Context Package Go Idioms

Incoming requests to a server should create a Context

• Create context early in processing task or request.
• Create a top level context

  func main(){
    ctx := context.Background()
  }
  

• http.Request value already contains a Context.

  func handleFunc(w http.ResponseWritter, req *http.Request){
    ctx, cancel = context.WithCancel(req.Context())
  }
  

Outgoing calls to servers should accept a Context

• Higher level calls need to tell lower level calls how long they are willing to wait. ```go // Create a context with a timeout of 100 milliseconds. ctx, cancel := context.WithTimeout(req.Context(), 100*time.Millsecond) defer cancel()

// Bind the new context into the request req = req.WithContext(ctx)

// Do will handle the context level timeout. resp, err := http.DefaultClient.Do(req) ```

• http.DefaultClient.Do() method to respect cancellation signal on timer expiry and return with error message.

Pass a Context to function performing I/O

• Any function that is performing I/O should accept a Context value as it's first parameter and respect any timeout or deadline configured by the caller.
• Any API's that takes a Context, the idiom is to have the first parameter accept the Context value.

Any change to a Context value creates a new Context value that is then propagated forward.

When a Context is cancelled, all Contexts derived from it are also cancelled.

• If a parent Context is cancelled, all children derived by that parent Context are cancelled as well.

Use TODO context if you are unsure about which Context to use

• If a function is not responsible for creating top level context.
• We need a temporary yop-level Context until we figured out where the actual Context will come from.

Use context value only for request-scoped data

• Do not use the Context value to pass data into a function which becomes essential for its successful execution.
• A function should be able to execute its logic with an empty Context value.

Summary

• Incoming requests to a server should create a Context.
• Outgoing calls to servers should accept a Context.
• Any function that performing I/O should accept a Context value.
• If a parent Context is cancelled, all children derived from it are also cancelled.
• Use TODO context if you are unsure about which Context to use.
• Use context values only for request-scoped data, not for passing optional parameters to functions.