Callback Magic with Go?

Published Jun 4, 2013

Go sounds great. You can write concurrent programs using goroutines and channels without ever touching that callback nonsense, right? Yeah!

I love FoundationDB. On the surface it’s extremely easy to use — it’s just a key-value store. All of the complexities are so well abstracted that you don’t have to worry about them. I wanted to try to use FoundationDB with Go. However, there’s no Go driver for it yet. I decided I should try to fix that.

FoundationDB’s C-API works with futures. There’s no way around them. Now, you can use futures in two ways. You can block and wait for a future to be ready, or you can set a callback (ew!).

Blocking doesn’t sound so bad with Go, right? If you have something that takes a long time, you’ll run it in a separate goroutine and keep going in your program. Goroutines are cheaper than threads so you don’t have to worry about a lot of overhead. I figured I could just launch a new goroutine and block for the future inside. Note that this doesn’t block the main program.

As it turns out, if you have a blocking system call in a goroutine, it will not yield to the scheduler. Keep in mind that a single OS thread will have multiple goroutines running on it. Go creates a maximum of N OS threads where N is the number of CPU cores you have available. If the Go runtime sees that a goroutine will not yield, it will spin up a new OS thread! These new threads don’t count towards the maximum.

So here’s the deal. If I end up creating 100 futures and block each of them in a separate goroutine, I’m essentially creating 100 OS threads. Suddenly, the idea of cheap goroutines is gone and I’m left with a poorly designed program. There’s only one option left: callbacks.

Here’s how to get futures working with Go. FoundationDB’s fdb_future_set_callback takes a function pointer and a callback argument that gets passed into the function. First you have to create a Go function that takes a pointer to a channel and sends something on the channel. This will be the callback. You’ll create a channel in your main program, and when you need the future to be ready, you’ll read something from the channel. If the future’s not ready, it’ll block. Otherwise, you’ll get a result immediately.

I know this is hard to follow. It takes a while to wrap your head around it. In the end, an end-user doesn’t really care. All of this callback magic is abstracted away and you end up with a really easy-to-use database driver.

Take a look at the entire prototype.