1.
5 Tips to build
long-lasting
Scala OSS
Kazuhiro Sera
@seratch
Scale By The Bay 2018

2.
@seratch
• My name is Kaz (Kazuhiro Sera)
• Happy with Scala for 8 years
• Came from Tokyo, Japan !
• Come and join ScalaMatsuri in June next year! 🙌
• My 2nd talk at Scal(ae) By The Bay 🙏
• Love open source software projects 😍
• Started ScalikeJDBC, Skinny Framework, AWScala, etc
• Took over the maintenance of json4s, Scalate
🍶 🍣 🍱

3.
What is ScalikeJDBC?
• My most long-lasting OSS (7 year history)
• A relational database client library in Scala
• Provide a “Scala-like” (Scala idiomatic) way
• Practical: easy-to-use APIs + less dependencies
• Brief history:
• Started as a simple lib similar to Twitter’s querulous
• Has more functionalities now
• Scala 2.10: String Interpolation, Macros 🎉
• SQL objects, one-to-x extraction, type-safe DSL

4.
Basic usage of ScalikeJDBC
• Only 3 steps to issue a database query:
• 1) Build a SQL object
• 2) Define a function to extract values from ResultSet
• 3) Perform SQL execution with an implicitly passed
database session (which can be transaction-wired)
1)> val q: SQL[Nothing, NoExtractor] = sql"select id, name from members limit 2"
2)> val toTuple: (WrappedResultSet) => (Int, Option[String]) =
| (rs) => (rs.get[Int]("id"), rs.get[Option[String]]("name"))
3)> val rows: Seq[(Int, Option[String])] = q.map(toTuple).list.apply()
[SQL Execution] select id, name from members limit 2; (0 ms)
rows: Seq[(Int, Option[String])] = List((1,Some(Alice)), (2,Some(Bob)))

5.
Other libraries?
• According to the number of stars,
• 1) Slick 2,124
• 2) Quill 1,311
• 3) Doobie 1,283
• 4) ScalikeJDBC 948
• ScalikeJDBC is ranked fourth (Not bad!)
• Try in-memory database example on scalikejdbc.org
• Any feedback? Reach out to me today 🤙

6.
ScalikeJDBC turned 7 yrs old!

7.
Happened during the time
• Scala binary versions: 5
• 2.9.1 ~ 2.9.2 ~ 2.10 ~ 2.11 ~ 2.12 ~ (2.13.0-M5 ~)
• (Prior to 2.10, every single patch version was bin-incompatible)
• JDK major versions: 5
• 7 ~ 8(LTS) ~ 9 ~ 10 ~ 11(LTS) ~
• JDBC API spec minor updates: 3
• 4.1 (JDK7) ~ 4.2 (JDK8) ~ 4.3 (JDK9)
• Added 4.2 APIs when dropping JDK7 support
• sbt migration: 0.13 to 1
• For the sbt plugin generating source code from database

8.
5 tips to build long-lasting OSS
• 1) Find your lifework project
• 2) Be careful to add Scala dependencies
• 3) Stick with binary compatibility
• 4) Provide cross builds
• 5) Have effective CI builds

9.
1) Find your lifework project
• Not specific to Scala OSS
• It’s all about the sustainability of voluntary works
• Two things to know 📝
• Focus on lifework projects
• Many projects terminated due to author’s social events
• e.g. Changing job, having a family, school graduation
• Can continue it regardless of those big changes?
• Difficulty to find successors
• Inconvenient truth; even if you have many users…
• Ideally, continue working on your project somehow

10.
2) Be careful to add Scala deps
• It’s unfortunate that I have to say this but it’s critical
• Two things to know 📝
• "You’re in the same boat with many others”
• 🙅 Major upgrade: have to wait for all releases; it causes
significant delay of your release
• 🙅 Cross builds: can’t do if just one of them doesn’t
• Suggestion: Choose Java over Scala inside
• Not cool… but I have to say it’s a realistic trade-off
• 🙆 Pros: Can make upgrade/cross builds much easier
• 🙅 Cons: Handle null values inside

11.
2’) Work with null in Scala
<<< Working with java.util.List >>>
scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._
scala> val j = java.util.Arrays.asList("A", "B", "C")
j: java.util.List[String] = [A, B, C]
scala> val s = j.asScala
s: scala.collection.mutable.Buffer[String] = Buffer(A, B, C)
scala> null.asInstanceOf[java.util.List[String]].asScala // use Option!
res0: scala.collection.mutable.Buffer[String] = null
<<< Working with primitives >>>
scala> val j: java.lang.Integer = null
j: Integer = null
scala> val s: Int = j // auto boxing
s: Int = 0
scala> val s = Option(j).map(Int.unbox)
s: Option[Int] = None
java.util.List can be null.
null.asScala returns null.
A null value can be unexpectedly
converted to the type’s default value.
(0 : scala.Int’s default value)

12.
3) Stick with bin-compatibility
• Bin-compatible?
• Normal situation; not see any runtime linkage errors
• If you break bin-compatibility:
• Painful runtime errors (NoSuchMethodError etc) 😿
• Scala compiler cannot detect runtime issues beforehand
• Two things to do 📝
• Define policies about bin-compatibility guarantee
• Once you released the first stable version
• Surely guarantee binary-compatibility
• Solution: Use MiMa

13.
3’) Use MiMa to ensure that
• Use MiMa (Migration Manager for Scala)
• Can detect syntactic incompatibilities
• Developed and maintained by Lightbend devs
// project/plugins.sbt
addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.3.0")
// build.sbt
project.settings(
MimaPlugin.mimaDefaultSettings ++ Seq(
mimaPreviousArtifacts := {
Set("0.1.0", "0.1.1").map { v =>
organization.value % s"${name.value}_${scalaBinaryVersion.value}" % v
}
},
test in Test := {
mimaReportBinaryIssues.value
(test in Test).value
}
)
)

14.
4) Provide cross builds
• Cross builds = publish against multiple Scala bin versions
• Reality: We cannot always use the latest Scala
• Apache Spark took 2 years to support Scala 2.12
• Remember the tip 2) “be careful to add Scala deps”
• Two things to do 📝
• Support latest stable major & its previous major
• To be specific, Scala 2.12 and 2.11 at this moment
• Don’t have many files under src/main/scala-X.Y
• Testing a plenty of code there is hard+time-consuming

15.
5) Have effective CI builds
• Why do we need it?
• To reduce review cost of pull requests
• To merge pull requests with confidence
• You need reliable evidences (regressions, bin-combat, code style, etc)
• Two things to do 📝
• Enable it anyway!
• You can use TravisCI/CircleCI for free
• Automate everything apart from code reviews
• Check bin-compatibility, consistency of code style, cross
builds, multiple JDKs, etc
• Configure matrix builds to cover all patterns

16.
5’) TravisCI Build Example
language: scala
sudo: false
scala:
- 2.11.12
- 2.12.7
- 2.13.0-M5
jdk:
- oraclejdk8
- openjdk11
matrix:
include:
- scala: 2.12.7
jdk: oraclejdk9
script:
sbt ++$TRAVIS_SCALA_VERSION scalafmtCheck test # code formatter check + test
• Scala 2.11 + JDK 8, 11 = 2 builds
• Scala 2.12 + JDK 8, 9, 11 = 3 builds
• Scala 2.13.0-M5 + JDK 8, 11 = 2 builds
Additional 1 matrix build
3 Scala versions * 2 JDKs
= 6 patterns
7 patterns in total

17.
Start with the sbt template!
• sbt project template (aka g8 template)
• Including all techniques in this talk
• Should be a good starting point for you 😎
mkdir my-awesome-library
cd my-awesome-library
sbt new seratch/long-lasting-scala.g8
sbt test

18.
Recap
• You found out about ScalikeJDBC
• Contact me at any time during the conf 🤙
• If you like it, increment the stars (1K soon!)
• 5 tips to build long-lasting Scala OSS
• 1) Find what interests you for years
• 2) Be careful about adding Scala dependencies
• 3) Stick with binary compatibility
• 4) Provide cross builds
• 5) Have effective CI builds