Before Advice

Born to be the bouncer of your functions


We will go though a real Before advice. This advice prints a trace with the function invocations.


Let’s check that our environment is ready to follow the tutorial!

  • Install beyond tool & clone the beyond-examples repository
    >> go get
    >> git clone
    >> cd before

Let’s do it!

> Define the advice

Before advices must implement the interface Before (

type Before interface {
  Before(ctx *context.BeyondContext)

Open file advice/tracing.go and have a look at type TracingAdvice.

type TracingAdvice struct {
  prefix string

func (a *TracingAdvice) Before(ctx *context.BeyondContext) {
  params := make([]string, ctx.Params().Count())
  ctx.Params().ForEach(func(index int, arg *context.Arg) {
    params[index] = fmt.Sprintf("%s:%v", arg.Name(), arg.Value())

Type TracingAdvice

This is our advice. We can build more reusable and customizable advices by making use of attributes (TracingAdvice has a prefix attribute)

Method Before:

It contains the code to be executed before intercepted functions are invoked.

> Register the advice

  • Write a function (or many) that returns the Before advice

The functions signature must be:

func() Before

Check the following functions, in file advice/tracing.go,

func NewTracingAdvice() api.Before {
  return &TracingAdvice{}

func NewTracingAdviceWithPrefix(prefix string) func() api.Before {
  return func() api.Before {
    return &TracingAdvice{
      prefix: prefix,

Keep in mind that Beyond ignores non-exported functions.

  • Register the above functions

Open file cmd/main.go and have a look at function Beyond().

func Beyond() *api.Beyond {
  return api.New().
    WithBefore(advice.NewTracingAdvice, "greeting.Hello(...)...").
    WithBefore(advice.NewTracingAdviceWithPrefix("beyond"), "greeting.Bye(...)...")

func main() {

Two functions will be intercepted:

  • Function NewTracingAdvice will be executed before function Hello in file greeting/greeting.go is invoked
  • Function NewTracingAdviceWithPrefix will be executed before Bye in file greeting/greeting.go is invoked.

We will learn more about how to register advices in section JoinPoint Expressions

> Beyond in action

This would be the normal behavior

>> go run cmd/main.go
Hey John
Bye John

but when we execute beyond command …

>> beyond run cmd/main.go
Hey John
[Beyond] greeting.Bye(firstName:John)
Bye John


I purpose you to implement a new advice to put in practice what we learnt in this article.

  • Create a new advice that transforms the string params to uppercase or lowercase.
  • This new advice will be applied to both greeting.Hello and greeting.Bye functions. For the Hello function the advice will transform the retrieved param to uppercase and for the function Bye the param will be transformed to lowercase.
  • The result when running beyond run cmd/main.go must be:
    >> beyond run cmd/main.go
    Hey JOHN
    Bye john

Hint To face the challenge you could find useful the next functions

  • ctx.Params().At(index int): It returns the *Arg in the provided position.
  • ctx.Params().SetAt(index int,value interface{}): It updates the value for the argument in the provided position.
  • ctx.Params().Get(paramName string): It returns the *Arg with the provided name.
  • ctx.Params().Set(paramName string,paramValue interface{}): It updates the value for the argument with the provided name.

Check sections BeyondContext for more details.

If you find any problem to resolve this challenge, don’t hesitate to drop me an email at and I will be happy to give you some help.

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