The use of context in Go can help you pass metadata through your program with helpful, related information about a call.
Let’s build an example where we set a context key, “stack”, which keeps a history of the function names called over the lifetime of the context.
As we pass the context object through a few layers of functions, we’ll append the name of the function to the value of the context key "stack".
package main
import (
"context"
"fmt"
)
func main() {
ctx := context.Background()
Handler(ctx)
}
func Handler(ctx context.Context) {
ctx = buildStackContext(ctx, "Handler")
Service(ctx)
}
func Service(ctx context.Context) {
ctx = buildStackContext(ctx, "Service")
Gateway(ctx)
}
func Gateway(ctx context.Context) {
ctx = buildStackContext(ctx, "Gateway")
// print the final value of "stack" on the context
fmt.Println(ctx.Value("stack"))
}
func buildStackContext(ctx context.Context, name string) context.Context {
// check if "stack" is initialized on the context
value, ok := ctx.Value("stack").(string)
if !ok {
return context.WithValue(ctx, "stack", name)
}
return context.WithValue(
ctx,
"stack",
fmt.Sprintf("%s:%s", value, name),
)
}
https://play.golang.org/p/Q-2AmWQ-bf6
In the code above, we initialize an empty context in our main function, then pass it down into some methods: Handler, Service and Gateway respectively.
In Gateway, we print the final value of the "stack" key on the context object, which is Handler:Service:Gateway.
You’ll notice we’ve had to hardcode the names of the functions ourselves which are appended to the "stack" context variable when we explicitly pass them into buildStackContext.
However, we can improve this.
By inspecting the Go runtime, we can programmatically look up the name of the function that calls buildStackContext and append that to the "stack" variable in the context:
package main
import (
"context"
"fmt"
"runtime"
"strings"
)
func main() {
ctx := context.Background()
Handler(ctx)
}
func Handler(ctx context.Context) {
ctx = buildStackContext(ctx)
Service(ctx)
}
func Service(ctx context.Context) {
ctx = buildStackContext(ctx)
Gateway(ctx)
}
func Gateway(ctx context.Context) {
ctx = buildStackContext(ctx)
// print the final value of "stack" on the context
fmt.Println(ctx.Value("stack"))
}
func buildStackContext(ctx context.Context) context.Context {
var name string
// inspect the runtime for the name of the caller
pc, _, _, ok := runtime.Caller(1)
details := runtime.FuncForPC(pc)
if ok && details != nil {
name = details.Name()
// break down the name,
// otherwise the package name will be included as well
// for example: "main.Handler"
split := strings.Split(name, ".")
name = split[len(split)-1]
}
// check if "stack" is initialized on the context
value, ok := ctx.Value("stack").(string)
if !ok {
return context.WithValue(ctx, "stack", name)
}
return context.WithValue(
ctx,
"stack",
fmt.Sprintf("%s:%s", value, name),
)
}
https://play.golang.org/p/AXOPYBr5SKF
The above code yields the same output, Handler:Service:Gateway, but it allows us to arbitrarily add more function calls or change function names and still get the expected stack of function calls:
package main
import (
"context"
"fmt"
"runtime"
"strings"
)
func main() {
ctx := context.Background()
Consumer(ctx)
}
func Consumer(ctx context.Context) {
ctx = buildStackContext(ctx)
Service(ctx)
}
func Service(ctx context.Context) {
ctx = buildStackContext(ctx)
Gateway(ctx)
}
func Gateway(ctx context.Context) {
ctx = buildStackContext(ctx)
Client(ctx)
}
func Client(ctx context.Context) {
ctx = buildStackContext(ctx)
// print the final value of "stack" on the context
fmt.Println(ctx.Value("stack"))
}
func buildStackContext(ctx context.Context) context.Context {
var name string
// inspect the runtime for the name of the caller
pc, _, _, ok := runtime.Caller(1)
details := runtime.FuncForPC(pc)
if ok && details != nil {
name = details.Name()
// break down the name, otherwise the package name will be included as well
// for example: "main.Handler"
split := strings.Split(name, ".")
name = split[len(split)-1]
}
// check if "stack" is initialized on the context
value, ok := ctx.Value("stack").(string)
if !ok {
return context.WithValue(ctx, "stack", name)
}
return context.WithValue(
ctx,
"stack",
fmt.Sprintf("%s:%s", value, name),
)
}
https://play.golang.org/p/Eb8eZ5AfWke
The above prints: Consumer:Service:Gateway:Client.
Using context in this way can provide useful namespacing in logs, making it easier to distinguish homogeneous log statements.