README.md

goroutines-demystified

A simple and easy explanation of Goroutines (Concurrency&Parallelism) with easy examples

1. Concurrency is not parallelism

Parallelism:

• Doing Multiple things at once
• Number of tasks you are doing in parallel is directly proportional to the number of cores in your system
• Order of execution is not predictable and we should not rely on them to execute in any specific order

Eg:

• Dreaming and sleeping at the same time
• Drinking and Dancing at the same time

2. Concurrency:

• Concurrency is nothing but designing of your code in to an independently executable function and should not have any side effects even if you run 100,000 times
• Running independently executable function separately and can communicate in between to synchronize their tasks

Eg: Multitasking on single core machine

Goroutines

• Goroutines referred as light weight thread of execution and can run in thousands and its not heavy as traditional threads (Referring to Java here)

• Goroutines takes few kb in stack size and the stack can grow or decrease according to the needs of the application. Where as a thread has to initialized with fixed stack size

• Goroutines are concurrent in nature and uses channels to communicate in between to synchronize instead of creating deadlocks or race conditions

• Any Concurrent programming requires an independently executable function and making that function to a goroutine is very simple

• Goroutine uses Channels to communicate with other goroutines

• Goroutines uses CSP (Communicating Sequential Processes) to talk to other goroutines by passing data between channels instead of sharing data

go func() {
}() //Anonymous goroutine

go hello() // Add a go keyword before a function creates a concurrently running goroutine

Let's write a easy goroutines to understand more about it

func main() {
	go helloWorld("vasu")
	// fmt.Println(runtime.NumGoroutine()) this gives information of how many goroutines exists at that point
	time.Sleep(time.Second) // why do you need this
}

func helloWorld(name string) {
	for i := 0; i < 3; i++ {
		fmt.Println("HelloWorld: " + name)
	}
}

why do you need time.Sleep(time.Second) ?

Because Main is also a goroutine here and its running concurrently and doesn't wait for other goroutines to finish because it doesn't know about other goroutines even exists

We Need Blocking!!!! Channels and Waitgroups are the way :)

3. Channels

• Channels are used to communicate between processes (goroutines) and its like a two ended pipe
• Channels send and receive are blocking by nature
• Data is communicated using channels and no shared data

Blocking Breaks Concurrency:

• Blocking leads to deadlocks
• Blocking can prevent scaling

Properties:

• Channel must have a type
• A Nil channel is of no use
• Sending data to closed channel will PANIC
• Sending to a channel will block the execution till receiver receives the inputted message, using this behavior we can write concurrent applications
• Waiting for a Receiver to receive from a channel having values will block (Deadlock)
• Sending to a channel but there is no receiver will block (Deadlock)

UnBuffered Channel:

• Channel with no capacity
• Sending data to closed channel will PANIC
• Sending to a channel with out receiver will create a leak (orphan goroutine where memory will never be released)
• Receiving from a channel where there is no data in the channel creates deadlock

Buffered Channel:

• Channel with a capacity
• Sends to a buffered channel are blocked only when the buffer is full
• Receives from a buffered channel are blocked only when the buffer is empty

4. Mutex:

Before going to Mutex we should have an idea about Critical Sections:

Critical Section is nothing but a section of code which contains mutation of a state of a shared variable

Mutex is meant for this reason, at a time only one thread (goroutine) enters into that critical section to avoid any race conditions

We can fix race conditions using both mutexes and buffered Channels of size 1 to ensure that one goroutine access the critical section at any point of time

mutex.Lock()  
a = a+1 
mutex.Unlock()  

ch := make(chan bool, 1)
ch <- true  
a = a+1 
<-ch

5. Waitgroups

• Waitgroups is used for waiting for a collection of goroutines to completes their tasks
• It provides a goroutine synchronization mechanism

wg.Add(i) - Adds i number of gorutines to the wait group 
wg.wait() - wait for all goroutines to finish

6. Select:

• Select statement blocks until one of the send/receive operation is ready. Takes any random one if both of them ready
• Select statement mainly used with the channels when you dont know which one is ready either send/receive
• Select statement default case is used to avoid panics if none of them ready and avoid blocking