A Recovering Java Developer Learns to Go

Matt Stine (@mstine) Cloud Foundry Platform Engineer at Pivotal

matt.stine@gmail.com http://www.mattstine.com

OFFICE HOURS Wednesday, 2:30 - 3:10 PM

Expo Hall (Table A)

I am not a Go expert.

I’m just learning to Go.

CF Architecture - January 2013

DEA Pool

Router

Cloud Controller

BOSH Director BOSH Agent

UAA/Login Servers Health Manager

Service Broker Node(s)

Messaging (NATS)

CLI Client

Ruby

Java/Spring

Go

CF Architecture - January 2014

Ruby

Java/Spring

Go

Loggregator

DEA Pool (Diego - Coming soon!)

Router

Cloud Controller

BOSH Director BOSH Agent

UAA/Login Servers Health Manager

Service Broker Node(s)

Messaging (NATS)

!

CLI Client

–Matt Stine

“Hey, wait! I just got done learning Ruby!”

• Mike Gehard: “Go Within Cloud Foundry”https://www.youtube.com/watch?v=d5aHr8VGU-8

• Onsi Fakhouri: “Diego: Re-envisioning the Elastic Runtime”https://www.youtube.com/watch?v=1OkmVTFhfLY

Go in Cloud Foundry

✓Hello World ✓Why Go? ✓Contrasts with Java:

- Features and Idioms - Packaging / Modularity - Types / OOP / Interfaces - Concurrency

Agenda

package main !import ( "fmt" ) !func main() { fmt.Println("Hello World") }

Hello WorldAll code goes in a package.

Give access to exported

stuff from other packages.

Function definition, main() is entrypoint.

Call an exported function!

Why Go?

Iron Triangle of Language Design

Effic

ient

Com

pilat

ion

Ease of Programming

Efficient ExecutionSystems Programming

• 2007-09-21: Go invented at Google by Robert Griesemer, Rob Pike, and Ken Thompson

• 2009-11-10: Go released as OSS

• 2012-03-28: Go 1.0 is Released

A History Lesson

Software Engineering in the LARGE

http://talks.golang.org/2012/splash.article

“Go is a programming language designed by Google to help solve Google's problems, and Google has big problems.”

Software Engineering in the LARGE

http://talks.golang.org/2012/splash.article

“Go is a programming language designed by Google to help solve Google's problems, and Google has big problems.”

✓ Safety and efficiency of astatically-typed, compiled language

✓ Productivity and feel of a dynamic,interpreted language

✓ Address modern compute environments:

- Multicore Processors

- Networked Systems

- Massive Computational Clusters

- Web Programming Model

Goals

https://www.flickr.com/photos/slackpics/4289782818

A Kit for

Systems Software

https://www.flickr.com/photos/pmiaki/6657150957

Differences from Java

https://www.flickr.com/photos/yukop/6778321940

• Features and Idioms

• Packaging / Modularity

• Types / OOP / Interfaces

• Concurrency

Contrasts with Java

Features and

Idioms

https://www.flickr.com/photos/yukop/6778321940

Multiple Return Valuesfunc vals() (int, int) { return 3, 7 } !func main() { a, b := vals() fmt.Println(a) fmt.Println(b) ! _, c := vals() fmt.Println(c) } GBE

Return a pair of values.

I

Ignore the first value returned.

Assign to multiple variables.

Closures

func intSeq() func() int { i := 0 return func() int { i += 1 return i } }

GBE

Closes over this state.

Closures

func main() { nextInt := intSeq() ! fmt.Println(nextInt()) fmt.Println(nextInt()) fmt.Println(nextInt()) ! newInts := intSeq() fmt.Println(newInts()) }

GBE

Captures its own value for i.

Increments own value of i.

Captures the value of i again!And increments it.

Where’s my java.util.List? Slicess := make([]string, 3) fmt.Println("emp:", s) !s[0] = "a" s[1] = "b" s[2] = "c" fmt.Println("set:", s) fmt.Println("get:", s[2]) !s = append(s, "d") s = append(s, "e", "f") fmt.Println("apd:", s)

GBE

Create an empty slice of strings (zero-valued).

Set value at index.

Get value at index.

Append function (not mutate in-place!).

Where’s my java.util.List? Slicesc := make([]string, len(s)) copy(c, s) fmt.Println("cpy:", c) !l := s[2:5] fmt.Println("sl1:", l) !l = s[:5] fmt.Println("sl2:", l) !t := []string{"g", "h", "i"} fmt.Println("dcl:", t)

GBE

Length function.Copy function.

Slicing function: index 2 (inclusive) to index 5 (exclusive).

Slicing function: index 0 (inclusive) to index 5 (exclusive).

Slice literals!

Where’s my java.util.Map? Mapsm := make(map[string]int) !m["k1"] = 7 m["k2"] = 13 !fmt.Println("map:", m) !v1 := m["k1"] fmt.Println("v1: ", v1) !fmt.Println("len:", len(m))

Create an empty map of string ! int.

Put values.

Get value.

Length function.

GBE

Where’s my java.util.Map? Mapsdelete(m, "k2") fmt.Println("map:", m) !_, prs := m["k2"] fmt.Println("prs:", prs) !n := map[string]int{"foo": 1, "bar": 2} fmt.Println("map:", n)

Delete function.

Optional second return indicating “presence.”

Map literals!GBE

Looping with Rangenums := []int{2, 3, 4} sum := 0 for _, num := range nums { sum += num } fmt.Println("sum:", sum) !for i, num := range nums { if num == 3 { fmt.Println("index:", i) } } !kvs := map[string]string{"a": "apple", "b": "banana"} for k, v := range kvs { fmt.Printf("%s -> %s\n", k, v) }

Discard first (index), sum second (value).

Keep both returns!

With maps, first = key, second = value.

GBE

We don’t need no stinkin’ exceptions…func f1(arg int) (int, error) { if arg == 42 { return -1, errors.New("can't work with 42") } return arg + 3, nil } !func main() { for _, i := range []int{7, 42} { if r, e := f1(i); e != nil { fmt.Println("f1 failed:", e) } else { fmt.Println("f1 worked:", r) } } }

Conventional: last return is error.

Makes an error with the provided message.

Return nil if there was no error.

Idiomatic inline error check.

GBE

(Semi)automatic Resource Managementfunc createFile(p string) *os.File { fmt.Println("creating") f, err := os.Create(p) if err != nil { panic(err) } return f }

func writeFile(f *os.File) { fmt.Println("writing") fmt.Fprintln(f, "data") !} !func closeFile(f *os.File) { fmt.Println("closing") f.Close() }

GBE

(Semi)automatic Resource Management

func main() { f := createFile("/tmp/defer.txt") defer closeFile(f) writeFile(f) }

Run after the function completes.

GBE

Packaging/Modularity

https://www.flickr.com/photos/yukop/6778321940

TL;DR

COMPLEX

SIMPLE

• Every class in a package

• Import classes explicitly

- import java.util.Map

• Import all classes in a package

- import java.util.*

• Statically import class static members:

- import static java.lang.Math.PI

- import static java.lang.Math.*

Java Packaging

• All types and functions belong to a package.

• Every source file must declare its package.

• Import packages to gain access to exported members.

Go Packaging

• public - any class in same package, or any importing class in a different package, can see

• default (“package private”) - any class in same package can see

• protected - any class in same package, or any subclass in a different package, can see

• private - no class other than this can see

• Scope indicated by prefixing name at declaration time.

Java Scoping

• exported - any code in an importing file can see

- exported names start with uppercase letter

- func Copy(src *[]byte, dest *[]byte)

• non-exported - only code in the same package can see

- non-exported names start with _ or lowercase letter

- func copy(src *[]byte, dest *[]byte)

- func _Copy(src *[]byte, dest *[]byte)

Go Scoping

• Conventional correspondence to directory paths (e.g. com.ms.foo should be at src/com/ms/foo) - tools expect this!

• Package paths do not have to be unique at compile or runtime (first dependency found/loaded wins!)

• Conventional correspondence to URL of author (e.g. my domain is www.mattstine.com, so my packages names start with com.mattstine) - but no actual relationship to source code location!

Java Naming

• Conventional correspondence to directory paths (e.g. github.com/go-martini/martini should be at src/github.com/go-martini/martini) - tools expect this!

• Package paths MUST BE UNIQUE across a $GOPATH.

• Package names do not have to be unique.

• Referring to imported names must be qualified by package name (e.g. sql.DB not just DB)…can locally alias (e.g. import dbstuff “database/sql”)

• Conventional correspondence to URL of code location (e.g. import http://github.com/joefitzgerald/cfenv as import “github.com/joefitzgerald/cfenv").

• Can “go get” remote packages - supports Git, SVN, Mercurial, Bazaar.

Go Naming

• Java admits:

- circular package dependencies

- dead imports

• Go rejects:

- circular package dependencies

- dead imports

Miscellany

Types/OOP/Interfaces

https://www.flickr.com/photos/yukop/6778321940

For realz this time…

structs FTW

type Point struct { X, Y float64 }

Define a type. Give it a name.

This type is a struct. (you can actually define others!)

Add stuff! (upcase exports apply here too!)

Methods are Functions!

func (p Point) Translate(xDist float64, yDist float64) Point { return Point{p.X + xDist, p.Y + yDist} }

Receiver argument!

Can define methods on pointers or values.

composition FTWtype Point struct { X, Y float64 } !const ( BLUE = iota RED = iota GREEN = iota ) !type ColorPoint struct { Point Point Color int }

Define an enumerated constant (closest to Java enum).

A ColorPoint has-a Point!

• I have Points.

• I have ColorPoints.

• ColorPoints are like Points, but they are not Points.

• But I want to compute the euclidean distance between them.

• What to do?

Problem

Interfaces Group Behaviors

type Positioner interface { Coordinates() Point } !type Distancer interface { DistanceTo(p Positioner) float64 }

It’s all about satisfaction…

Java = explicit !Go = implicit

Calculating Distancefunc distanceBetween(a Positioner, b Positioner) float64 { p := a.Coordinates() q := b.Coordinates() sqOfXDist := math.Pow(p.X-q.X, 2) sqOfYDist := math.Pow(p.Y-q.Y, 2) return math.Sqrt(sqOfXDist + sqOfYDist) }

Point Satisfies Distancer and Positioner

func (p Point) Coordinates() Point { return p } !func (p Point) DistanceTo(pos Positioner) float64 { return distanceBetween(p, pos) }

ColorPoint Satisfies Distancer and Positioner

func (cp ColorPoint) Coordinates() Point { return cp.Point } !func (cp ColorPoint) DistanceTo(pos Positioner) float64 { return distanceBetween(cp, pos) }

Behavior Not Taxonomy

Animal Satisfies Distancer and Positionerfunc (a Animal) Coordinates() point.Point { return point.Point{X: a.X, Y: a.Y} } !func (a Animal) DistanceTo(pos point.Positioner) float64 { thing := pos.Coordinates() sqOfXDist := math.Pow(a.X-thing.X, 2) sqOfYDist := math.Pow(a.Y-thing.Y, 2) return math.Sqrt(sqOfXDist + sqOfYDist) }

Go!p = point.Point{X: 1, Y: 2} q := point.ColorPoint{Point: point.Point{X: 1, Y: 4}, Color: point.BLUE} !fmt.Printf("Dist b/w p and q = %v\n", p.DistanceTo(q)) fmt.Printf("Dist b/w q and p = %v\n", q.DistanceTo(p)) !penguin := animal.Animal{Name: "penguin", X: 1, Y: 1} seal := animal.Animal{Name: "seal", X: 1, Y: 4} !fmt.Printf("Dist b/w penguin and seal = %v\n", penguin.DistanceTo(seal)) fmt.Printf("Dist b/w penguin and point = %v\n", penguin.DistanceTo(p))

Concurrency

https://www.flickr.com/photos/yukop/6778321940

• Parallelism = leveraging simultaneous execution of work to perform many things at once. Limited to number of processors/cores you have.

• Concurrency = composition of work to manage many things at once. No theoretical limit.

• Rob Pike: “Concurrency is Not Parallelism”http://www.youtube.com/watch?v=cN_DpYBzKso

Concurrency vs Parallelism

• Java - Threads - OS managed - Share address space with other threads in same process

• Go - Goroutines - user-space managed by language runtime - multiplexed onto pool of OS threads

Parallelism - How?

• Java - Shared memory - Locking

• Go - Can share memory (see http://golang.org/pkg/sync) - But there is a better way!

Synchronization?

– http://golang.org/doc/effective_go.html

“Do not communicate by sharing memory; instead, share memory by communicating.”

Goroutinesfunc f(from string) { for i := 0; i < 3; i++ { fmt.Println(from, ":", i) } } !func main() { f("direct") ! go f("goroutine") ! go func(msg string) { fmt.Println(msg) }("going") } GBE

Synchronous

Asynchronous

Asynchronous and Anonymous

Channelsfunc main() { messages := make(chan string) ! go func() { messages <- "ping" }() ! msg := <-messages fmt.Println(msg) }

GBE

Create a new channel.

Sending

Receiving

Channel Bufferingfunc main() { messages := make(chan string, 2) ! messages <- "buffered" messages <- "channel" ! fmt.Println(<-messages) fmt.Println(<-messages) }

GBE

Make a channel that will buffer two values.

Send twice

Receive twice

Channel Synchronizationfunc worker(done chan bool) { fmt.Print("working...") time.Sleep(time.Second) fmt.Println("done") done <- true } !func main() { done := make(chan bool, 1) go worker(done) <-done } GBE

Notify receive that I’m done.

Run worker on a goroutine, pass “done” channel.

Block until msg received!

Selectc1 := make(chan string) c2 := make(chan string) !go func() { time.Sleep(time.Second * 1) c1 <- "one" }() go func() { time.Sleep(time.Second * 2) c2 <- "two" }()

GBE

Create two channels.

Create two goroutines; each sends message to different channel.

Selectfor i := 0; i < 2; i++ { select { case msg1 := <-c1: fmt.Println("received", msg1) case msg2 := <-c2: fmt.Println("received", msg2) } }

GBE

Await both messages simultaneously!

Print each as it arrives!

Closing Channelsjobs := make(chan int, 5) done := make(chan bool) !go func() { for { j, more := <-jobs if more { fmt.Println("received job", j) } else { fmt.Println("received all jobs") done <- true return } } }()

GBE

Job channel for sending work.Done channel to indicate all work complete.

Receive jobs - more will be false if jobs is closed.

If no more jobs, say that I’m done!

Closing Channelsfor j := 1; j <= 3; j++ { jobs <- j fmt.Println("sent job", j) } close(jobs) fmt.Println("sent all jobs") !<-done

GBE

Send the jobs to the worker.

Close the jobs channel.

Block until the worker is finished.

Range Over Channels

func main() { queue := make(chan string, 2) queue <- "one" queue <- "two" close(queue) ! for elem := range queue { fmt.Println(elem) } }

GBE

Pull messages off channel for each iteration of the loop.

• Features and Idioms

• Packaging / Modularity

• Types / OOP / Interfaces

• Concurrency

Contrasts with Java

Thank You!!!Matt Stine (@mstine)

Cloud Foundry Platform Engineer at Pivotal matt.stine@gmail.com

http://www.mattstine.com

OFFICE HOURS Wednesday, 2:30 - 3:10 PM

Expo Hall (Table A)

Code samples marked “GBE” at https://gobyexample.com are by Mark McGranaghan and are Creative Commons Attribution 3.0 Unported licensed (http://creativecommons.org/licenses/by/3.0). !

The Go Gopher logo was created by Renee French and is Creative Commons Attribution 3.0 Unported licensed (http://creativecommons.org/licenses/by/3.0). !

The Java Duke logo is BSD licensed (http://opensource.org/licenses/bsd-license.php).