but make sure to use them on a good design! For example, the class above has Int and String attributes. These types can host an infinity of values. Particularly these values can be rather big! Generators take these at random, so you may find the Bible in one of those strings. Instead, your design should have types that allow only the values you want (for example limiting the length of the string and the range of the integers).


With this knowledge you can start testing performance. You will surely catch your biggest bottlenecks. Most of your time will go in setting up your generators. This is worthy because you can also use them for property testing (and setting up data in a test environment too!). So run your Ammonite, paste the code and start generating right now.

Happy performing!