sbt 2.0: go big (Scala Days 2025 edition)

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Eugene Yokota (eed3si9n), Net
fl
ix
go big
sbt 2.0

An idea of the ultimate build tool
Vision sbt 2.0
1. Simplify build.sbt DSL
2. Use Sc
a
l
a
3 constructs
3. C
a
ching
4. s...

Simplicity
• Get the b
a
sics right
•
• Flexibility
a
nd minim
a
lism
•
1

• Get the b
a
sics right
• Compile, test, publish, IDE integr
a
tion
• Flexibility
a
nd minim
a
lism
• Settings, t
a
sks, ...

Complex things possible (hard af)
Simple things should be easy
/
/
set the Scala ve
r
sion
scalaVe
r
sion
:
=
"3.7.2"
lib
...

Uni
fi
ed build and plugin language via language extension
The DSL (domain-speci
f
ic language) strategy
empty build.sbt J...

Uni
fi
ed build and plugin language via language extension
The DSL (domain-speci
f
ic language) strategy
empty build.sbt J...

Fearless concurrency without for comprehension since 2013
tasks are direct-style effects
/
/
set the Scala ve
r
sion
scala...

Fearless concurrency without for comprehension
tasks provide structural concurrency
• Minim
a
lity
• aaa / Compile / compi...

sbt 1.x pitfall: root project is just another project
scalaVe
r
sion
:
=
"2.13.16"
lazy val sub1 = p
r
oject
.settings(nam...

sbt 1.x pitfall: No dynamic dispatch
ThisBuild / o
r
ganization
:
=
name.value + "aa"
lazy val sub1 = p
r
oject
.settings(...

sbt 2.x: common settings
o
r
ganization
:
=
name.value + "aa"
lazy val sub1 = p
r
oject
.settings(name
:
=
"sub1")
lazy va...

built-in projectMatrix
No more +compile and ++3.x
lazy val sub1 = p
r
ojectMat
r
ix
.settings(
name
:
=
"sub1"
)
.jvmPlatf...

sbt query
_ sbt query
In sbt 2.0, we will extend the sl
a
sh synt
a
x
a
bit to
a
llow
fi
ltering bec
a
use we might not w
...

Scala 3
Constructs
2

2
sbt 1.x (Sc
a
l
a
2.12) sbt 2.x (Sc
a
l
a
3.7.x)
M
a
cros
Often inline
Quoted m
a
cro
String Op
a
que types
AList, KList...

.value
macros often can be an inline
implicit def mac
r
oValueI[T](@dep
r
ecated("unused", "") in: Initialize[A1])
:
Mac
r...

conditional tasks
if expression is Selective
/
/
In ea
r
lie
r
sbt 1.x
publish
:
=
(Def.taskDyn {
val skp = (publish / ski...

Caching
• Fl
a
ttening the sc
a
l
a
bility curve
3

where n is the number of members
build time and test time likely grow quadratically*
• Every d
a
y everyone
a
dds some cod...

_
sbt 2.x (remote) caching
• sbt 1.x executes t
a
sks in p
a
r
a
llel
• In sbt 2.x, t
a
sks c
a
n be c
a
ched
• sbt 2.x c
...

Disk cache and/or remote cache
Bazel-compatible caching
• You c
a
n
a
dminister
a
n open source
remote c
a
che, or choose ...

sbt2.x:testresults
willbecachedonCI
• The secret to f
a
st CI is to
a
void executing the tests
• test becomes testQuick, w...

Memoization 101
sbt 2.x cached task
• Look for the
a
ggreg
a
ted h
a
sh in the c
a
che store
• Return persisted result if ...

_
sbt 2.x: tasks provide structural caching
• Minim
a
lity
• aaa / Compile / compile runs once ever, for the s
a
me
SHA-25...

All tasks are cached: AST forms Merkle tree
sbt 2.x: tasks provide structural caching
/
/
task 3 is cached by default
task...

Caching problems

At least as hard as the serialization problem
caching is serialization-hard
task3
:
=
{
val t1 = task1.value
val t2 = task...

caching is hermeticity-hard
• For e
ff
ective c
a
ching, we need
a
super
repe
a
t
a
ble build. In B
a
zel lingo,
a
build
t...

sbt 2.x provides multiple ways to opt out of caching
opting-out of caching
1. Per t
a
sk inst
a
nce
val someKey = taskKey[...

File is often ambiguous
caching is
f
ile-serialization-hard
• File c
a
n me
a
n sever
a
l
things
1. The
f
ile p
a
th
2. M
...

File is often ambiguous
caching is
f
ile-serialization-hard
• File c
a
n me
a
n sever
a
l
things
1. The
f
ile p
a
th
2. M
...

At least as hard as the IO problem
caching is IO-effect-hard
someTask
:
=
{
val conv = f
i
leConve
r
te
r
.value
val out: ...

What are some of the observations?
caching is worth it
•From 45 min to 3 min 3 sec "In re
a
l life, wh
a
t engineers exper...

sbtn + sbt server
4

"Because a build is just a giant, shared, mutable, state device." — Josh Suereth
sbt server
• Multiple clients (users, IDE...

Testing with Pekko (42 subprojects + too many local plugins)
sbt server: Performance
sbt 2.0.0-RC3
Cold lo
a
d
time sbt
-
...

_
client-side run
IDE
sbtn
sbt server
a
pp

Why sbt?
5

Lifting boats

Survey says...
Most widely-used build tool in Scala
• JetBr
a
ins survey 2023 s
a
ys used by 82% of the respondents regul
...

Common solutions for common problems
A hub station
Problems Solutions
Sc
a
l
a
ch
a
nging bincomp
a
t frequently Cross bui...

How can I get help / help?
Get involved
• Try it out: sbt.version=2.0.0-RC3
• Improve the docs: 'The Book of sbt'
• Help o...

An idea of the ultimate build tool
Vision sbt 2.0
1. Simplify build.sbt DSL
2. Use Sc
a
l
a
3 constructs
3. C
a
ching
4. s...

Questions?
Speci
a
l th
a
nks to the former
m
a
int
a
iners M
a
rk H
a
rr
a
h, Josh Suereth,
D
a
le Wijn
a
nd
Plugin p
a
r...

About the author
• I'm Eugene (eed3si9n), third-gener
a
tion m
a
int
a
iner for sbt in my free time
• At Net
f
lix, I work...

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