prakharsrivastav / go-context

Understanding context cancellation and timeouts in golang

Understanding context in go

This repository intends to explain how timeout and cancellations can be handled usingcontext package in go. This repository supports the examples discussed in this blog post.

Creating new context

• context.Background() : Use when creating fresh default context.
• context.Todo() : For future use.

Deriving context from an existing context

• context.WithCancel(parentCtx) : Provides a new context with a CancelFunc. CancelFunc can be used for explicit cancellation.
• context.WithDeadline(parentCtx) : Provides a new context that will automatically cancel after a durations. Also provides CancelFunc for explicit cancellation.
• context.WithTimeout(parentCtx) : Provides a new context that will expire at a given timestamp. Also provides CancelFunc for explicit cancellation.

Cancelling a context

1. cancelling explicitly

func main() {
    _,cancel := context.WithCancel(parentCtx)
    // call the cancel to cancel the context
    cancel()
}

2. cancelling after few seconds

func main() {
    // cancel the context in 10 seconds
    _,cancel := context.WithTimeout(parentCtx, time.Second * 10) 
    
    // cancel explicitly (when function returns)
    defer cancel()  
}

3. cancelling at a given time

func main() {
    // cancel the context at given time
    _,cancel := context.WithDeadline(parentCtx, time.Now().Add(2 * time.Second)) 

    // cancel explicitly (when function returns)
    defer cancel()
}

go-context's Issues

Calling cancel() more than once

Hi,

First, thanks for a great write-up on Context and Cancellation and the example code. I just wanted to add that in my opinion, cancel() can be called as often as required and all calls after the first are simple ignored. Have a look at the source of the context package. I believe it is considered best practice to defer cancel() as soon as the new cancellable context is created which is guaranteed to call cancel() even if it was called in the intervening lines.

I must say that I am no expert and would love to be corrected

I did enjoy reading your examples and many thanks for that.