Fetch value from net/context in Golang

Question

I am using Golang net/context package for passing on the ID's wrapped in a context object from one service to another. I am able to pass the context object successfully but to actually retrieve the value of a particular key, the context.Value(key) always returns a nil. I am not sure why, but this is what I have done so far:

if ctx != nil {
        fmt.Println(ctx)
        fmt.Println("Found UUID, forwarding it")

        id, ok := ctx.Value(0).(string)  // This always returns a nil and thus ok is set to false
        if ok {
            fmt.Println("id found %s", id)
            req.headers.Set("ID", id)
        }
    }  

ctx is of the type context.Context and on printing it I get:

context.Background.WithValue(0, "12345")

I am interested in fetching the value "12345" from the context. From the Golang net/context documentation (https://blog.golang.org/context) , the Value() takes in a key of interface{} type and returns a interface{} and hence I typecast is to .(string). Could anyone help with this ?

Answer 1

Your context key is isn't an int, which is the default type the untyped constant 0 will be assigned to when passed to Value in an interface{}.

c := context.Background()

v := context.WithValue(c, int32(0), 1234)
fmt.Println(v.Value(int64(0)))  // prints <nil>
fmt.Println(v.Value(int32(0)))  // print 1234

You need to set and extract the value with the correct type as well. You need to define a single type that you will always use as keys. I often times define helper functions to extract the context values and do the type assertion, which in your case could also serve to normalize the key type.