Context Package As Data Bag

• Context Package can used to transport request-scoped data down the call graph.
• context.WithValue() provides a way to associate request-scoped values with Context.

context.WithValue() and context.Value()

• context.WithValue() - is used to associate request-scoped data with context.
• context.Value - is used to extract the value given a key from the context.

Parent Goroutine

type userIDType string
ctx := context.WithValue(context.Background(),userIDType("userIDKey"),"Kimi")

Child Goroutine

userId := context.Value(userIDType("userIDKey")).(userIDType)

exercise

witvalue() https://go.dev/play/p/kGdlPUgbTxL

package main

import (
    "context"
    "fmt"
)

type database map[string]bool
type userIDKey string

var db database = database{
    "Kimi": true,
}

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    processRequest(ctx, "Kimi")
}

func processRequest(ctx context.Context, userid string) {
    // send userID information to checkMemberShip through context for
    // map lookup.
    vctx := context.WithValue(ctx, userIDKey("userIDKey"), userid)
    ch := checkMemberShip(vctx)
    status := <-ch
    fmt.Printf("membership status of userid : %s : %v\n", userid, status)
}

// checkMemberShip - takes context as input.
// extracts the user id information from context.
// spins a goroutine to do map lookup
// sends the result on the returned channel.
func checkMemberShip(ctx context.Context) <-chan bool {
    ch := make(chan bool)
    go func() {
        defer close(ch)
        // do some database lookup
        userid := ctx.Value(userIDKey("userIDKey")).(string)
        status := db[userid]
        ch <- status
    }()
    return ch
}