Go lang global variables without goroutines overwriting

Question

I'm writing a CMS in Go and have a session type (user id, page contents to render, etc). Ideally I'd like that type to be a global variable so I'm not having to propagate it through all the nested functions, however having a global variable like that would obviously mean that each new session would overwrite it's predecessor, which, needlessly to say, would be an epic fail.

Some languages to offer a way of having globals within threads that are preserved within that thread (ie the value of that global is sandboxed within that thread). While I'm aware that Goroutines are not threading, I just wondered if there was a similar method at my disposal or if I'd have to pass a local pointer of my session type down through the varies nested routines.

I'm guessing channels wouldn't do this? From what I can gather (and please correct me if I'm wrong here), but they're basically just a safe way of sharing global variables?

edit: I'd forgotten about this question! Anyhow, an update for anyone who is curious. This question was written back when I was new to Go and the CMS was basically my first project. I was coming from a C background with familiarity with POSIX thread but I quickly realised a better approach was to write the code in a mode functional design with session objects passed down as pointers in function parameters. This gave me both the context-sensitive local scope I was after while also minimizing the amount to data I was copying about. However being a 7 year old project and one that was at the start of my transition to Go, it's fair to say the project could do with a major rewrite anyway as there are a lot of mistakes made. That's a concern for another day though - currently it works and I have enough other projects on the go at.

Answer 1

You'll want to use something like a Context:

http://blog.golang.org/context

Basically, the pattern is to create a Context for each unique thing you want to do. (A web request in your case.) Use context.WithValue to embed multiple variables in the context. Then always pass it as the first parameter to other methods that are doing further work in other goroutines.

Getting the variable you need out of the context is a matter of calling context.Value from within any goroutine. From the above link:

A Context is safe for simultaneous use by multiple goroutines. Code can pass a single Context to any number of goroutines and cancel that Context to signal all of them.

I had an implementation where I was explicitly sending variables as method parameters, and I discovered that embedding these variables using contexts significantly cleaned up my code.

Using a Context also helps because it provides ways to end long-running tasks by using channels, select, and a concept called a "done channel." See this article for a great basic review and implementation:

http://blog.golang.org/pipelines

I'd recommend reading the pipelines article first for a good flavor of how to manage communication among goroutines, then the context article for a better idea of how to level-up and start embedding variables to pass around.

Good luck!

Answer 2

Don't use global variables. Use Go goroutine-local variables.

go-routine Id..

There are already goroutine-local variables: they are called function arguments, function return values, and local variables.

Russ

Answer 3

If you have more than one user, then wouldn't you need that info for each connection? So I would think that you'd have a struct per connected user. It would be idiomatic Go to pass a pointer to that struct when setting up the worker goroutine, or passing the pointer over a channel.