Shared variables in Go

The fun continues

https://talks.obedmr.com/

Based on:

The Go Programming Language book

Presentation code and examples:

https://github.com/CodersSquad/hello-gophers

Race Conditions

A race condition is a situation in which the program does not give the correct result for some interleavings of the operations of multiple goroutines.

 var balance int

 func Deposit(amount int) {
     balance = balance + amount
 }

 func Balance() int {
     return balance
 }

Execution example

 // Alice
 go func() {
     bank.Deposit(200)                // A1
     fmt.Println("=", bank.Balance()) // A2
 }()

 // Bob
 go bank.Deposit(100)                 // B

 Alice first              Bob first        Alice/Bob/Alice
           0                      0                      0
   A1    200              B     100              A1    200
   A2 "= 200"             A1    300              B     300
   B     300              A2 "= 300"             A2 "= 300"

Goroutine Monitor

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

 package bank // Package bank provides a concurrency-safe bank with one account.

 var deposits = make(chan int) // send amount to deposit
 var balances = make(chan int) // receive balance
 func Deposit(amount int) { deposits <- amount }
 func Balance() int       { return <-balances }

 func teller() {
     var balance int // balance is confined to teller goroutine
     for {
         select {
         case amount := <-deposits:
             balance += amount
         case balances <- balance:
         }
     }
 }

 func init() {
     go teller() // start the monitor goroutine
 }

Mutual Exclusion

Remember the concurrent web crawler example? we may use a buffered channel as a counting semaphore to ensure that no more than 20 goroutines made simultaneous HTTP requests.

A semaphore that counts only to 1 is called a binary semaphore.

 var (
     sema    = make(chan struct{}, 1) // a binary semaphore guarding balance
     balance int
 )

 func Deposit(amount int) {
     sema <- struct{}{} // acquire token
     balance = balance + amount
     <-sema // release token
 }

 func Balance() int {
     sema <- struct{}{} // acquire token
     b := balance
     <-sema // release token
     return b
 }

Mutual Exclusion with Mutexes

 import "sync"

 var (
     mu      sync.Mutex // guards balance
     balance int
 )


 func Deposit(amount int) {
     mu.Lock()
     balance = balance + amount
     mu.Unlock()
 }

 func Balance() int {
     mu.Lock()
     defer mu.Unlock()
     return balance
 }
  • Why we used defer?

Read/Write Mutexes

Imagine the case of thousands of Balance requests. What will happen?

 var mu sync.RWMutex
 var balance int

 func Balance() int {
     mu.RLock() // readers lock
     defer mu.RUnlock()
     return balance
 }

The Balance function now calls the RLock and RUnlock methods to acquire and releases multiple readers or shared lock.

The Deposit function, which is unchanged, calls the mu.Lock and mu.Unlock methods to acquire and release a writer or exclusive lock.

Memory Synchronization

 var x, y int
 go func() {
     x = 1                   // A1
     fmt.Print("y:", y, " ") // A2
 }()
 go func() {
     y = 1                   // B1
     fmt.Print("x:", x, " ") // B2
 }()

Possible outputs:

 y:0 x:1    x:1 y:1    x:0 y:0 // This is a weird one
 x:0 y:1    y:1 x:1    y:0 x:0 // This is another weird one

That's why Synchronization patterns are required on this type of concurrent operations.

Let's code: Goroutines vs OS Threads

Follow instructions from:

https://github.com/CodersSquad/go-goroutines-vs-threads

Golang Race Detector

Take a look on:

https://blog.golang.org/race-detector

Thanks

  • Obed N Muñoz Reynoso
    • Cloud Software Engineer
    • obed.n.munoz@gmail | tec | intel.com