Go http client setup for multiple endpoints? - go

Question

I reuse the http client connection to make external calls to a single endpoint. An excerpt of the program is shown below:
var AppCon MyApp
func New(user, pass string, platformURL *url.URL, restContext string) (*MyApp, error) {
if AppCon == (MyApp{}) {
AppCon = MyApp{
user: user,
password: pass,
URL: platformURL,
Client: &http.Client{Timeout: 30 * time.Second},
RESTContext: restContext,
}
cj, err := cookiejar.New(nil)
if err != nil {
return &AppCon, err
}
AppCon.cookie = cj
}
return &AppCon, nil
}
// This is an example only. There are many more functions which accept *MyApp as a pointer.
func(ma *MyApp) GetUser(name string) (string, error){
// Return user
}
func main(){
for {
// Get messages from a queue
// The message returned from the queue provide info on which methods to call
// 'm' is a struct with message metadata
c, err := New(m.un, m.pass, m.url)
go func(){
// Do something i.e c.GetUser("123456")
}()
}
}
I now have the requirement to set up a client connections with different endpoints/credentials received via queue messages.
The problem I foresee is I can't just simply modify AppCon with the new endpoint details since a pointer to MyApp is returned, resulting in resetting c. This can impact a goroutine making a HTTP call to an unintended endpoint. To make matters non trivial, the program is not meant to have awareness of the endpoints (I was considering using a switch statement) but rather receive what it needs via queue messages.
Given the issues I've called out are correct, are there any recommendations on how to solve it?
EDIT 1
Based on the feedback provided, I am inclined to believe this will solve my problem:
Remove the use of a Singleton of MyApp
Decouple the http client from MyApp which will enable it for reuse
var httpClient *http.Client
func New(user, pass string, platformURL *url.URL, restContext string) (*MyApp, error) {
AppCon = MyApp{
user: user,
password: pass,
URL: platformURL,
Client: func() *http.Client {
if httpClient == nil {
httpClient = &http.Client{Timeout: 30 * time.Second}
}
return httpClient
}()
RESTContext: restContext,
}
return &AppCon, nil
}
// This is an example only. There are many more functions which accept *MyApp as a pointer.
func(ma *MyApp) GetUser(name string) (string, error){
// Return user
}
func main(){
for {
// Get messages from a queue
// The message returned from the queue provide info on which methods to call
// 'm' is a struct with message metadata
c, err := New(m.un, m.pass, m.url)
// Must pass a reference
go func(c *MyApp){
// Do something i.e c.GetUser("123456")
}(c)
}
}

Answer 1

Disclaimer: this is not a direct answer to your question but rather an attempt to direct you to a proper way of solving your problem.
Try to avoid a singleton pattern for you MyApp. In addition, New is misleading, it doesn't actually create a new object every time. Instead you could be creating a new instance every time, while preserving the http client connection.
Don't use constructions like this: AppCon == (MyApp{}), one day you will shoot in your leg doing this. Use instead a pointer and compare it to nil.
Avoid race conditions. In your code you start a goroutine and immediately proceed to the new iteration of the for loop. Considering you re-use the whole MyApp instance, you essentially introduce a race condition.
Using cookies, you make your connection kinda stateful, but your task seems to require stateless connections. There might be something wrong in such an approach.