Context

Why

• Timeout

• Deadline

• Channel

• Share data

Scenarios

• Web request
• HTTP request
• gRPC

How to use

Create a context

ctx, cancel := context.Background()
ctx, cancel := context.TODO() // less common

Use withValue to share data

package main

import (
    "context"
    "fmt"
)

func main() {
    ctx := context.Background()
    ctx = addValue(ctx)
    readValue(ctx)
}

func addValue(ctx context.Context) context.Context {
    return context.WithValue(ctx, "key", "test-value")
}

func readValue(ctx context.Context) {
    val := ctx.Value("key")
    fmt.Println(val) // test-value
}

Use cancellation to terminate all process in the same pipeline

func operation1(ctx context.Context) error {
	// Let's assume that this operation failed for some reason
	// We use time.Sleep to simulate a resource intensive operation
	time.Sleep(100 * time.Millisecond)
	return errors.New("failed")
}

func operation2(ctx context.Context) {
	// We use a similar pattern to the HTTP server
	// that we saw in the earlier example
	select {
	case <-time.After(500 * time.Millisecond):
		fmt.Println("done")
	case <-ctx.Done():
		fmt.Println("halted operation2")
	}
}

func main() {
	// Create a new context
	ctx := context.Background()
	// Create a new context, with its cancellation function
	// from the original context
	ctx, cancel := context.WithCancel(ctx)

	// Run two operations: one in a different go routine
	go func() {
		err := operation1(ctx)
		// If this operation returns an error
		// cancel all operations using this context
		if err != nil {
			cancel()
		}
	}()

	// Run operation2 with the same context we use for operation1
	operation2(ctx)
}

Use timeout and deadline so that app does not wait forever

// The context will be cancelled after 3 seconds
// If it needs to be cancelled earlier, the `cancel` function can
// be used, like before
ctx, cancel := context.WithTimeout(ctx, 3*time.Second)

// Setting a context deadline is similar to setting a timeout, except
// you specify a time when you want the context to cancel, rather than a duration.
// Here, the context will be cancelled on 2009-11-10 23:00:00
ctx, cancel := context.WithDeadline(ctx, time.Date(2009, time.November, 10, 23, 0, 0, 0, time.UTC))

Reference