我应该为下游服务调用创建一个新的上下文还是仅传递上游上下文？

Question

The process is like this: for each incoming HTTP request, I have an early common handler which will create the context for the request, also there are some other common handlers which will do some pre-processing/parsing operations then put some data in the context for future usages, eventually will call one downstream grpc API.

Now I cannot make my mind on this:

1. I just simply pass the context created by the early common handler to the grpc API call, the concern is the downstream API doesn't need those data previously I put in the context, so I plan to use the WithValue(parent Context, key, val interface{}) Context API to set them all to nil before making the grpc call;

2. I create a totally new context outgoingCtx := metadata.NewOutgoingContext(context.Background(), md), then upon the early context ctx.Done() I called the Cancel() on this outgoingCtx;

Which option is better, or it doesn't really make any difference, even for very high concurrency case?

Answer 1

Context values are not transported across network boundaries. It's not possible since there is no way to get a list of all values from a context without knowing all the keys.

What is transported is the cancelation signal. That is, if the client of your service gives up before an RPC completes, upstream services can stop their work as well. Whether or not that is desired is a case-by-case decision: you may not want to cancel sending email after registration has already completed, but it's usually totaly fine to cancel read-only operations.